Era
ZKsync
FoundryLayer 2
500,000 USDC
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Submission Details
Severity: medium
Valid

Merkle Proof Verification Path Inconsistency Enables Cross-Chain Message Verification Bypass

cheatc0d33

Summary

A critical inconsistency exists in Matter Labs' Merkle Tree implementation where competing validation logics for single-node trees create a race condition in the security model. While calculateRootPaths correctly validates single-node scenarios, both calculateRoot and calculateRootMemory implement a conflicting validation pattern that breaks the fundamental security assumptions of cross-chain message verification.

This vulnerability manifests in L1->L2 message processing where single-node Merkle trees (which are valid and common in certain bridge scenarios) would be rejected by calculateRoot but accepted by calculateRootPaths. The inconsistency creates an attack vector where a malicious actor could force message processing down specific codepaths by manipulating the tree structure, potentially leading to cross-chain message verification failures or bypasses.

Most critically, single-node Merkle trees are commonly used in bridge implementations for individual message verification, batch processing of single messages, and recovery scenarios. The presence of dual validation paths breaks the security invariant that a valid Merkle proof should be consistently verifiable across the entire system.

Proof of Concept

The vulnerability stems from competing validations in two locations:

calculateRootPaths accepts valid single-node cases:

https://github.com/Cyfrin/2024-10-zksync/blob/cfc1251de29379a9548eeff1eea3c78267288356/era-contracts/l1-contracts/contracts/common/libraries/Merkle.sol#L71

// Location: contracts/common/libraries/Merkle.sol#L82
if (pathLength == 0 && (_startIndex != 0 || levelLen != 1)) {
revert MerklePathEmpty();
}

While _validatePathLengthForSingleProof used by calculateRoot rejects ALL single-node cases:

https://github.com/Cyfrin/2024-10-zksync/blob/cfc1251de29379a9548eeff1eea3c78267288356/era-contracts/l1-contracts/contracts/common/libraries/Merkle.sol#L125

// Location: contracts/common/libraries/Merkle.sol#L132
function _validatePathLengthForSingleProof(uint256 _index, uint256 _pathLength) private pure {
if (_pathLength == 0) {
revert MerklePathEmpty(); // Always reverts
}
// ... additional validation
}

Here's a proof-of-concept that demonstrates how this affects bridge message verification:

contract MerkleBridgeExploit {
bytes32 private constant EMPTY_STRING_KECCAK = 0xc5d2460186f7233c927e7db2dcc703c0e500b653ca82273b7bfad8045d85a470;
function exploitInconsistentValidation() external {
// Simulate a single cross-chain message
bytes32 messageHash = keccak256(abi.encodePacked("cross_chain_message"));
// Set up minimal valid proof components
bytes32[] memory emptyPath = new bytes32[]();
uint256 index = 0;
// Try direct calculation (fails)
try Merkle.calculateRoot(emptyPath, index, messageHash) returns (bytes32 root1) {
revert("Should have failed - validation inconsistency");
} catch Error(string memory) {
// Expected failure path
}
​
// Set up for batch path calculation
bytes32[] memory messages = new bytes32[]();
messages[0] = messageHash;
// Calculate via batch path (succeeds)
bytes32 root2 = Merkle.calculateRootPaths(
emptyPath,
emptyPath,
index,
messages
);
// Proof that the same message/proof combination produces different results
console.log("Message verification status mismatch detected");
}
}

To exploit this in a bridge context:

  1. Attacker constructs a cross-chain message that must be verified via Merkle proof

  2. Forces message batching into a single-node tree

  3. Message verification fails or succeeds based on which validation path is taken

  4. This breaks the fundamental assumption that valid messages should always verify consistently

The real-world impact is particularly severe for L1->L2 bridges where inconsistent message verification could lead to:

  • Stuck cross-chain messages

  • Potential double spending if redundant verification paths exist

  • Denial of service for legitimate single-message proofs

  • Increased gas costs from forced message batching to avoid the inconsistency

Recommended mitigation steps

The fix requires unifying the validation logic to maintain consistent security invariants across all verification paths. Implement a new unified validation function:

function validateMerkleProof(uint256 _index, uint256 _pathLength, uint256 _totalLeaves) private pure {
// Special case: Valid single-node tree
if (_pathLength == 0) {
if (_index == 0 && _totalLeaves == 1) {
return;
}
revert MerklePathEmpty();
}
​
// Standard multi-node validation
if (_pathLength >= 256) {
revert MerklePathOutOfBounds();
}
// Safe index validation incorporating total leaves
uint256 maxIndex;
unchecked {
// Safe even in unchecked because _pathLength < 256
maxIndex = (1 << _pathLength) - 1;
}
if (_index > maxIndex || _index >= _totalLeaves) {
revert MerkleIndexOutOfBounds();
}
}

Then modify both proof verification paths to use this unified validation:

function calculateRoot(bytes32[] calldata _path, uint256 _index, bytes32 _itemHash)
internal pure returns (bytes32)
{
validateMerkleProof(_index, _path.length, 1);
if (_path.length == 0) {
return _itemHash; // Single-node case
}
// Existing merkle path calculation...
}
​
function calculateRootPaths(
bytes32[] memory _startPath,
bytes32[] memory _endPath,
uint256 _startIndex,
bytes32[] memory _itemHashes
) internal pure returns (bytes32) {
validateMerkleProof(_startIndex, _startPath.length, _itemHashes.length);
// Existing batch verification logic...
}
Updates

Lead Judging Commences

inallhonesty Lead Judge 5 months ago
Submission Judgement Published
Validated
Assigned finding tags:

Path Length Validation Bug Breaks Single-Node Merkle Tree Security Guarantees

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