A practical security guide for web developers

The intended audience

Security issues happen for two reasons -

1. Developers who have just started and cannot really tell a difference between using MD5 or bcrypt.
2. Developers who know stuff but forget/ignore them.

Our detailed explanations should help the first type while we hope our checklist helps the second one create more secure systems. This is by no means a comprehensive guide, it just covers stuff based on the most common issues we have discovered in the past.

Contents

1. The Security Checklist
2. What can go wrong?
3. Securely transporting stuff: HTTPS explained
4. Authentication: I am who I say I am
   4.1 Form based authentication
   4.2 Basic authentication
   4.3 One is not enough, 2 factor, 3 factor, ....
   4.4 Why use insecure text messages? Introducing HOTP & TOTP
   4.5 Handling password resets
5. Authorization: What am I allowed to do?
   5.1 Token based Authorization
   5.2 OAuth & OAuth2
   5.3 JWT
6. Data Validation and Sanitation: Never trust user input
   6.1 Validating and Sanitizing Inputs
   6.2 Sanitizing Outputs
   6.3 Cross Site Scripting
   6.4 Injection Attacks
   6.5 User uploads
   6.6 Tamper-proof user inputs
7. Plaintext != Encoding != Encryption != Hashing
   7.1 Common encoding schemes
   7.2 Encryption
   7.3 Hashing & One way functions
   7.4 Hashing speeds cheatsheet
8. Passwords: dadada, 123456 and cute@123
   8.1 Password policies
   8.2 Storing passwords
   8.3 Life without passwords
9. Public Key Cryptography
10. Sessions: Remember me, please
    10.1 Where to save state?
    10.2 Invalidating sessions
    10.3 Cookie monster & you
11. Fixing security, one header at a time
    11.1 Secure web headers
    11.2 Data integrity check for 3rd party code
    11.3 Certificate Pinning
12. Configuration mistakes
    12.1 Provisioning in cloud: Ports, Shodan & AWS
    12.2 Honey, you left the debug mode on
    12.3 Logging (or not logging)
    12.4 Monitoring
    12.5 Principle of least privilege
    12.6 Rate limiting & Captchas
    12.7 Storing project secrets and passwords in a file
    12.8 DNS: Of subdomains and forgotten pet-projects
    12.9 Patching & Updates
13. Attacks: When the bad guys arrive
    13.1 Clickjacking
    13.2 Cross Site Request Forgery
    13.3 Denial of Service
    13.4 Server Side Request Forgery
14. Stats about vulnerabilities discovered in Internet Companies
15. On reinventing the wheel, and making it square
    15.1 Security libraries and packages for Python
    15.2 Security libraries and packages for Node/JS
    15.3 Learning resources
16. Maintaining a good security hygiene
17. Security Vs Usability
18. Back to Square 1: The Security Checklist explained

Who are we?

We are full stack developers who just grew tired by watching how developers were lowering the barrier to call something a hack by writing unsecure code. In the past six months we have prevented leaks of more than 15 million credit card details, personal details of over 45 million users and potentially saved companies from shutting down. Recently, we discovered an issue that could result in system takeover and data leak in a bitcoin institution. We have helped several startups secure their systems, most of them for free, sometimes without even getting a thank you in response :)

If you disagree with something or find a bug please open an issue or file a PR. Alternatively, you can talk to us on [email protected]