Context

This repository is the basis for the blog post found here: https://blog.sigmaprime.io/solidity-security.html.

It forms the basis of the Smart Contract Security section in the Mastering Ethereum book: https://github.com/ethereumbook/ethereumbook

Summary

This post aims to be a relatively in-depth and up-to-date introductory post detailing the past mistakes that have been made by Solidity developers in an effort to prevent future devs from repeating history.

Although in its infancy, Solidity has had widespread adoption and is used to compile the byte-code in many Ethereum smart contracts we see today. There have been a number of harsh lessons learnt by developers and users alike in discovering the nuances of the language and the EVM. This post aims to be a relatively in-depth and up-to-date introductory post detailing the past mistakes that have been made by Solidity developers in an effort to prevent future devs from repeating history.

As this is a rapidly changing area, I've put this post on Github to encourage anyone/everyone to contribute to this post or submit issues for the errors that I've surely made.

Ethereum Wiki - Safety
Solidity Docs - Security Considerations
Consensus - Ethereum Smart Contract Best Practices
History of Ethereum Security Vulnerabilities, Hacks and Their Fixes
Decentralized Application Security Project (DASP) Top 10 of 2018
A Survey of attacks on Ethereum Smart Contracts
Ethereum Smart Contract Security
Lessons Learnt from the Underhanded Solidity Contest

1. Re-Entrancy

One of the features of Ethereum smart contracts is the ability to call and utilise code of other external contracts. Contracts also typically handle ether, and as such often send ether to various external user addresses. The operation of calling external contracts, or sending ether to an address, requires the contract to submit an external call. These external calls can be hijacked by attackers whereby they force the contract to execute further code (i.e. through a fallback function) , including calls back into itself. Thus the code execution "re-enters" the contract. Attacks of this kind were used in the infamous DAO hack.

For further reading on re-entrancy attacks, see Reentrancy Attack On Smart Contracts and Consensus - Ethereum Smart Contract Best Practices.

The Vulnerability

This attack can occur when a contract sends ether to an unknown address. An attacker can carefully construct a contract at an external address which contains malicious code in the fallback function. Thus, when a contract sends ether to this address, it will invoke the malicious code. Typically the malicious code executes a function on the vulnerable contract, performing operations not expected by the developer. The name "re-entrancy" comes from the fact that the external malicious contract calls back a function on the vulnerable contract and "re-enters" code execution at an arbitrary location on the vulnerable contract.

To clarify this, consider the simple vulnerable contract, which acts as an Ethereum vault that allows depositors to only withdraw 1 ether per week.

EtherStore.sol:

contract EtherStore {

    uint256 public withdrawalLimit = 1 ether;
    mapping(address => uint256) public lastWithdrawTime;
    mapping(address => uint256) public balances;

    function depositFunds() public payable {
        balances[msg.sender] += msg.value;
    }

    function withdrawFunds (uint256 _weiToWithdraw) public {
        require(balances[msg.sender] >= _weiToWithdraw);
        // limit the withdrawal
        require(_weiToWithdraw <= withdrawalLimit);
        // limit the time allowed to withdraw
        require(now >= lastWithdrawTime[msg.sender] + 1 weeks);
        require(msg.sender.call.value(_weiToWithdraw)());
        balances[msg.sender] -= _weiToWithdraw;
        lastWithdrawTime[msg.sender] = now;
    }
 }

This contract has two public functions. depositFunds() and withdrawFunds(). The depositFunds() function simply increments the senders balances. The withdrawFunds() function allows the sender to specify the amount of wei to withdraw. It will only succeed if the requested amount to withdraw is less than 1 ether and a withdrawal hasn't occurred in the last week. Or does it?...

The vulnerability comes on line [17] where we send the user their requested amount of ether. Consider a malicious attacker creating the following contract,

Attack.sol:

import "EtherStore.sol";

contract Attack {
  EtherStore public etherStore;

  // initialise the etherStore variable with the contract address
  constructor(address _etherStoreAddress) {
      etherStore = EtherStore(_etherStoreAddress);
  }

  function pwnEtherStore() public payable {
      // attack to the nearest ether
      require(msg.value >= 1 ether);
      // send eth to the depositFunds() function
      etherStore.depositFunds.value(1 ether)();
      // start the magic
      etherStore.withdrawFunds(1 ether);
  }

  function collectEther() public {
      msg.sender.transfer(this.balance);
  }

  // fallback function - where the magic happens
  function () payable {
      if (etherStore.balance > 1 ether) {
          etherStore.withdrawFunds(1 ether);
      }
  }
}

Let us see how this malicious contract can exploit our EtherStore contract. The attacker would create the above contract (let's say at the address 0x0...123) with the EtherStore's contract address as the constructor parameter. This will initialize and point the public variable etherStore to the contract we wish to attack.

The attacker would then call the pwnEtherStore() function, with some amount of ether (greater than or equal to 1), let's say 1 ether for this example. In this example we assume a number of other users have deposited ether into this contract, such that it's current balance is 10 ether. The following would then occur:

Attack.sol - Line [15] - The depositFunds() function of the EtherStore contract will be called with a msg.value of 1 ether (and a lot of gas). The sender (msg.sender) will be our malicious contract (0x0...123). Thus, balances[0x0..123] = 1 ether.
Attack.sol - Line [17] - The malicious contract will then call the withdrawFunds() function of the EtherStore contract with a parameter of 1 ether. This will pass all the requirements (Lines [12]-[16] of the EtherStore contract) as we have made no previous withdrawals.
EtherStore.sol - Line [17] - The contract will then send 1 ether back to the malicious contract.
Attack.sol - Line [25] - The ether sent to the malicious contract will then execute the fallback function.
Attack.sol - Line [26] - The total balance of the EtherStore contract was 10 ether and is now 9 ether so this if statement passes.
Attack.sol - Line [27] - The fallback function then calls the EtherStore withdrawFunds() function again and "re-enters" the EtherStore contract.
EtherStore.sol - Line [11] - In this second call to withdrawFunds(), our balance is still 1 ether as line [18] has not yet been executed. Thus, we still have balances[0x0..123] = 1 ether. This is also the case for the lastWithdrawTime variable. Again, we pass all the requirements.
EtherStore.sol - Line [17] - We withdraw another 1 ether.
Steps 4-8 will repeat - until EtherStore.balance >= 1 as dictated by line [26] in Attack.sol.
Attack.sol - Line [26] - Once there less 1 (or less) ether left in the EtherStore contract, this if statement will fail. This will then allow lines [18] and [19] of the EtherStore contract to be executed (for each call to the withdrawFunds() function).
EtherStore.sol - Lines [18] and [19] - The balances and lastWithdrawTime mappings will be set and the execution will end.

The final result, is that the attacker has withdrawn all (bar 1) ether from the EtherStore contract, instantaneously with a single transaction.

Preventative Techniques

There are a number of common techniques which help avoid potential re-entrancy vulnerabilities in smart contracts. The first is to ( whenever possible) use the built-in transfer() function when sending ether to external contracts. The transfer function only sends 2300 gas with the external call, which isn't enough for the destination address/contract to call another contract (i.e. re-enter the sending contract).

The second technique is to ensure that all logic that changes state variables happen before ether is sent out of the contract (or any external call). In the EtherStore example, lines [18] and [19] of EtherStore.sol should be put before line [17]. It is good practice to place any code that performs external calls to unknown addresses as the last operation in a localised function or piece of code execution. This is known as the checks-effects-interactions pattern.

A third technique is to introduce a mutex. That is, to add a state variable which locks the contract during code execution, preventing reentrancy calls.

Applying all of these techniques (all three are unnecessary, but is done for demonstrative purposes) to EtherStore.sol, gives the re-entrancy-free contract:

contract EtherStore {

    // initialise the mutex
    bool reEntrancyMutex = false;
    uint256 public withdrawalLimit = 1 ether;
    mapping(address => uint256) public lastWithdrawTime;
    mapping(address => uint256) public balances;

    function depositFunds() public payable {
        balances[msg.sender] += msg.value;
    }

    function withdrawFunds (uint256 _weiToWithdraw) public {
        require(!reEntrancyMutex);
        require(balances[msg.sender] >= _weiToWithdraw);
        // limit the withdrawal
        require(_weiToWithdraw <= withdrawalLimit);
        // limit the time allowed to withdraw
        require(now >= lastWithdrawTime[msg.sender] + 1 weeks);
        balances[msg.sender] -= _weiToWithdraw;
        lastWithdrawTime[msg.sender] = now;
        // set the reEntrancy mutex before the external call
        reEntrancyMutex = true;
        msg.sender.transfer(_weiToWithdraw);
        // release the mutex after the external call
        reEntrancyMutex = false;
    }
 }

Real-World Example: The DAO

The DAO (Decentralized Autonomous Organization) was one of the major hacks that occurred in the early development of Ethereum. At the time, the contract held over $150 million USD. Re-entrancy played a major role in the attack which ultimately lead to the hard-fork that created Ethereum Classic (ETC). For a good analysis of the DAO exploit, see Phil Daian's post.

2. Arithmetic Over/Under Flows

The Ethereum Virtual Machine (EVM) specifies fixed-size data types for integers. This means that an integer variable, only has a certain range of numbers it can represent. A uint8 for example, can only store numbers in the range [0,255]. Trying to store 256 into a uint8 will result in 0. If care is not taken, variables in Solidity can be exploited if user input is unchecked and calculations are performed which result in numbers that lie outside the range of the data type that stores them.

For further reading on arithmetic over/under flows, see How to Secure Your Smart Contracts, Ethereum Smart Contract Best Practices and Ethereum, Solidity and integer overflows: programming blockchains like 1970

The Vulnerability

An over/under flow occurs when an operation is performed that requires a fixed size variable to store a number (or piece of data) that is outside the range of the variable's data type.

For example, subtracting 1 from a uint8 (unsigned integer of 8 bits, i.e. only positive) variable that stores 0 as it's value, will result in the number 255. This is an underflow. We have assigned a number below the range of the uint8, the result wraps around and gives the largest number a uint8 can store. Similarly, adding 2^8=256 to a uint8 will leave the variable unchanged as we have wrapped around the entire length of the uint (for the mathematicians, this is similar to adding $2\pi$ to the angle of a trigonometric function, $\sin(x) = \sin(x+2\pi)$). Adding numbers larger than the data type's range is called an overflow. For clarity, adding 257 to a uint8 that currently has a zero value will result in the number 1. It's sometimes instructive to think of fixed type variables being cyclic, where we start again from zero if we add numbers above the largest possible stored number, and vice-versa for zero (where we start counting down from the largest number the more we subtract from 0).

These kinds of numerical caveats allow attackers to misuse code and create unexpected logic flows. For example, consider the time locking contract below.

TimeLock.sol:

contract TimeLock {

    mapping(address => uint) public balances;
    mapping(address => uint) public lockTime;

    function deposit() public payable {
        balances[msg.sender] += msg.value;
        lockTime[msg.sender] = now + 1 weeks;
    }

    function increaseLockTime(uint _secondsToIncrease) public {
        lockTime[msg.sender] += _secondsToIncrease;
    }

    function withdraw() public {
        require(balances[msg.sender] > 0);
        require(now > lockTime[msg.sender]);
        uint transferValue = balances[msg.sender];
        balances[msg.sender] = 0;
        msg.sender.transfer(transferValue);
    }
}

This contract is designed to act like a time vault, where users can deposit ether into the contract and it will be locked there for at least a week. The user may extend the wait time to longer than 1 week if they choose, but once deposited, the user can be sure their ether is locked in safely for at least a week. Or can they?...

In the event a user is forced to hand over their private key (think hostage situation) a contract such as this may be handy to ensure ether is unobtainable in short periods of time. If a user had locked in 100 ether in this contract and handed their keys over to an attacker, an attacker could use an overflow to receive the ether, regardless of the lockTime.

The attacker could determine the current lockTime for the address they now hold the key for (its a public variable). Let's call this userLockTime. They could then call the increaseLockTime function and pass as an argument the number 2^256 - userLockTime. This number would be added to the current userLockTime and cause an overflow, resetting lockTime[msg.sender] to 0. The attacker could then simply call the withdraw function to obtain their reward.

Let's look at another example, this one from the Ethernaut Challanges.

SPOILER ALERT: If you've not yet done the Ethernaut challenges, this gives a solution to one of the levels.

pragma solidity ^0.4.18;

contract Token {

  mapping(address => uint) balances;
  uint public totalSupply;

  function Token(uint _initialSupply) {
    balances[msg.sender] = totalSupply = _initialSupply;
  }

  function transfer(address _to, uint _value) public returns (bool) {
    require(balances[msg.sender] - _value >= 0);
    balances[msg.sender] -= _value;
    balances[_to] += _value;
    return true;
  }

  function balanceOf(address _owner) public constant returns (uint balance) {
    return balances[_owner];
  }
}

This is a simple token contract which employs a transfer() function, allowing participants to move their tokens around. Can you see the error in this contract?

The flaw comes in the transfer() function. The require statement on line [13] can be bypassed using an underflow. Consider a user that has no balance. They could call the transfer() function with any non-zero _value and pass the require statement on line [13]. This is because balances[msg.sender] is zero (and a uint256) so subtracting any positive amount (excluding 2^256) will result in a positive number due to the underflow we described above. This is also true for line [14], where our balance will be credited with a positive number. Thus, in this example, we have achieved free tokens due to an underflow vulnerability.

Preventative Techniques

The (currently) conventional technique to guard against under/overflow vulnerabilities is to use or build mathematical libraries which replace the standard math operators; addition, subtraction and multiplication (division is excluded as it doesn't cause over/under flows and the EVM reverts on division by 0).

OppenZepplin have done a great job in building and auditing secure libraries which can be leveraged by the Ethereum community. In particular, their Safe Math Library is a reference or library to use to avoid under/over flow vulnerabilities.

To demonstrate how these libraries are used in Solidity, let us correct the TimeLock contract, using Open Zepplin's SafeMath library. The over flow-free contract would become:

library SafeMath {

  function mul(uint256 a, uint256 b) internal pure returns (uint256) {
    if (a == 0) {
      return 0;
    }
    uint256 c = a * b;
    assert(c / a == b);
    return c;
  }

  function div(uint256 a, uint256 b) internal pure returns (uint256) {
    // assert(b > 0); // Solidity automatically throws when dividing by 0
    uint256 c = a / b;
    // assert(a == b * c + a % b); // There is no case in which this doesn't hold
    return c;
  }

  function sub(uint256 a, uint256 b) internal pure returns (uint256) {
    assert(b <= a);
    return a - b;
  }

  function add(uint256 a, uint256 b) internal pure returns (uint256) {
    uint256 c = a + b;
    assert(c >= a);
    return c;
  }
}

contract TimeLock {
    using SafeMath for uint; // use the library for uint type
    mapping(address => uint256) public balances;
    mapping(address => uint256) public lockTime;

    function deposit() public payable {
        balances[msg.sender] = balances[msg.sender].add(msg.value);
        lockTime[msg.sender] = now.add(1 weeks);
    }

    function increaseLockTime(uint256 _secondsToIncrease) public {
        lockTime[msg.sender] = lockTime[msg.sender].add(_secondsToIncrease);
    }

    function withdraw() public {
        require(balances[msg.sender] > 0);
        require(now > lockTime[msg.sender]);
        uint transferValue = balances[msg.sender];
        balances[msg.sender] = 0;
        msg.sender.transfer(transferValue);
    }
}

Notice that all standard math operations have been replaced by the those defined in the SafeMath library. The TimeLock contract no longer performs any operation which is capable of doing an under/over flow.

Real-World Examples: PoWHC and Batch Transfer Overflow (CVE-2018–10299)

A 4chan group decided it was a great idea to build a ponzi scheme on Ethereum, written in Solidity. They called it the Proof of Weak Hands Coin (PoWHC). Unfortunately it seems that the author(s) of the contract hadn't seen over/under flows before and consequently, 866 ether was liberated from its contract. A good overview of how the underflow occurs (which is not too dissimilar to the Ethernaut challenge above) is given in Eric Banisadar's post.

Some developers also implemented a batchTransfer() function into some ERC20 token contracts. The implementation contained an overflow. This post explains it, however I think the title is misleading, in that it has nothing to do with the ERC20 standard, rather some ERC20 token contracts have a vulnerable batchTransfer() function implemented.

3. Unexpected Ether

Typically when ether is sent to a contract, it must execute either the fallback function, or another function described in the contract. There are two exceptions to this, where ether can exist in a contract without having executed any code. Contracts which rely on code execution for every ether sent to the contract can be vulnerable to attacks where ether is forcibly sent to a contract.

For further reading on this, see How to Secure Your Smart Contracts: 6 and Solidity security patterns - forcing ether to a contract .

The Vulnerability

A common defensive programming technique that is useful in enforcing correct state transitions or validating operations is invariant-checking. This technique involves defining a set of invariants (metrics or parameters that should not change) and checking these invariants remain unchanged after a single (or many) operation(s). This is typically good design, provided the invariants being checked are in fact invariants. One example of an invariant is the totalSupply of a fixed issuance ERC20 token. As no functions should modify this invariant, one could add a check to the transfer() function that ensures the totalSupply remains unmodified to ensure the function is working as expected.

In particular, there is one apparent invariant, that may be tempting to use but can in fact be manipulated by external users (regardless of the rules put in place in the smart contract) .This is the current ether stored in the contract. Often when developers first learn Solidity they have the misconception that a contract can only accept or obtain ether via payable functions. This misconception can lead to contracts that have false assumptions about the ether balance within them which can lead to a range of vulnerabilities. The smoking gun for this vulnerability is the (incorrect) use of this.balance. As we will see, incorrect uses of this.balance can lead to serious vulnerabilities of this type.

There are two ways in which ether can (forcibly) be sent to a contract without using a payable function or executing any code on the contract. These are listed below.

Self Destruct / Suicide

Any contract is able to implement the selfdestruct(address) function, which removes all bytecode from the contract address and sends all ether stored there to the parameter-specified address. If this specified address is also a contract, no functions (including the fallback) get called. Therefore, the selfdestruct() function can be used to forcibly send ether to any contract regardless of any code that may exist in the contract. This is inclusive of contracts without any payable functions. This means, any attacker can create a contract with a selfdestruct() function, send ether to it, call selfdestruct(target) and force ether to be sent to a target contract. Martin Swende has an excellent blog post describing some quirks of the self-destruct opcode (Quirk #2) along with a description of how client nodes were checking incorrect invariants which could have lead to a rather catastrophic nuking of clients.

Pre-sent Ether

The second way a contract can obtain ether without using a selfdestruct() function or calling any payable functions is to pre-load the contract address with ether. Contract addresses are deterministic, in fact the address is calculated from the keccak256 (sometimes synonomous with SHA3) hash of the address creating the contract and the transaction nonce which creates the contract. Specifically, it is of the form: address = sha3(rlp.encode([account_address,transaction_nonce])) (see Keyless Ether for some fun use cases of this). This means, anyone can calculate what a contract address will be before it is created and thus send ether to that address. When the contract does get created it will have a non-zero ether balance.

Let's explore some pitfalls that can arise given the above knowledge.

Consider the overly-simple contract,

EtherGame.sol:

contract EtherGame {

    uint public payoutMileStone1 = 3 ether;
    uint public mileStone1Reward = 2 ether;
    uint public payoutMileStone2 = 5 ether;
    uint public mileStone2Reward = 3 ether;
    uint public finalMileStone = 10 ether;
    uint public finalReward = 5 ether;

    mapping(address => uint) redeemableEther;
    // users pay 0.5 ether. At specific milestones, credit their accounts
    function play() public payable {
        require(msg.value == 0.5 ether); // each play is 0.5 ether
        uint currentBalance = this.balance + msg.value;
        // ensure no players after the game as finished
        require(currentBalance <= finalMileStone);
        // if at a milestone credit the players account
        if (currentBalance == payoutMileStone1) {
            redeemableEther[msg.sender] += mileStone1Reward;
        }
        else if (currentBalance == payoutMileStone2) {
            redeemableEther[msg.sender] += mileStone2Reward;
        }
        else if (currentBalance == finalMileStone ) {
            redeemableEther[msg.sender] += finalReward;
        }
        return;
    }

    function claimReward() public {
        // ensure the game is complete
        require(this.balance == finalMileStone);
        // ensure there is a reward to give
        require(redeemableEther[msg.sender] > 0);
        uint transferValue = redeemableEther[msg.sender];
        redeemableEther[msg.sender] = 0;
        msg.sender.transfer(transferValue);
    }
 }

This contract represents a simple game (which would naturally invoke race-conditions) whereby players send 0.5 ether quanta to the contract in hope to be the player that reaches one of three milestones first. Milestone's are denominated in ether. The first to reach the milestone may claim a portion of the ether when the game has ended. The game ends when the final milestone (10 ether) is reached and users can claim their rewards.

The issues with the EtherGame contract come from the poor use of this.balance in both lines [14] (and by association [16]) and [32]. A mischievous attacker could forcibly send a small amount of ether, let's say 0.1 ether via the selfdestruct() function (discussed above) to prevent any future players from reaching a milestone. As all legitimate players can only send 0.5 ether increments, this.balance would no longer be half integer numbers, as it would also have the 0.1 ether contribution. This prevents all the if conditions on lines [18], [21] and [24] from being true.

Even worse, a vengeful attacker who missed a milestone, could forcibly send 10 ether (or an equivalent amount of ether that pushes the contract's balance above the finalMileStone) which would lock all rewards in the contract forever. This is because the claimReward() function will always revert, due to the require on line [32] (i.e. this.balance is greater than finalMileStone).

Preventative Techniques

This vulnerability typically arises from the misuse of this.balance. Contract logic, when possible, should avoid being dependent on exact values of the balance of the contract because it can be artificially manipulated. If applying logic based on this.balance, ensure to account for unexpected balances.

If exact values of deposited ether are required, a self-defined variable should be used that gets incremented in payable functions, to safely track the deposited ether. This variable will not be influenced by the forced ether sent via a selfdestruct() call.

With this in mind, a corrected version of the EtherGame contract could look like:

contract EtherGame {

    uint public payoutMileStone1 = 3 ether;
    uint public mileStone1Reward = 2 ether;
    uint public payoutMileStone2 = 5 ether;
    uint public mileStone2Reward = 3 ether;
    uint public finalMileStone = 10 ether;
    uint public finalReward = 5 ether;
    uint public depositedWei;

    mapping (address => uint) redeemableEther;

    function play() public payable {
        require(msg.value == 0.5 ether);
        uint currentBalance = depositedWei + msg.value;
        // ensure no players after the game as finished
        require(currentBalance <= finalMileStone);
        if (currentBalance == payoutMileStone1) {
            redeemableEther[msg.sender] += mileStone1Reward;
        }
        else if (currentBalance == payoutMileStone2) {
            redeemableEther[msg.sender] += mileStone2Reward;
        }
        else if (currentBalance == finalMileStone ) {
            redeemableEther[msg.sender] += finalReward;
        }
        depositedWei += msg.value;
        return;
    }

    function claimReward() public {
        // ensure the game is complete
        require(depositedWei == finalMileStone);
        // ensure there is a reward to give
        require(redeemableEther[msg.sender] > 0);
        uint transferValue = redeemableEther[msg.sender];
        redeemableEther[msg.sender] = 0;
        msg.sender.transfer(transferValue);
    }
 }

Here, we have just created a new variable, depositedWei which keeps track of the known ether deposited, and it is this variable to which we perform our requirements and tests. Notice, that we no longer have any reference to this.balance.

Real-World Example: Unknown

I'm yet to find an example of this that has been exploited in the wild. However, a few examples of exploitable contracts were given in the Underhanded Solidity Contest.

4. Delegatecall

The CALL and DELEGATECALL opcodes are useful in allowing Ethereum developers to modularise their code. Standard external message calls to contracts are handled by the CALL opcode whereby code is run in the context of the external contract/function. The DELEGATECALL opcode is identical to the standard message call, except that the code executed at the targeted address is run in the context of the calling contract along with the fact that msg.sender and msg.value remain unchanged. This feature enables the implementation of libraries whereby developers can create reusable code for future contracts.

Although the differences between these two opcodes are simple and intuitive, the use of DELEGATECALL can lead to unexpected code execution.

For further reading, see Ethereum Stack Exchange Question, Solidity Docs and How to Secure Your Smart Contracts: 6.

The Vulnerability

The context preserving nature of DELEGATECALL has proved that building vulnerability-free custom libraries is not as easy as one might think. The code in libraries themselves can be secure and vulnerability-free however when run in the context of another application new vulnerabilities can arise. Let's see a fairly complex example of this, using Fibonacci numbers.

Consider the following library which can generate the Fibonacci sequence and sequences of similar form. FibonacciLib.sol¹

// library contract - calculates fibonacci-like numbers;
contract FibonacciLib {
    // initializing the standard fibonacci sequence;
    uint public start;
    uint public calculatedFibNumber;

    // modify the zeroth number in the sequence
    function setStart(uint _start) public {
        start = _start;
    }

    function setFibonacci(uint n) public {
        calculatedFibNumber = fibonacci(n);
    }

    function fibonacci(uint n) internal returns (uint) {
        if (n == 0) return start;
        else if (n == 1) return start + 1;
        else return fibonacci(n - 1) + fibonacci(n - 2);
    }
}

This library provides a function which can generate the n-th Fibonacci number in the sequence. It allows users to change the starting number of the sequence (start) and calculate the n-th Fibonacci-like numbers in this new sequence.

Let's now consider a contract that utilises this library.

FibonacciBalance.sol:

contract FibonacciBalance {

    address public fibonacciLibrary;
    // the current fibonacci number to withdraw
    uint public calculatedFibNumber;
    // the starting fibonacci sequence number
    uint public start = 3;
    uint public withdrawalCounter;
    // the fibonancci function selector
    bytes4 constant fibSig = bytes4(sha3("setFibonacci(uint256)"));

    // constructor - loads the contract with ether
    constructor(address _fibonacciLibrary) public payable {
        fibonacciLibrary = _fibonacciLibrary;
    }

    function withdraw() {
        withdrawalCounter += 1;
        // calculate the fibonacci number for the current withdrawal user
        // this sets calculatedFibNumber
        require(fibonacciLibrary.delegatecall(fibSig, withdrawalCounter));
        msg.sender.transfer(calculatedFibNumber * 1 ether);
    }

    // allow users to call fibonacci library functions
    function() public {
        require(fibonacciLibrary.delegatecall(msg.data));
    }
}

This contract allows a participant to withdraw ether from the contract, with the amount of ether being equal to the Fibonacci number corresponding to the participants withdrawal order; i.e., the first participant gets 1 ether, the second also gets 1, the third gets 2, the forth gets 3, the fifth 5 and so on (until the balance of the contract is less than the Fibonacci number being withdrawn).

There are a number of elements in this contract that may require some explanation. Firstly, there is an interesting-looking variable, fibSig. This holds the first 4 bytes of the Keccak (SHA-3) hash of the string "setFibonacci(uint256)". This is known as the function selector and is put into calldata to specify which function of a smart contract will be called. It is used in the delegatecall function on line [21] to specify that we wish to run the setFibonacci(uint256) function. The second argument in delegatecall is the parameter we are passing to the function. Secondly, we assume that the address for the FibonacciLib library is correctly referenced in the constructor (section External Contract Referencing discuss some potential vulnerabilities relating to this kind of contract reference initialisation).

Can you spot any error(s) in this contract? If you put this into remix, fill it with ether and call withdraw(), it will likely revert.

You may have noticed that the state variable start is used in both the library and the main calling contract. In the library contract, start is used to specify the beginning of the Fibonacci sequence and is set to 0, whereas it is set to 3 in the FibonacciBalance contract. You may also have noticed that the fallback function in the FibonacciBalance contract allows all calls to be passed to the library contract, which allows for the setStart() function of the library contract to be called also. Recalling that we preserve the state of the contract, it may seem that this function would allow you to change the state of the start variable in the local FibonnacciBalance contract. If so, this would allow one to withdraw more ether, as the resulting calculatedFibNumber is dependent on the start variable (as seen in the library contract). In actual fact, the setStart() function does not (and cannot) modify the start variable in the FibonacciBalance contract. The underlying vulnerability in this contract is significantly worse than just modifying the start variable.

Before discussing the actual issue, we take a quick detour to understanding how state variables (storage variables) actually get stored in contracts. State or storage variables (variables that persist over individual transactions) are placed into slots sequentially as they are introduced in the contract. (There are some complexities here, and I encourage the reader to read Layout of State Variables in Storage for a more thorough understanding).

As an example, let's look at the library contract. It has two state variables, start and calculatedFibNumber. The first variable is start, as such it gets stored into the contract's storage at slot[0] (i.e. the first slot). The second variable, calculatedFibNumber, gets placed in the next available storage slot, slot[1]. If we look at the function setStart(), it takes an input and sets start to whatever the input was. This function is therefore setting slot[0] to whatever input we provide in the setStart() function. Similarly, the setFibonacci() function sets calculatedFibNumber to the result of fibonacci(n). Again, this is simply setting storage slot[1] to the value of fibonacci(n).

Now let's look at the FibonacciBalance contract. Storage slot[0] now corresponds to fibonacciLibrary address and slot[1] corresponds to calculatedFibNumber. It is in this incorrect mapping that the vulnerability occurs. delegatecall preserves contract context. This means that code that is executed via delegatecall will act on the state (i.e. storage) of the calling contract.

Now notice that in withdraw() on line [21] we execute, fibonacciLibrary.delegatecall(fibSig,withdrawalCounter). This calls the setFibonacci() function, which as we discussed, modifies storage slot[1], which in our current context is calculatedFibNumber. This is as expected (i.e. after execution, calculatedFibNumber gets adjusted). However, recall that the start variable in the FibonacciLib contract is located in storage slot[0], which is the fibonacciLibrary address in the current contract. This means that the function fibonacci() will give an unexpected result. This is because it references start (slot[0]) which in the current calling context is the fibonacciLibrary address (which will often be quite large, when interpreted as a uint). Thus it is likely that the withdraw() function will revert as it will not contain uint(fibonacciLibrary) amount of ether, which is what calculatedFibNumber will return.

Even worse, the FibonacciBalance contract allows users to call all of the fibonacciLibrary functions via the fallback function on line [26]. As we discussed earlier, this includes the setStart() function. We discussed that this function allows anyone to modify or set storage slot[0]. In this case, storage slot[0] is the fibonacciLibrary address. Therefore, an attacker could create a malicious contract (an example of one is given below), convert the address to a uint (this can be done in python easily using int('<address>',16)) and then call setStart(<attack_contract_address_as_uint>). This will change fibonacciLibrary to the address of the attack contract. Then, whenever a user calls withdraw() or the fallback function, the malicious contract will run (which can steal the entire balance of the contract) because we've modified the actual address for fibonacciLibrary. An example of such an attack contract would be,

contract Attack {
    uint storageSlot0; // corresponds to fibonacciLibrary
    uint storageSlot1; // corresponds to calculatedFibNumber

    // fallback - this will run if a specified function is not found
    function() public {
        storageSlot1 = 0; // we set calculatedFibNumber to 0, so that if withdraw
        // is called we don't send out any ether.
        <attacker_address>.transfer(this.balance); // we take all the ether
    }
 }

Notice that this attack contract modifies the calculatedFibNumber by changing storage slot[1]. In principle, an attacker could modify any other storage slots they choose to perform all kinds of attacks on this contract. I encourage all readers to put these contracts into Remix and experiment with different attack contracts and state changes through these delegatecall functions.

It is also important to notice that when we say that delegatecall is state-preserving, we are not talking about the variable names of the contract, rather the actual storage slots to which those names point. As you can see from this example, a simple mistake, can lead to an attacker hijacking the entire contract and its ether.

Preventative Techniques

Solidity provides the library keyword for implementing library contracts (see the Solidity Docs for further details). This ensures the library contract is stateless and non-self-destructable. Forcing libraries to be stateless mitigates the complexities of storage context demonstrated in this section. Stateless libraries also prevent attacks whereby attackers modify the state of the library directly in order to affect the contracts that depend on the library's code. As a general rule of thumb, when using DELEGATECALL pay careful attention to the possible calling context of both the library contract and the calling contract, and whenever possible, build state-less libraries.

Real-World Example: Parity Multisig Wallet (Second Hack)

The Second Parity Multisig Wallet hack is an example of how the context of well-written library code can be exploited if run in its non-intended context. There are a number of good explanations of this hack, such as this overview: Parity MultiSig Hacked. Again by Anthony Akentiev, this stack exchange question and An In-Depth Look at the Parity Multisig Bug.

To add to these references, let's explore the contracts that were exploited. The library and wallet contract can be found on the parity github here.

Let's look at the relevant aspects of this contract. There are two contracts of interest contained here, the library contract and the wallet contract.

The library contract,

contract WalletLibrary is WalletEvents {

  ...

  // throw unless the contract is not yet initialized.
  modifier only_uninitialized { if (m_numOwners > 0) throw; _; }

  // constructor - just pass on the owner array to the multiowned and
  // the limit to daylimit
  function initWallet(address[] _owners, uint _required, uint _daylimit) only_uninitialized {
    initDaylimit(_daylimit);
    initMultiowned(_owners, _required);
  }

  // kills the contract sending everything to `_to`.
  function kill(address _to) onlymanyowners(sha3(msg.data)) external {
    suicide(_to);
  }

  ...

}

and the wallet contract,

contract Wallet is WalletEvents {

  ...

  // METHODS

  // gets called when no other function matches
  function() payable {
    // just being sent some cash?
    if (msg.value > 0)
      Deposit(msg.sender, msg.value);
    else if (msg.data.length > 0)
      _walletLibrary.delegatecall(msg.data);
  }

  ...

  // FIELDS
  address constant _walletLibrary = 0xcafecafecafecafecafecafecafecafecafecafe;
}

Notice that the Wallet contract essentially passes all calls to the WalletLibrary contract via a delegate call. The constant _walletLibrary address in this code snippet acts as a placeholder for the actually deployed WalletLibrary contract (which was at 0x863DF6BFa4469f3ead0bE8f9F2AAE51c91A907b4).

The intended operation of these contracts was to have a simple low-cost deployable Wallet contract whose code base and main functionality was in the WalletLibrary contract. Unfortunately, the WalletLibrary contract is itself a contract and maintains it's own state. Can you see why this might be an issue?

It is possible to send calls to the WalletLibrary contract itself. Specifically, the WalletLibrary contract could be initialised, and become owned. A user did this, by calling initWallet() function on the WalletLibrary contract, becoming an owner of the library contract. The same user, subsequently called the kill() function. Because the user was an owner of the Library contract, the modifier passed and the library contract suicided. As all Wallet contracts in existence refer to this library contract and contain no method to change this reference, all of their functionality, including the ability to withdraw ether is lost along with the WalletLibrary contract. More directly, all ether in all parity multi-sig wallets of this type instantly become lost or permanently unrecoverable.

5. Default Visibilities

Functions in Solidity have visibility specifiers which dictate how functions are allowed to be called. The visibility determines whether a function can be called externally by users, by other derived contracts, only internally or only externally. There are four visibility specifiers, which are described in detail in the Solidity Docs. Functions default to public allowing users to call them externally. Incorrect use of visibility specifiers can lead to some devestating vulernabilities in smart contracts as will be discussed in this section.

The Vulnerability

The default visibility for functions is public. Therefore functions that do not specify any visibility will be callable by external users. The issue comes when developers mistakenly ignore visibility specifiers on functions which should be private (or only callable within the contract itself).

Let's quickly explore a trivial example.

contract HashForEther {

    function withdrawWinnings() {
        // Winner if the last 8 hex characters of the address are 0.
        require(uint32(msg.sender) == 0);
        _sendWinnings();
     }

     function _sendWinnings() {
         msg.sender.transfer(this.balance);
     }
}

This simple contract is designed to act as an address guessing bounty game. To win the balance of the contract, a user must generate an Ethereum address whose last 8 hex characters are 0. Once obtained, they can call the WithdrawWinnings() function to obtain their bounty.

Unfortunately, the visibility of the functions have not been specified. In particular, the _sendWinnings() function is public and thus any address can call this function to steal the bounty.

Preventative Techniques

It is good practice to always specify the visibility of all functions in a contract, even if they are intentionally public. Recent versions of Solidity will now show warnings during compilation for functions that have no explicit visibility set, to help encourage this practice.

Real-World Example: Parity MultiSig Wallet (First Hack)

In the first Parity multi-sig hack, about $31M worth of Ether was stolen from primarily three wallets. A good recap of exactly how this was done is given by Haseeb Qureshi in this post.

Essentially, the multi-sig wallet (which can be found here) is constructed from a base Wallet contract which calls a library contract containing the core functionality (as was described in Real-World Example: Parity Multisig (Second Hack)). The library contract contains the code to initialise the wallet as can be seen from the following snippet

contract WalletLibrary is WalletEvents {

  ...

  // METHODS

  ...

  // constructor is given number of sigs required to do protected "onlymanyowners" transactions
  // as well as the selection of addresses capable of confirming them.
  function initMultiowned(address[] _owners, uint _required) {
    m_numOwners = _owners.length + 1;
    m_owners[1] = uint(msg.sender);
    m_ownerIndex[uint(msg.sender)] = 1;
    for (uint i = 0; i < _owners.length; ++i)
    {
      m_owners[2 + i] = uint(_owners[i]);
      m_ownerIndex[uint(_owners[i])] = 2 + i;
    }
    m_required = _required;
  }

  ...

  // constructor - just pass on the owner array to the multiowned and
  // the limit to daylimit
  function initWallet(address[] _owners, uint _required, uint _daylimit) {
    initDaylimit(_daylimit);
    initMultiowned(_owners, _required);
  }
}

Notice that neither of the functions have explicitly specified a visibility. Both functions default to public. The initWallet() function is called in the wallets constructor and sets the owners for the multi-sig wallet as can be seen in the initMultiowned() function. Because these functions were accidentally left public, an attacker was able to call these functions on deployed contracts, resetting the ownership to the attackers address. Being the owner, the attacker then drained the wallets of all their ether, to the tune of $31M.

6. Entropy Illusion

All transactions on the Ethereum blockchain are deterministic state transition operations. Meaning that every transaction modifies the global state of the Ethereum ecosystem and it does so in a calculable way with no uncertainty. This ultimately means that inside the blockchain ecosystem there is no source of entropy or randomness. There is no rand() function in Solidity. Achieving decentralised entropy (randomness) is a well established problem and many ideas have been proposed to address this (see for example, RandDAO or using a chain of Hashes as described by Vitalik in this post).

The Vulnerability

Some of the first contracts built on the Ethereum platform were based around gambling. Fundamentally, gambling requires uncertainty (something to bet on), which makes building a gambling system on the blockchain (a deterministic system) rather difficult. It is clear that the uncertainty must come from a source external to the blockchain. This is possible for bets amongst peers (see for example the commit-reveal technique), however, it is significantly more difficult if you want to implement a contract to act as the house (like in blackjack our roulette). A common pitfall is to use future block variables, such as hashes, timestamps, blocknumber or gas limit. The issue with these are that they are controlled by the miner who mines the block and as such are not truly random. Consider, for example, a roulette smart contract with logic that returns a black number if the next block hash ends in an even number. A miner (or miner pool) could bet \$1M on black. If they solve the next block and find the hash ends in an odd number, they would happily not publish their block and mine another until they find a solution with the block hash being an even number (assuming the block reward and fees are less than $1M). Using past or present variables can be even more devastating as Martin Swende demonstrates in his excellent blog post. Furthermore, using solely block variables mean that the pseudo-random number will be the same for all transactions in a block, so an attacker can multiply their wins by doing many transactions within a block (should there be a maximum bet).

Preventative Techniques

The source of entropy (randomness) must be external to the blockchain. This can be done amongst peers with systems such as commit-reveal, or via changing the trust model to a group of participants (such as in RandDAO). This can also be done via a centralised entity, which acts as a randomness oracle. Block variables (in general, there are some exceptions) should not be used to source entropy as they can be manipulated by miners.

Real-World Example: PRNG Contracts

Arseny Reutov wrote a blog post after he analysed 3649 live smart contracts which were using some sort of pseudo random number generator (PRNG) and found 43 contracts which could be exploited.

7. External Contract Referencing

One of the benefits of the Ethereum global computer is the ability to re-use code and interact with contracts already deployed on the network. As a result, a large number of contracts reference external contracts and in general operation use external message calls to interact with these contracts. These external message calls can mask malicious actors intentions in some non-obvious ways, which we will discuss.

The Vulnerability

In Solidity, any address can be cast as a contract regardless of whether the code at the address represents the contract type being cast. This can be deceiving, especially when the author of the contract is trying to hide malicious code. Let us illustrate this with an example:

Consider a piece of code which rudimentarily implements the Rot13 cipher.

Rot13Encryption.sol:

//encryption contract
contract Rot13Encryption {

   event Result(string convertedString);

    //rot13 encrypt a string
    function rot13Encrypt (string text) public {
        uint256 length = bytes(text).length;
        for (var i = 0; i < length; i++) {
            byte char = bytes(text)[i];
            //inline assembly to modify the string
            assembly {
                char := byte(0,char) // get the first byte
                if and(gt(char,0x6D), lt(char,0x7B)) // if the character is in [n,z], i.e. wrapping.
                { char:= sub(0x60, sub(0x7A,char)) } // subtract from the ascii number a by the difference char is from z.
                if iszero(eq(char, 0x20)) // ignore spaces
                {mstore8(add(add(text,0x20), mul(i,1)), add(char,13))} // add 13 to char.
            }
        }
        emit Result(text);
    }

    // rot13 decrypt a string
    function rot13Decrypt (string text) public {
        uint256 length = bytes(text).length;
        for (var i = 0; i < length; i++) {
            byte char = bytes(text)[i];
            assembly {
                char := byte(0,char)
                if and(gt(char,0x60), lt(char,0x6E))
                { char:= add(0x7B, sub(char,0x61)) }
                if iszero(eq(char, 0x20))
                {mstore8(add(add(text,0x20), mul(i,1)), sub(char,13))}
            }
        }
        emit Result(text);
    }
}

This code simply takes a string (letters a-z, without validation) and encrypts it by shifting each character 13 places to the right (wrapping around 'z'); i.e. 'a' shifts to 'n' and 'x' shifts to 'k'. The assembly in here is not important, so don't worry if it doesn't make any sense at this stage.

Consider the following contract which uses this code for its encryption,

import "Rot13Encryption.sol";

// encrypt your top secret info
contract EncryptionContract {
    // library for encryption
    Rot13Encryption encryptionLibrary;

    // constructor - initialise the library
    constructor(Rot13Encryption _encryptionLibrary) {
        encryptionLibrary = _encryptionLibrary;
    }

    function encryptPrivateData(string privateInfo) {
        // potentially do some operations here
        encryptionLibrary.rot13Encrypt(privateInfo);
     }
 }

The issue with this contract is that the encryptionLibrary address is not public or constant. Thus the deployer of the contract could have given an address in the constructor which points to this contract:

//encryption contract
contract Rot26Encryption {

   event Result(string convertedString);

    //rot13 encrypt a string
    function rot13Encrypt (string text) public {
        uint256 length = bytes(text).length;
        for (var i = 0; i < length; i++) {
            byte char = bytes(text)[i];
            //inline assembly to modify the string
            assembly {
                char := byte(0,char) // get the first byte
                if and(gt(char,0x6D), lt(char,0x7B)) // if the character is in [n,z], i.e. wrapping.
                { char:= sub(0x60, sub(0x7A,char)) } // subtract from the ascii number a by the difference char is from z.
                if iszero(eq(char, 0x20)) // ignore spaces
                {mstore8(add(add(text,0x20), mul(i,1)), add(char,26))} // add 13 to char.
            }
        }
        emit Result(text);
    }

    // rot13 decrypt a string
    function rot13Decrypt (string text) public {
        uint256 length = bytes(text).length;
        for (var i = 0; i < length; i++) {
            byte char = bytes(text)[i];
            assembly {
                char := byte(0,char)
                if and(gt(char,0x60), lt(char,0x6E))
                { char:= add(0x7B, sub(char,0x61)) }
                if iszero(eq(char, 0x20))
                {mstore8(add(add(text,0x20), mul(i,1)), sub(char,26))}
            }
        }
        emit Result(text);
    }
}

which implements the rot26 cipher (shifts each character by 26 places, get it? :p). Again, there is no need to understand the assembly in this contract. The deployer could have also linked the following contract:

contract Print{
    event Print(string text);

    function rot13Encrypt(string text) public {
        emit Print(text);
    }
 }

If the address of either of these contracts were given in the constructor, the encryptPrivateData() function would simply produce an event which prints the unencrypted private data. Although in this example a library-like contract was set in the constructor, it is often the case that a privileged user (such as an owner) can change library contract addresses. If a linked contract doesn't contain the function being called, the fallback function will execute. For example, with the line encryptionLibrary.rot13Encrypt(), if the contract specified by encryptionLibrary was:

 contract Blank {
     event Print(string text);
     function () {
         emit Print("Here");
         //put malicious code here and it will run
     }
 }

then an event with the text "Here" would be emitted. Thus if users can alter contract libraries, they can in principle get users to unknowingly run arbitrary code.

Note: Don't use encryption contracts such as these, as the input parameters to smart contracts are visible on the blockchain. Also the Rot cipher is not a recommended encryption technique :p

Preventative Techniques

As demonstrated above, vulnerability free contracts can (in some cases) be deployed in such a way that they behave maliciously. An auditor could publicly verify a contract and have it's owner deploy it in a malicious way, resulting in a publicly audited contract which has vulnerabilities or malicious intent.

There are a number of techniques which prevent these scenarios.

One technique, is to use the new keyword to create contracts. In the example above, the constructor could be written like:

    constructor() {
        encryptionLibrary = new Rot13Encryption();
    }

This way an instance of the referenced contract is created at deployment time and the deployer cannot replace the Rot13Encryption contract with anything else without modifying the smart contract.

Another solution is to hard code any external contract addresses if they are known.

In general, code that calls external contracts should always be looked at carefully. As a developer, when defining external contracts, it can be a good idea to make the contract addresses public (which is not the case in the honey-pot example given below) to allow users to easily examine which code is being referenced by the contract. Conversely, if a contract has a private variable contract address it can be a sign of someone behaving maliciously (as shown in the real-world example). If a privileged (or any) user is capable of changing a contract address which is used to call external functions, it can be important (in a decentralised system context) to implement a time-lock or voting mechanism to allow users to see which code is being changed or to give participants a chance to opt in/out with the new contract address.

Real-World Example: Re-Entrancy Honey Pot

A number of recent honey pots have been released on the mainnet. These contracts try to outsmart Ethereum hackers who try to exploit the contracts, but who in turn end up getting ether lost to the contract they expect to exploit. One example employs the above attack by replacing an expected contract with a malicious one in the constructor. The code can be found here:

pragma solidity ^0.4.19;

contract Private_Bank
{
    mapping (address => uint) public balances;
    uint public MinDeposit = 1 ether;
    Log TransferLog;

    function Private_Bank(address _log)
    {
        TransferLog = Log(_log);
    }

    function Deposit()
    public
    payable
    {
        if(msg.value >= MinDeposit)
        {
            balances[msg.sender]+=msg.value;
            TransferLog.AddMessage(msg.sender,msg.value,"Deposit");
        }
    }

    function CashOut(uint _am)
    {
        if(_am<=balances[msg.sender])
        {
            if(msg.sender.call.value(_am)())
            {
                balances[msg.sender]-=_am;
                TransferLog.AddMessage(msg.sender,_am,"CashOut");
            }
        }
    }

    function() public payable{}

}

contract Log
{
    struct Message
    {
        address Sender;
        string  Data;
        uint Val;
        uint  Time;
    }

    Message[] public History;
    Message LastMsg;

    function AddMessage(address _adr,uint _val,string _data)
    public
    {
        LastMsg.Sender = _adr;
        LastMsg.Time = now;
        LastMsg.Val = _val;
        LastMsg.Data = _data;
        History.push(LastMsg);
    }
}

This post by one reddit user explains how they lost 1 ether to this contract by trying to exploit the re-entrancy bug they expected to be present in the contract.

8. Short Address/Parameter Attack

This attack is not specifically performed on Solidity contracts themselves but on third party applications that may interact with them. I add this attack for completeness and to be aware of how parameters can be manipulated in contracts.

For further reading, see The ERC20 Short Address Attack Explained, ICO Smart contract Vulnerability: Short Address Attack or this reddit post.

The Vulnerability

When passing parameters to a smart contract, the parameters are encoded according to the ABI specification. It is possible to send encoded parameters that are shorter than the expected parameter length (for example, sending an address that is only 38 hex chars (19 bytes) instead of the standard 40 hex chars (20 bytes)). In such a scenario, the EVM will pad 0's to the end of the encoded parameters to make up the expected length.

This becomes an issue when third party applications do not validate inputs. The clearest example is an exchange which doesn't verify the address of an ERC20 token when a user requests a withdrawal. This example is covered in more detail in Peter Venesses' post, The ERC20 Short Address Attack Explained mentioned above.

Consider, the standard ERC20 transfer function interface, noting the order of the parameters,

function transfer(address to, uint tokens) public returns (bool success);

Now consider, an exchange, holding a large amount of a token (let's say REP) and a user wishes to withdraw their share of 100 tokens. The user would submit their address, 0xdeaddeaddeaddeaddeaddeaddeaddeaddeaddead and the number of tokens, 100. The exchange would encode these parameters in the order specified by the transfer() function, i.e. address then tokens. The encoded result would be a9059cbb000000000000000000000000deaddeaddeaddeaddeaddeaddeaddeaddeaddead0000000000000 000000000000000000000000000000000056bc75e2d63100000. The first four bytes (a9059cbb) are the transfer() function signature/selector, the second 32 bytes are the address, followed by the final 32 bytes which represent the uint256 number of tokens. Notice that the hex 56bc75e2d63100000 at the end corresponds to 100 tokens (with 18 decimal places, as specified by the REP token contract).

Ok, so now let's look at what happens if we were to send an address that was missing 1 byte (2 hex digits). Specifically, let's say an attacker sends 0xdeaddeaddeaddeaddeaddeaddeaddeaddeaddeas an address (missing the last two digits) and the same 100 tokens to withdraw. If the exchange doesn't validate this input, it would get encoded as a9059cbb000000000000000000000000deaddeaddeaddeaddeaddeaddeaddeaddeadde00000000000000 00000000000000000000000000000000056bc75e2d6310000000. The difference is subtle. Note that 00 has been padded to the end of the encoding, to make up for the short address that was sent. When this gets sent to the smart contract, the address parameters will read as 0xdeaddeaddeaddeaddeaddeaddeaddeaddeadde00 and the value will be read as 56bc75e2d6310000000 (notice the two extra 0's). This value is now, 25600 tokens (the value has been multiplied by 256). In this example, if the exchange held this many tokens, the user would withdraw 25600 tokens (whilst the exchange thinks the user is only withdrawing 100) to the modified address. Obviously the attacker won't possess the modified address in this example, but if the attacker were to generate any address which ended in 0's (which can be easily brute forced) and used this generated address, they could easily steal tokens from the unsuspecting exchange.

Preventative Techniques

I suppose it is obvious to say that validating all inputs before sending them to the blockchain will prevent these kinds of attacks. It should also be noted that parameter ordering plays an important role here. As padding only occurs at the end, careful ordering of parameters in the smart contract can potentially mitigate some forms of this attack.

Real-World Example: Unknown

I do not know of any publicised attack of this kind in the wild.

9. Unchecked CALL Return Values

There a number of ways of performing external calls in solidity. Sending ether to external accounts is commonly performed via the transfer() method. However, the send() function can also be used and, for more versatile external calls, the CALL opcode can be directly employed in solidity. The call() and send() functions return a boolean indicating if the call succeeded or failed. Thus these functions have a simple caveat, in that the transaction that executes these functions will not revert if the external call (initialised by call() or send()) fails, rather the call() or send() will simply return false. A common pitfall arises when the return value is not checked, rather the developer expects a revert to occur.

For further reading, see DASP Top 10 and Scanning Live Ethereum Contracts for the "Unchecked-Send" Bug.

The Vulnerability

Consider the following example:

contract Lotto {

    bool public payedOut = false;
    address public winner;
    uint public winAmount;

    // ... extra functionality here

    function sendToWinner() public {
        require(!payedOut);
        winner.send(winAmount);
        payedOut = true;
    }

    function withdrawLeftOver() public {
        require(payedOut);
        msg.sender.send(this.balance);
    }
}

This contract represents a Lotto-like contract, where a winner receives winAmount of ether, which typically leaves a little left over for anyone to withdraw.

The bug exists on line [11] where a send() is used without checking the response. In this trivial example, a winner whose transaction fails (either by running out of gas or being a contract that intentionally throws in the fallback function) allows payedOut to be set to true (regardless of whether ether was sent or not). In this case, the public can withdraw the winner's winnings via the withdrawLeftOver() function.

Preventative Techniques

Whenever possible, use the transfer() function rather than send() as transfer() will revert if the external transaction reverts. If send() is required, always ensure to check the return value.

An even more robust recommendation is to adopt a withdrawal pattern. In this solution, each user is burdened with calling an isolated function (i.e. a withdraw function) which handles the sending of ether out of the contract and therefore independently deals with the consequences of failed send transactions. The idea is to logically isolate the external send functionality from the rest of the code base and place the burden of potentially failed transaction to the end-user who is calling the withdraw function.

Real-World Example: Etherpot and King of the Ether

Etherpot was a smart contract lottery, not too dissimilar to the example contract mentioned above. The solidity code for etherpot, can be found here: lotto.sol. The primary downfall of this contract was due to an incorrect use of block hashes (only the last 256 block hashes are useable, see Aakil Fernandes's post about how Etherpot failed to implement this correctly). However this contract also suffered from an unchecked call value. Notice the function, cash() on line [80] of lotto.sol:

...
  function cash(uint roundIndex, uint subpotIndex){

        var subpotsCount = getSubpotsCount(roundIndex);

        if(subpotIndex>=subpotsCount)
            return;

        var decisionBlockNumber = getDecisionBlockNumber(roundIndex,subpotIndex);

        if(decisionBlockNumber>block.number)
            return;

        if(rounds[roundIndex].isCashed[subpotIndex])
            return;
        //Subpots can only be cashed once. This is to prevent double payouts

        var winner = calculateWinner(roundIndex,subpotIndex);
        var subpot = getSubpot(roundIndex);

        winner.send(subpot);

        rounds[roundIndex].isCashed[subpotIndex] = true;
        //Mark the round as cashed
}
...

Notice that on line [21] the send function's return value is not checked, and the following line then sets a boolean indicating the winner has been sent their funds. This bug can allow a state where the winner does not receive their ether, but the state of the contract can indicate that the winner has already been paid.

A more serious version of this bug occurred in the King of the Ether. An excellent post-mortem of this contract has been written which details how an unchecked failed send() could be used to attack the contract.

10. Race Conditions / Front Running

The combination of external calls to other contracts and the multi-user nature of the underlying blockchain gives rise to a variety of potential Solidity pitfalls whereby users race code execution to obtain unexpected states. Re-Entrancy is one example of such a race condition. In this section we will talk more generally about different kinds of race conditions that can occur on the Ethereum blockchain. There is a variety of good posts on this subject, a few are: Ethereum Wiki - Safety, DASP - Front-Running and the Consensus - Smart Contract Best Practices.

The Vulnerability

As with most blockchains, Ethereum nodes pool transactions and form them into blocks. The transactions are only considered valid once a miner has solved a consensus mechanism (currently ETHASH PoW for Ethereum). The miner who solves the block also chooses which transactions from the pool will be included in the block, this is typically ordered by the gasPrice of a transaction. In here lies a potential attack vector. An attacker can watch the transaction pool for transactions which may contain solutions to problems, modify or revoke the attacker's permissions or change a state in a contract which is undesirable for the attacker. The attacker can then get the data from this transaction and create a transaction of their own with a higher gasPrice and get their transaction included in a block before the original.

Let's see how this could work with a simple example. Consider the contract FindThisHash.sol:

contract FindThisHash {
    bytes32 constant public hash = 0xb5b5b97fafd9855eec9b41f74dfb6c38f5951141f9a3ecd7f44d5479b630ee0a;

    constructor() public payable {} // load with ether

    function solve(string solution) public {
        // If you can find the pre image of the hash, receive 1000 ether
        require(hash == sha3(solution));
        msg.sender.transfer(1000 ether);
    }
}

Imagine this contract contains 1000 ether. The user who can find the pre-image of the sha3 hash 0xb5b5b97fafd9855eec9b41f74dfb6c38f5951141f9a3ecd7f44d5479b630ee0a can submit the solution and retrieve the 1000 ether. Let's say one user figures out the solution is Ethereum!. They call solve() with Ethereum! as the parameter. Unfortunately an attacker has been clever enough to watch the transaction pool for anyone submitting a solution. They see this solution, check it's validity, and then submit an equivalent transaction with a much higher gasPrice than the original transaction. The miner who solves the block will likely give the attacker preference due to the higher gasPrice and accept their transaction before the original solver. The attacker will take the 1000 ether and the user who solved the problem will get nothing (there is no ether left in the contract).

A more realistic problem comes in the design of the future Casper implementation. The Casper proof of stake contracts invoke slashing conditions where users who notice validators double-voting or misbehaving are incentivised to submit proof that they have done so. The validator will be punished and the user rewarded. In such a scenario, it is expected that miners and users will front-run all such submissions of proof, and this issue must be addressed before the final release.

Preventative Techniques

There are two classes of users who can perform these kinds of front-running attacks. Users (who modify the gasPrice of their transactions) and miners themselves (who can re-order the transactions in a block how they see fit). A contract that is vulnerable to the first class (users), is significantly worse-off than one vulnerable to the second (miners) as miner's can only perform the attack when they solve a block, which is unlikely for any individual miner targeting a specific block. Here I'll list a few mitigation measures with relation to which class of attackers they may prevent.

One method that can be employed is to create logic in the contract that places an upper bound on the gasPrice. This prevents users from increasing the gasPrice and getting preferential transaction ordering beyond the upper-bound. This preventative measure only mitigates the first class of attackers (arbitrary users). Miners in this scenario can still attack the contract as they can order the transactions in their block however they like, regardless of gas price.

A more robust method is to use a commit-reveal scheme, whenever possible. Such a scheme dictates users send transactions with hidden information (typically a hash). After the transaction has been included in a block, the user sends a transaction revealing the data that was sent (the reveal phase). This method prevents both miners and users from frontrunning transactions as they cannot determine the contents of the transaction. This method however, cannot conceal the transaction value (which in some cases is the valuable information that needs to be hidden). The ENS smart contract allowed users to send transactions, whose committed data included the amount of ether they were willing to spend. Users could then send transactions of arbitrary value. During the reveal phase, users were refunded the difference between the amount sent in the transaction and the amount they were willing to spend.

A further suggestion by Lorenz, Phil, Ari and Florian is to use Submarine Sends. An efficient implementation of this idea requires the CREATE2 opcode, which currently hasn't been adopted, but seems likely in upcoming hard forks.

Real-World Examples: ERC20 and Bancor

The ERC20 standard is quite well-known for building tokens on Ethereum. This standard has a potential frontrunning vulnerability which comes about due to the approve() function. A good explanation of this vulnerability can be found here.

The standard specifies the approve() function as:

function approve(address _spender, uint256 _value) returns (bool success)

This function allows a user to permit other users to transfer tokens on their behalf. The frontrunning vulnerability comes in the scenario when a user, Alice, approves her friend, Bob to spend 100 tokens. Alice later decides that she wants to revoke Bob's approval to spend 100 tokens, so she creates a transaction that sets Bob's allocation to 50 tokens. Bob, who has been carefully watching the chain, sees this transaction and builds a transaction of his own spending the 100 tokens. He puts a higher gasPrice on his transaction than Alice's and gets his transaction prioritised over hers. Some implementations of approve() would allow Bob to transfer his 100 tokens, then when Alice's transaction gets committed, resets Bob's approval to 50 tokens, in effect giving Bob access to 150 tokens. The mitigation strategies of this attack are given here in the document linked above.

Another prominent, real-world example is Bancor. Ivan Bogatty and his team documented a profitable attack on the initial Bancor implementation. His blog post and Devon 3 talk discuss in detail how this was done. Essentially, prices of tokens are determined based on transaction value, users can watch the transaction pool for Bancor transactions and front run them to profit from the price differences. This attack has been addressed by the Bancor team.

11. Denial Of Service (DOS)

This category is very broad, but fundamentally consists of attacks where users can leave the contract inoperable for a small period of time, or in some cases, permanently. This can trap ether in these contracts forever, as was the case with the Second Parity MultiSig hack

The Vulnerability

There are various ways a contract can become inoperable. Here I will only highlight some potentially less-obvious Blockchain nuanced Solidity coding patterns that can lead to attackers performing DOS attacks.

1. External calls without gas stipends - It may be the case that you wish to make an external call to an unknown contract and continue processing the transaction regardless whether that call fails or not. Typically this is achieved by using the CALL opcode, which does not revert the transaction if the call fails (see Unchecked CALL Return Values for further details and examples). Let us consider a simple example, where we have a contract wallet, that slowly trickles out ether when the withdraw() function is called. A partner can add their address and spend gas to call the withdraw, giving both the partner and the owner 1% of the total contract balance.

contract TrickleWallet {

    address public partner; // withdrawal partner - pay the gas, split the withdraw
    address public constant owner = 0xA9E;
    uint timeLastWithdrawn;
    mapping(address => uint) withdrawPartnerBalances; // keep track of partners balances

    function setWithdrawPartner(address _partner) public {
        require(partner == '0x0' || msg.sender == partner);
        partner = _partner;
    }

    // withdraw 1% to recipient and 1% to owner
    function withdraw() public {
        uint amountToSend = address(this).balance/100;
        // perform a call without checking return
        // the recipient can revert, the owner will still get their share
        partner.call.value(amountToSend)();
        owner.transfer(amountToSend);
        // keep track of last withdrawal time
        timeLastWithdrawn = now;
        withdrawPartnerBalances[partner] += amountToSend;
    }

    // allow deposit of funds
    function() payable {}

    // convenience function
    function contractBalance() view returns (uint) {
        return address(this).balance;
    }
}

Notice that on line [17] we perform an external call sending 1% of the contract balance to a user-specified account. The reason the CALL opcode is used, is to ensure that the owner still gets paid, even if the external call reverts. The issue is that the transaction will send all of its gas (in reality, only most of the transaction gas is sent, some is left to finish processing the call) to the external call. If the user were malicious they could create a contract that would consume all the gas, and force all transactions to withdraw() to fail, due to running out of gas.

For example, consider the following malicious contract that consumes all gas,

contract ConsumeAllGas {
    function () payable {
        // an assert consumes all transaction gas, unlike a
        //revert which returns the remaining gas
        assert(1==2);
    }
}

If a withdrawal partner decided they didn't like the owner of the contract. They could set the partner address to this contract and lock all the funds in the TrickleWallet contract forever.

To prevent such DOS attack vectors, ensure a gas stipend is specified in an external call, to limit the amount of gas that that transaction can use. In our example, we could remedy this attack by changing line [17] to:

partner.call.gas(50000).value(amountToSend)();

This modification allows only 50,000 gas to be spent on the external transaction. The owner may set a gas price larger than this, in order to have their transaction complete, regardless of how much the external transaction uses.

2. Looping through externally manipulated mappings or arrays - In my adventures I've seen various forms of this kind of pattern. Typically it appears in scenarios where an owner wishes to distribute tokens amongst their investors, and do so with a distribute()-like function as can be seen in the example contract:

contract DistributeTokens {
    address public owner; // gets set somewhere
    address[] investors; // array of investors
    uint[] investorTokens; // the amount of tokens each investor gets

    // ... extra functionality, including transfertoken()

    function invest() public payable {
        investors.push(msg.sender);
        investorTokens.push(msg.value * 5); // 5 times the wei sent
        }

    function distribute() public {
        require(msg.sender == owner); // only owner
        for(uint i = 0; i < investors.length; i++) {
            // here transferToken(to,amount) transfers "amount" of tokens to the address "to"
            transferToken(investors[i],investorTokens[i]);
        }
    }
}

Notice that the loop in this contract runs over an array which can be artificially inflated. An attacker can create many user accounts making the investor array large. In principle this can be done such that the gas required to execute the for loop exceeds the block gas limit, essentially making the distribute() function inoperable.

3. Owner operations - Another common pattern is where owners have specific privileges in contracts and must perform some task in order for the contract to proceed to the next state. One example would be an ICO contract that requires the owner to finalize() the contract which then allows tokens to be transferable, i.e.

bool public isFinalized = false;
address public owner; // gets set somewhere

function finalize() public {
    require(msg.sender == owner);
    isFinalized == true;
}

// ... extra ICO functionality

// overloaded transfer function
function transfer(address _to, uint _value) returns (bool) {
    require(isFinalized);
    super.transfer(_to,_value)
}

...

In such cases, if a privileged user loses their private keys, or becomes inactive, the entire token contract becomes inoperable. In this case, if the owner cannot call finalize() no tokens can be transferred; i.e. the entire operation of the token ecosystem hinges on a single address.

4. Progressing state based on external calls - Contracts are sometimes written such that in order to progress to a new state requires sending ether to an address, or waiting for some input from an external source. These patterns can lead to DOS attacks, when the external call fails or is prevented for external reasons. In the example of sending ether, a user can create a contract which does not accept ether. If a contract requires ether to be withdrawn (consider a time-locking contract that requires all ether to be withdrawn before being useable again) in order to progress to a new state, the contract will never achieve the new state as ether can never be sent to the user's contract which does not accept ether.

Preventative Techniques

In the first example, contracts should not loop through data structures that can be artificially manipulated by external users. A withdrawal pattern is recommended, whereby each of the investors call a withdraw function to claim tokens independently.

In the second example a privileged user was required to change the state of the contract. In such examples (wherever possible) a fail-safe can be used in the event that the owner becomes incapacitated. One solution could be setting up the owner as a multisig contract. Another solution is to use a timelock, where the require on line [13] could include a time-based mechanism, such as require(msg.sender == owner || now > unlockTime) which allows any user to finalise after a period of time, specified by unlockTime. This kind of mitigation technique can be used in the third example also. If external calls are required to progress to a new state, account for their possible failure and potentially add a time-based state progression in the event that the desired call never comes.

Note: Of course there are centralised alternatives to these suggestions where one can add a maintenanceUser who can come along and fix problems with DOS-based attack vectors if need be. Typically these kinds of contracts contain trust issues over the power of such an entity, but that is not a conversation for this section.

Real-World Examples: GovernMental

GovernMental was an old Ponzi scheme that accumulated quite a large amount of ether. In fact, at one point it had accumulated 1100 ether. Unfortunately, it was susceptible to the DOS vulnerabilities mentioned in this section. This Reddit Post describes how the contract required the deletion of a large mapping in order to withdraw the ether. The deletion of this mapping had a gas cost that exceeded the block gas limit at the time, and thus was not possible to withdraw the 1100 ether. The contract address is 0xF45717552f12Ef7cb65e95476F217Ea008167Ae3 and you can see from transaction 0x0d80d67202bd9cb6773df8dd2020e7190a1b0793e8ec4fc105257e8128f0506b that the 1100 ether was finally obtained with a transaction that used 2.5M gas (after the block gas limit allowed such a transaction).

12. Block Timestamp Manipulation

Block timestamps have historically been used for a variety of applications, such as entropy for random numbers (see the Entropy Illusion section for further details), locking funds for periods of time and various state-changing conditional statements that are time-dependent. Miner's have the ability to adjust timestamps slightly which can prove to be quite dangerous if block timestamps are used incorrectly in smart contracts.

Some useful references for this are: The Solidity Docs, this Stack Exchange Question.

The Vulnerability

block.timestamp or its alias now can be manipulated by miners if they have some incentive to do so. Let's construct a simple game, which would be vulnerable to miner exploitation,

roulette.sol:

contract Roulette {
    uint public pastBlockTime; // Forces one bet per block

    constructor() public payable {} // initially fund contract

    // fallback function used to make a bet
    function () public payable {
        require(msg.value == 10 ether); // must send 10 ether to play
        require(now != pastBlockTime); // only 1 transaction per block
        pastBlockTime = now;
        if(now % 15 == 0) { // winner
            msg.sender.transfer(this.balance);
        }
    }
}

This contract behaves like a simple lottery. One transaction per block can bet 10 ether for a chance to win the balance of the contract. The assumption here is that, block.timestamp is uniformly distributed about the last two digits. If that were the case, there would be a 1/15 chance of winning this lottery.

However, as we know, miners can adjust the timestamp, should they need to. In this particular case, if enough ether pooled in the contract, a miner who solves a block is incentivised to choose a timestamp such that block.timestamp or now modulo 15 is 0. In doing so they may win the ether locked in this contract along with the block reward. As there is only one person allowed to bet per block, this is also vulnerable to front-running attacks.

In practice, block timestamps are monotonically increasing and so miners cannot choose arbitrary block timestamps (they must be larger than their predecessors). They are also limited to setting blocktimes not too far in the future as these blocks will likely be rejected by the network (nodes will not validate blocks whose timestamps are in the future).

Preventative Techniques

Block timestamps should not be used for entropy or generating random numbers - i.e. they should not be the deciding factor (either directly or through some derivation) for winning a game or changing an important state (if assumed to be random).

Time-sensitive logic is sometimes required; i.e. unlocking contracts (timelocking), completing an ICO after a few weeks or enforcing expiry dates. It is sometimes recommend to use block.number (see the Solidity docs) and an average block time to estimate times; .i.e. 1 week with a 10 second block time, equates to approximately, 60480 blocks. Thus, specifying a block number at which to change a contract state can be more secure as miners are unable to manipulate the block number as easily. The BAT ICO contract employed this strategy.

This can be unnecessary if contracts aren't particularly concerned with miner manipulations of the block timestamp, but it is something to be aware of when developing contracts.

Real-World Example: GovernMental

GovernMental was an old Ponzi scheme that accumulated quite a large amount of ether. It was also vulnerable to a timestamp-based attack. The contract payed out to the player who was the last player to join (for at least one minute) in a round. Thus, a miner who was a player, could adjust the timestamp (to a future time, to make it look like a minute had elapsed) to make it appear that the player was the last to join for over a minute (even though this is not true in reality). More detail on this can be found in the History of Ethereum Security Vulnerabilities Post by Tanya Bahrynovska.

13. Constructors with Care

Constructors are special functions which often perform critical, privileged tasks when initialising contracts. Before solidity v0.4.22 constructors were defined as functions that had the same name as the contract that contained them. Thus, when a contract name gets changed in development, if the constructor name isn't changed, it becomes a normal, callable function. As you can imagine, this can (and has) lead to some interesting contract hacks.

For further reading, I suggest the reader attempt the Ethernaught Challenges (in particular the Fallout level).

The Vulnerability

If the contract name gets modified, or there is a typo in the constructor's name such that it no longer matches the name of the contract, the constructor will behave like a normal function. This can lead to dire consequences, especially if the constructor is performing privileged operations. Consider the following contract

contract OwnerWallet {
    address public owner;

    //constructor
    function ownerWallet(address _owner) public {
        owner = _owner;
    }

    // fallback. Collect ether.
    function () payable {}

    function withdraw() public {
        require(msg.sender == owner);
        msg.sender.transfer(this.balance);
    }
}

This contract collects ether and only allows the owner to withdraw all the ether by calling the withdraw() function. The issue arises due to the fact that the constructor is not exactly named after the contract. Specifically, ownerWallet is not the same as OwnerWallet. Thus, any user can call the ownerWallet() function, set themselves as the owner and then take all the ether in the contract by calling withdraw().

Preventative Techniques

This issue has been primarily addressed in the Solidity compiler in version 0.4.22. This version introduced a constructor keyword which specifies the constructor, rather than requiring the name of the function to match the contract name. Using this keyword to specify constructors is recommended to prevent naming issues as highlighted above.

Real-World Example: Rubixi

Rubixi (contract code) was another pyramid scheme that exhibited this kind of vulnerability. It was originally called DynamicPyramid but the contract name was changed before deployment to Rubixi. The constructor's name wasn't changed, allowing any user to become the creator. Some interesting discussion related to this bug can be found on this Bitcoin Thread. Ultimately, it allowed users to fight for creator status to claim the fees from the pyramid scheme. More detail on this particular bug can be found here.

14. Uninitialised Storage Pointers

The EVM stores data either as storage or as memory. Understanding exactly how this is done and the default types for local variables of functions is highly recommended when developing contracts. This is because it is possible to produce vulnerable contracts by inappropriately initialising variables.

To read more about storage and memory in the EVM, see the Solidity Docs: Data Location, Solidity Docs: Layout of State Variables in Storage, Solidity Docs: Layout in Memory.

This section is based off the excellent post by Stefan Beyer. Further reading on this topic can be found from Sefan's inspiration, which is this reddit thread.

The Vulnerability

Local variables within functions default to storage or memory depending on their type. Uninitialised local storage variables can point to other unexpected storage variables in the contract, leading to intentional (i.e. the developer intentionally puts them there to attack later) or unintentional vulnerabilities.

Let's consider the following, relatively simple name registrar contract:

// A Locked Name Registrar
contract NameRegistrar {

    bool public unlocked = false;  // registrar locked, no name updates

    struct NameRecord { // map hashes to addresses
        bytes32 name;
        address mappedAddress;
    }

    mapping(address => NameRecord) public registeredNameRecord; // records who registered names
    mapping(bytes32 => address) public resolve; // resolves hashes to addresses

    function register(bytes32 _name, address _mappedAddress) public {
        // set up the new NameRecord
        NameRecord newRecord;
        newRecord.name = _name;
        newRecord.mappedAddress = _mappedAddress;

        resolve[_name] = _mappedAddress;
        registeredNameRecord[msg.sender] = newRecord;

        require(unlocked); // only allow registrations if contract is unlocked
    }
}

This simple name registrar has only one function. When the contract is unlocked, it allows anyone to register a name (as a bytes32 hash) and map that name to an address. Unfortunately, this registrar is initially locked and the require on line [23] prevents register() from adding name records. There is however a vulnerability in this contract, that allows name registration regardless of the unlocked variable.

To discuss this vulnerability, first we need to understand how storage works in Solidity. As a high level overview (without any proper technical detail - I suggest reading the Solidity docs for a proper review), state variables are stored sequentially in slots as they appear in the contract (they can be grouped together, but not in this example, so we won't worry about that). Thus, unlocked exists in slot 0, registeredNameRecord exists in slot 1 and resolve in slot 2 etc. Each of these slots are of byte size 32 (there are added complexities with mappings which we ignore for now). The boolean unlocked will look like 0x000...0 (64 0's, excluding the 0x) for false or 0x000...1(63 0's) for true. As you can see, there is a significant waste of storage in this particular example.

The next piece of information that we need, is that Solidity defaults complex data types, such as structs, to storage when initialising them as local variables. Therefore, newRecord on line [16] defaults to storage. The vulnerability is caused by the fact that newRecord is not initialised. Because it defaults to storage, it becomes a pointer to storage and because it is uninitialised, it points to slot 0 (i.e. where unlocked is stored). Notice that on lines [17] and [18] we then set nameRecord.name to _name and nameRecord.mappedAddress to _mappedAddress, this in effect changes the storage location of slot 0 and slot 1 which modifies both unlocked and the storage slot associated with registeredNameRecord.

This means that unlocked can be directly modified, simply by the bytes32 _name parameter of the register() function. Therefore, if the last byte of _name is non-zero, it will modify the last byte of storage slot 0 and directly change unlocked to true. Such _name values will pass the require() on line [23] as we are setting unlocked to true. Try this in Remix. Notice the function will pass if you use a _name of the form: 0x0000000000000000000000000000000000000000000000000000000000000001

Preventative Techniques

The Solidity compiler raises uninitialised storage variables as warnings, thus developers should pay careful attention to these warnings when building smart contracts. The current version of mist (0.10), doesn't allow these contracts to be compiled. It is good practice to explicitly use the memory or storage keywords when dealing with complex types to ensure they behave as expected. As of Solidity version 0.5.0 use of memory and storage are mandatory.

Real-World Examples: Honey Pots: OpenAddressLottery and CryptoRoulette

A honey pot named OpenAddressLottery (contract code) was deployed that used this uninitialised storage variable querk to collect ether from some would-be hackers. The contract is rather in-depth, so I will leave the discussion to this reddit thread where the attack is quite clearly explained.

Another honey pot, CryptoRoulette (contract code) also utilises this trick to try and collect some ether. If you can't figure out how the attack works, see An analysis of a couple Ethereum honeypot contracts for an overview of this contract and others.

15. Floating Points and Precision

As of this writing (Solidity v0.4.24), fixed point or floating point numbers are not supported. This means that floating point representations must be made with the integer types in Solidity. This can lead to errors/vulnerabilities if not implemented correctly.

For further reading, see Ethereum Contract Security Techniques and Tips - Rounding with Integer Division,

The Vulnerability

As there is no fixed point type in Solidity, developers are required to implement their own using the standard integer data types. There are a number of pitfalls developers can run into during this process. I will try to highlight some of these in this section.

Let's begin with a code example (let's ignore any over/under flow issues for simplicity).

contract FunWithNumbers {
    uint constant public tokensPerEth = 10;
    uint constant public weiPerEth = 1e18;
    mapping(address => uint) public balances;

    function buyTokens() public payable {
        uint tokens = msg.value/weiPerEth*tokensPerEth; // convert wei to eth, then multiply by token rate
        balances[msg.sender] += tokens;
    }

    function sellTokens(uint tokens) public {
        require(balances[msg.sender] >= tokens);
        uint eth = tokens/tokensPerEth;
        balances[msg.sender] -= tokens;
        msg.sender.transfer(eth*weiPerEth); //
    }
}

This simple token buying/selling contract has some obvious problems in the buying and selling of tokens. Although the mathematical calculations for buying and selling tokens are correct, the lack of floating point numbers will give erroneous results. For example, when buying tokens on line [7], if the value is less than 1 ether the initial division will result in 0, leaving the final multiplication 0 (i.e. 200 wei divided by 1e18 weiPerEth equals 0). Similarly, when selling tokens, any tokens less than 10 will also result in 0 ether. In fact, rounding here is always down, so selling 29 tokens, will result in 2 ether.

The issue with this contract is that the precision is only to the nearest ether (i.e. 1e18 wei). This can sometimes get tricky when dealing with decimals in ERC20 tokens when you need higher precisions.

Preventative Techniques

Keeping the right precision in your smart contracts is very important, especially when dealing ratios and rates which reflect economic decisions.

You should ensure that any ratios or rates you are using allow for large numerators in fractions. For example, we used the rate tokensPerEth in our example. It would have been better to use weiPerTokens which would be a large number. To solve for the amount of tokens we could do msg.value/weiPerTokens. This would give a more precise result.

Another tactic to keep in mind, is to be mindful of order of operations. In the above example, the calculation to purchase tokens was msg.value/weiPerEth*tokenPerEth. Notice that the division occurs before the multiplication. This example would have achieved a greater precision if the calculation performed the multiplication first and then the division, i.e. msg.value*tokenPerEth/weiPerEth.

Finally, when defining arbitrary precision for numbers it can be a good idea to convert variables into higher precision, perform all mathematical operations, then finally when needed, convert back down to the precision for output. Typically uint256's are used (as they are optimal for gas usage) which give approximately 60 orders of magnitude in their range, some of which can be dedicated to the precision of mathematical operations. It may be the case that it is better to keep all variables in high precision in solidity and convert back to lower precisions in external apps (this is essentially how the decimals variable works in ERC20 Token contracts). To see examples of how this can be done and the libraries to do this, I recommend looking at the Maker DAO DSMath. They use some funky naming, WADs and RAYs but the concept is useful.

Real-World Example: Ethstick

I couldn't find a good example where rounding has caused a severe issue in a contract, but I'm sure there are plenty out there. Feel free to update this if you have a good one in mind.

For lack of a good example, I want to draw your attention to Ethstick mainly because I like the cool naming within the contract. This contract doesn't use any extended precision, however, it deals with wei. So this contract will have issues of rounding, but only at the wei level of precision. It has some more serious flaws, but these are relating back to the difficulty in getting entropy on the blockchain (see Entropy Illusion). For a further discussion on the Ethstick contract, I'll refer you to another post of Peter Venesses, Ethereum Contracts Are Going to be Candy For Hackers.

16. Tx.Origin Authentication

Solidity has a global variable, tx.origin which traverses the entire call stack and returns the address of the account that originally sent the call (or transaction). Using this variable for authentication in smart contracts leaves the contract vulnerable to a phishing-like attack.

For further reading, see Stack Exchange Question, Peter Venesses's Blog and Solidity - Tx.Origin attacks.

The Vulnerability

Contracts that authorise users using the tx.origin variable are typically vulnerable to phishing attacks which can trick users into performing authenticated actions on the vulnerable contract.

Consider the simple contract,

contract Phishable {
    address public owner;

    constructor (address _owner) {
        owner = _owner;
    }

    function () public payable {} // collect ether

    function withdrawAll(address _recipient) public {
        require(tx.origin == owner);
        _recipient.transfer(this.balance);
    }
}

Notice that on line [11] this contract authorises the withdrawAll() function using tx.origin. This contract allows for an attacker to create an attacking contract of the form,

import "Phishable.sol";

contract AttackContract {

    Phishable phishableContract;
    address attacker; // The attackers address to receive funds.

    constructor (Phishable _phishableContract, address _attackerAddress) {
        phishableContract = _phishableContract;
        attacker = _attackerAddress;
    }

    function () payable {
        phishableContract.withdrawAll(attacker);
    }
}

To utilise this contract, an attacker would deploy it, and then convince the owner of the Phishable contract to send this contract some amount of ether. The attacker may disguise this contract as their own private address and social engineer the victim to send some form of transaction to the address. The victim, unless being careful, may not notice that there is code at the attacker's address, or the attacker may pass it off as being a multisignature wallet or some advanced storage wallet (remember source code of public contracts is not available by default).

In any case, if the victim sends a transaction (with enough gas) to the AttackContract address, it will invoke the fallback function, which in turn calls the withdrawAll() function of the Phishable contract, with the parameter attacker. This will result in the withdrawal of all funds from the Phishable contract to the attacker address. This is because the address that first initialised the call was the victim (i.e. the owner of the Phishable contract). Therefore, tx.origin will be equal to owner and the require on line [11] of the Phishable contract will pass.

Preventative Techniques

tx.origin should not be used for authorisation in smart contracts. This isn't to say that the tx.origin variable should never be used. It does have some legitimate use cases in smart contracts. For example, if one wanted to deny external contracts from calling the current contract, they could implement a require of the from require(tx.origin == msg.sender). This prevents intermediate contracts being used to call the current contract, limiting the contract to regular code-less addresses.

Real-World Example: Not Known

I do not know of any publicised exploits of this form in the wild.

Ethereum Quirks

I intend to populate this section with various interesting quirks that get discovered by the community. These are kept in this blog as they may aid in smart contract development if one were to utilize these quirks in practice.

Keyless Ether

Contract addresses are deterministic, meaning that they can be calculated prior to actually creating the address. This is the case for addresses creating contracts and also for contracts spawning other contracts. In fact, a created contract's address is determined by:

keccak256(rlp.encode([<account_address>, <transaction_nonce>])

Essentially, a contract's address is just the keccak256 hash of the account that created it concatenated with the accounts transaction nonce². The same is true for contracts, except contracts nonce's start at 1 whereas address's transaction nonce's start at 0.

This means that given an Ethereum address, we can calculate all the possible contract addresses that this address can spawn. For example, if the address 0x123000...000 were to create a contract on its 100th transaction, it would create the contract address keccak256(rlp.encode[0x123...000, 100]), which would give the contract address, 0xed4cafc88a13f5d58a163e61591b9385b6fe6d1a.

What does this all mean? This means that you can send ether to a pre-determined address (one which you don't own the private key to, but know that one of your accounts can create a contract to). You can send ether to that address and then retrieve the ether by later creating a contract which gets spawned over the same address. The constructor could be used to return all your pre-sent ether. Thus if someone were to obtain all your Ethereum private keys, it would be difficult for the attacker to discover that your Ethereum addresses also have access to this hidden ether. In fact, if the attacker spent too many transaction such that the nonce required to access your ether is used, it is impossible to recover your hidden ether.

Let me clarify this with a contract.

contract KeylessHiddenEthCreator {
    uint public currentContractNonce = 1; // keep track of this contracts nonce publicly (it's also found in the contracts state)

    // determine future addresses which can hide ether.
    function futureAddresses(uint8 nonce) public view returns (address) {
        if(nonce == 0) {
            return address(keccak256(0xd6, 0x94, this, 0x80));
        }
        return address(keccak256(0xd6, 0x94, this, nonce));
    // need to implement rlp encoding properly for a full range of nonces
    }

    // increment the contract nonce or retrieve ether from a hidden/key-less account
    // provided the nonce is correct
    function retrieveHiddenEther(address beneficiary) public returns (address) {
        currentContractNonce +=1;
        return new RecoverContract(beneficiary);
    }

    function () payable {} // Allow ether transfers (helps for playing in remix)
}

contract RecoverContract {
    constructor(address beneficiary) {
        selfdestruct(beneficiary); // don't deploy code. Return the ether stored here to the beneficiary.
    }
 }

This contract allows you to store keyless ether (relatively safely, in the sense you can't accidentally miss the nonce)³. The futureAddresses() function can be used to calculate the first 127 contract addresses that this contract can spawn, by specifying the nonce. If you send ether to one of these addresses, it can be later recovered by calling the retrieveHiddenEther() enough times. For example, if you choose nonce=4 (and send ether to the associated address), you will need to call retrieveHiddenEther() four times and it will recover the ether to the beneficiary address.

This can be done without a contract. You can send ether to addresses that can be created from one of your standard Ethereum accounts and recover it later, at the correct nonce. Be careful however, if you accidentally surpass the transaction nonce that is required to recover your ether, your funds will be lost forever.

For more information on some more advanced tricks you can do with this quirk, I recommend reading Martin Swende's post.

One Time Addresses

Ethereum transaction signing uses the Elliptic Curve Digital Signing Algorithm (ECDSA). Conventionally, in order to send a verified transaction on Ethereum, you sign a message with your Ethereum private key, which authorises spending from your account. In slightly more detail, the message that you sign is the components of the Ethereum transaction, specifically, the to, value, gas, gasPrice, nonce and data fields. The result of an Ethereum signature is three numbers, v, r and s. I won't go into detail about what each of these represent, instead I refer the interested readers to the ECDSA wiki page (which describes r and s) and the Ethereum Yellow Paper (Appendix F - which describes v) and finally EIP155 for the current use of v.

So we know that an Ethereum transaction signature consists of a message and the numbers v, r and s. We can check if a signature is valid, by using the message (i.e. transaction details), r and s to derive an Ethereum address. If the derived Ethereum address matches the from field of the transaction, then we know that r and s were created by someone who owns (or has access to) the private key for the from field and thus the signature is valid.

Consider now, that we don't own a private key, but instead make up values for r and s for an arbitrary transaction. Consider we have a transaction, with the parameters:

{to: "0xa9e", value: 10e18, nonce: 0}

I've ignored the other parameters. This transaction will send 10 ether to the 0xa9e address. Now let's say we make up some numbers r and s (these have specific ranges) and a v. If we derive the Ethereum address related to these made up numbers we will get a random Ethereum address, let's call it 0x54321. Knowing this address, we could send 10 ether to the 0x54321 address (without owning the private key for the address). At any point in the future, we could send the transaction,

{to: "0xa9e", value: 10e18, nonce: 0, from: "0x54321"}

along with the signature, i.e. the v, r and s we made up. This will be a valid transaction, because the derived address will match our from field. This allows us to spend our money from this random address (0x54321) to the address we chose 0xa9e. Thus we have managed to store ether in an address that we do not have the private key and used a one-time transaction to recover the ether.

This quirk can also be used to send ether to a large number of people in a trustless manner, as Nick Johnson describes in How to send Ether to 11,440 people.

Single Transaction Airdrops

An Airdrop refers to the process of distributing tokens amongst a large group of people. Traditionally, airdrops have been processed via a large number of transactions where each transaction updates either a single or a batch of user's balances. This can be costly and strenuous on the Ethereum blockchain. There is an alternative method, in which many users balances can be credited with tokens using a single transaction.

This technique is explained in more detail by its proposer, RicMoo in his post: Merkle Air-Drops: Make Love, Not War.

The idea is to create a Merkle Tree which contains (as leaf nodes) all the addresses and balances of users to be credited tokens. This will be done off-chain. The merkle tree can be given out publicly (again off-chain). A smart contract can then be created containing the root hash of the merkle tree which allows users to submit merkle-proofs to obtain their tokens. Thus a single transaction (the one used to create the contract, or to simply store the Merkle tree root hash), allows all credited users to redeem their airdropped tokens.

RicMoo in his post also provides an example of a function which can accept Merkle Proofs and credit a user's balance:

function redeem(uint256 index, address recipient,
                uint256 amount, bytes32[] merkleProof) public {

    // Make sure this has not been redeemed
    uint256 redeemedBlock = _redeemed[index / 256];
    uint256 redeemedMask = (uint256(1) << uint256(index % 256));
    require((redeemedBlock & redeemedMask) == 0);

    // Mark it as redeemed (if we fail, we revert)
    _redeemed[index / 256] = redeemedBlock | redeemedMask;

    // Compute the merkle root from the merkle proof
    bytes32 node = keccak256(index, recipient, amount);
    uint256 path = index;
    for (uint16 i = 0; i < merkleProof.length; i++) {
        if ((path & 0x01) == 1) {
            node = keccak256(merkleProof[i], node);
        } else {
            node = keccak256(node, merkleProof[i]);
        }
        path /= 2;
    }

    // Check the resolved merkle proof matches our merkle root
    require(node == _rootHash);

    // Redeem!
    _balances[recipient] += amount;
    _totalSupply += amount;
    Transfer(0, recipient, amount);
}

This function could be built into a token contract to allow future airdrops. The only transaction required to credit all user's balances, would be the transaction that sets the Merkle tree root.

List of Interesting Crypto Related Hacks/Bugs

SpankChain
CoinDash
SmartBillions
Exchange Didn't add "0x" to payload

Footnotes

This code was modified from web3j ↩
A transaction nonce is like a transaction counter. It increments ever time a transaction is sent from your account. ↩
Do not deploy this contract to store any real ether. It is for demonstrative purposes only. It has no inherent privileges, anyone can recover your ether if you deploy and use it. ↩

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Context

Summary

Table of Contents

References / Further Reading List

1. Re-Entrancy

The Vulnerability

Preventative Techniques

Real-World Example: The DAO

2. Arithmetic Over/Under Flows

The Vulnerability

Preventative Techniques

Real-World Examples: PoWHC and Batch Transfer Overflow (CVE-2018–10299)

3. Unexpected Ether

The Vulnerability

Self Destruct / Suicide

Pre-sent Ether

Preventative Techniques

Real-World Example: Unknown

4. Delegatecall

The Vulnerability

Preventative Techniques

Real-World Example: Parity Multisig Wallet (Second Hack)

5. Default Visibilities

The Vulnerability

Preventative Techniques

Real-World Example: Parity MultiSig Wallet (First Hack)

6. Entropy Illusion

The Vulnerability

Preventative Techniques

Real-World Example: PRNG Contracts

7. External Contract Referencing

The Vulnerability

Preventative Techniques

Real-World Example: Re-Entrancy Honey Pot

8. Short Address/Parameter Attack

The Vulnerability

Preventative Techniques

Real-World Example: Unknown

9. Unchecked CALL Return Values

The Vulnerability

Preventative Techniques

Real-World Example: Etherpot and King of the Ether

10. Race Conditions / Front Running

The Vulnerability

Preventative Techniques

Real-World Examples: ERC20 and Bancor

11. Denial Of Service (DOS)

The Vulnerability

Preventative Techniques

Real-World Examples: GovernMental

12. Block Timestamp Manipulation

The Vulnerability

Preventative Techniques

Real-World Example: GovernMental

13. Constructors with Care

The Vulnerability

Preventative Techniques

Real-World Example: Rubixi

14. Uninitialised Storage Pointers

The Vulnerability

Preventative Techniques

Real-World Examples: Honey Pots: OpenAddressLottery and CryptoRoulette

15. Floating Points and Precision

The Vulnerability

Preventative Techniques

Real-World Example: Ethstick

16. Tx.Origin Authentication

The Vulnerability

Preventative Techniques

Real-World Example: Not Known

Ethereum Quirks

Keyless Ether

One Time Addresses

Single Transaction Airdrops