Update docs

.changeset/shy-crews-teach.md

@@ -0,0 +1,5 @@
+---
+'openzeppelin-solidity': patch
+---
+
+`MerkleProof`: Fix a bug in `processMultiProof` and `processMultiProofCalldata` that allows proving arbitrary leaves if the tree contains a node with value 0 at depth 1.

contracts/utils/cryptography/MerkleProof.sol

@@ -121,10 +121,11 @@ library MerkleProof {
         // `hashes` array. At the end of the process, the last hash in the `hashes` array should contain the root of
         // the merkle tree.
         uint256 leavesLen = leaves.length;
+        uint256 proofLen = proof.length;
         uint256 totalHashes = proofFlags.length;
 
         // Check proof validity.
-        require(leavesLen + proof.length - 1 == totalHashes, "MerkleProof: invalid multiproof");
+        require(leavesLen + proofLen - 1 == totalHashes, "MerkleProof: invalid multiproof");
 
         // The xxxPos values are "pointers" to the next value to consume in each array. All accesses are done using
         // `xxx[xxxPos++]`, which return the current value and increment the pointer, thus mimicking a queue's "pop".
@@ -146,6 +147,7 @@ library MerkleProof {
         }
 
         if (totalHashes > 0) {
+            require(proofPos == proofLen, "MerkleProof: invalid multiproof");
             unchecked {
                 return hashes[totalHashes - 1];
             }
@@ -173,10 +175,11 @@ library MerkleProof {
         // `hashes` array. At the end of the process, the last hash in the `hashes` array should contain the root of
         // the merkle tree.
         uint256 leavesLen = leaves.length;
+        uint256 proofLen = proof.length;
         uint256 totalHashes = proofFlags.length;
 
         // Check proof validity.
-        require(leavesLen + proof.length - 1 == totalHashes, "MerkleProof: invalid multiproof");
+        require(leavesLen + proofLen - 1 == totalHashes, "MerkleProof: invalid multiproof");
 
         // The xxxPos values are "pointers" to the next value to consume in each array. All accesses are done using
         // `xxx[xxxPos++]`, which return the current value and increment the pointer, thus mimicking a queue's "pop".
@@ -198,6 +201,7 @@ library MerkleProof {
         }
 
         if (totalHashes > 0) {
+            require(proofPos == proofLen, "MerkleProof: invalid multiproof");
             unchecked {
                 return hashes[totalHashes - 1];
             }

test/utils/cryptography/MerkleProof.test.js

@@ -1,11 +1,8 @@
-require('@openzeppelin/test-helpers');
-
 const { expectRevert } = require('@openzeppelin/test-helpers');
+const { expect } = require('chai');
 const { MerkleTree } = require('merkletreejs');
 const keccak256 = require('keccak256');
 
-const { expect } = require('chai');
-
 const MerkleProof = artifacts.require('$MerkleProof');
 
 contract('MerkleProof', function () {
@@ -176,5 +173,28 @@ contract('MerkleProof', function () {
       expect(await this.merkleProof.$multiProofVerify([root], [], root, [])).to.equal(true);
       expect(await this.merkleProof.$multiProofVerifyCalldata([root], [], root, [])).to.equal(true);
     });
+
+    it('reverts processing manipulated proofs with a zero-value node at depth 1', async function () {
+      // Create a merkle tree that contains a zero leaf at depth 1
+      const leaves = [keccak256('real leaf'), Buffer.alloc(32, 0)];
+      const merkleTree = new MerkleTree(leaves, keccak256, { sortPairs: true });
+
+      const root = merkleTree.getRoot();
+
+      // Now we can pass any  ** malicious ** fake leaves as valid!
+      const maliciousLeaves = ['some', 'malicious', 'leaves'].map(keccak256).sort(Buffer.compare);
+      const maliciousProof = [leaves[0], leaves[0]];
+      const maliciousProofFlags = [true, true, false];
+
+      await expectRevert(
+        this.merkleProof.$multiProofVerify(maliciousProof, maliciousProofFlags, root, maliciousLeaves),
+        'MerkleProof: invalid multiproof',
+      );
+
+      await expectRevert(
+        this.merkleProof.$multiProofVerifyCalldata(maliciousProof, maliciousProofFlags, root, maliciousLeaves),
+        'MerkleProof: invalid multiproof',
+      );
+    });
   });
 });