From 5c349edd679cd71bf96f7bbfacdcb0781cd56275 Mon Sep 17 00:00:00 2001
From: Sander Bosma <sanderbosma@gmail.com>
Date: Fri, 28 Jul 2023 17:29:44 +0200
Subject: [PATCH] fix: check coinbase position in block

---
 crates/btc-relay/src/lib.rs   | 10 ++++++++
 crates/btc-relay/src/tests.rs | 46 +++++++++++++++++++++++++++++++++++
 2 files changed, 56 insertions(+)

diff --git a/crates/btc-relay/src/lib.rs b/crates/btc-relay/src/lib.rs
index 6162d7441f..9844414331 100644
--- a/crates/btc-relay/src/lib.rs
+++ b/crates/btc-relay/src/lib.rs
@@ -316,6 +316,8 @@ pub mod pallet {
         WrongForkBound,
         /// Weight bound exceeded
         BoundExceeded,
+        /// Coinbase tx must be the first transaction in the block
+        InvalidCoinbasePosition,
     }
 
     /// Store Bitcoin block headers
@@ -608,6 +610,14 @@ impl<T: Config> Pallet<T> {
         let user_proof_result = Self::verify_merkle_proof(unchecked_transaction.user_tx_proof)?;
         let coinbase_proof_result = Self::verify_merkle_proof(unchecked_transaction.coinbase_proof)?;
 
+        // make sure the the coinbase tx is the first tx in the block. Otherwise a fake coinbase
+        // could be included in a leaf-node attack. Related:
+        // https://bitslog.com/2018/06/09/leaf-node-weakness-in-bitcoin-merkle-tree-design/ .
+        ensure!(
+            coinbase_proof_result.transaction_position == 0,
+            Error::<T>::InvalidCoinbasePosition
+        );
+
         // Make sure the coinbase tx is for the same block as the user tx
         ensure!(
             user_proof_result.extracted_root == coinbase_proof_result.extracted_root,
diff --git a/crates/btc-relay/src/tests.rs b/crates/btc-relay/src/tests.rs
index ac65cf9526..c5bbc93113 100644
--- a/crates/btc-relay/src/tests.rs
+++ b/crates/btc-relay/src/tests.rs
@@ -1491,6 +1491,52 @@ fn get_chain_from_id_ok() {
     });
 }
 
+#[test]
+fn fake_coinbase_gets_rejected() {
+    let target = U256::from(2).pow(254.into());
+    let some_address = BtcAddress::P2PKH(H160::from_str(&"66c7060feb882664ae62ffad0051fe843e318e85").unwrap());
+
+    run_test(|| {
+        let transaction = TransactionBuilder::new()
+            .with_version(2)
+            .add_input(TransactionInputBuilder::new().build())
+            .add_output(TransactionOutput::payment(100, &some_address.clone()))
+            .build();
+
+        // build a block with two coinbase transactions
+        let block = BlockBuilder::new()
+            .with_coinbase(&some_address, 50, 25) // this one will be index 1
+            .with_coinbase(&some_address, 50, 0) // this one will be index 0
+            .add_transaction(transaction)
+            .mine(target)
+            .unwrap();
+        assert_ok!(BTCRelay::_initialize(3, block.header, 0));
+
+        let fake_coinbase = block.transactions[1].clone();
+        let fake_coinbase_proof = block.merkle_proof(&[fake_coinbase.tx_id()]).unwrap();
+        let user_tx = block.transactions[2].clone();
+        let user_tx_proof = block.merkle_proof(&[user_tx.tx_id()]).unwrap();
+
+        let full_proof = FullTransactionProof {
+            coinbase_proof: PartialTransactionProof {
+                transaction: fake_coinbase,
+                tx_encoded_len: u32::MAX,
+                merkle_proof: fake_coinbase_proof,
+            },
+            user_tx_proof: PartialTransactionProof {
+                transaction: user_tx,
+                tx_encoded_len: u32::MAX,
+                merkle_proof: user_tx_proof,
+            },
+        };
+
+        assert_err!(
+            BTCRelay::_verify_transaction_inclusion(full_proof, Some(0)),
+            Error::<Test>::InvalidCoinbasePosition
+        );
+    })
+}
+
 #[test]
 fn store_generated_block_headers() {
     let target = U256::from(2).pow(254.into());