GHSA-wprv-93r4-jj2p OpenZeppelin Contracts using MerkleProof multiproofs may allow proving arbitrary leaves for specific trees

• npm @openzeppelin/contracts: >= 4.7.0, < 4.9.2 → fixed in 4.9.2 • npm @openzeppelin/contracts-upgradeable: >= 4.7.0, < 4.9.2 → fixed in 4.9.2

Impact When the `verifyMultiProof`, `verifyMultiProofCalldata`, `processMultiProof`, or `processMultiProofCalldata` functions are in use, it is possible to construct merkle trees that allow forging a valid multiproof for an arbitrary set of leaves. A contract may be vulnerable if it uses multiproofs for verification and the merkle tree that is processed includes a node with value 0 at depth 1 (just under the root). This could happen inadvertently for balanced trees with 3 leaves or less, if the leaves are not hashed. This could happen deliberately if a malicious tree builder includes such a node in the tree. A contract is not vulnerable if it uses single-leaf proving (`verify`, `verifyCalldata`, `processProof`, or `processProofCalldata`), or if it uses multiproofs with a known tree that has hashed leaves. Standard merkle trees produced or validated with the @openzeppelin/merkle-tree library are safe. Patches The problem has been patched in 4.9.2. Workarounds If you are using multiproofs: When constructing merkle trees hash the leaves and do not insert empty nodes in your trees. Using the @openzeppelin/merkle-tree package eliminates this issue. Do not accept user-provided merkle roots without reconstructing at least the first level of the tree. Verify the merkle tree structure by reconstructing it from the leaves.