CVE-2023-34459
MediumCVSS 5.3Exploitation Probability (EPSS)
Low risk29th percentile - higher than 29% of all known CVEs
Summary
The OpenZeppelin Contracts library, from version 4.7.0 to 4.9.2, contains a vulnerability that allows the construction of merkle trees enabling the forging of valid multiproofs for any set of leaves. The vulnerability occurs when using multiproof functions and the merkle tree contains a node with a value of 0 at depth 1.
Risk Assessment
Organizations using multiproofs in contracts may be exposed to attacks that allow proof forgery, potentially leading to unauthorized transactions or data manipulation.
Recommendation
It is recommended to upgrade to version 4.9.2 or to implement workarounds such as hashing leaves and avoiding the insertion of empty nodes in trees. The structure of the merkle tree should also be verified by reconstructing it from the leaves.
Original NVD description (English source)
OpenZeppelin Contracts is a library for smart contract development. Starting in version 4.7.0 and prior to version 4.9.2, when the `verifyMultiProof`, `verifyMultiProofCalldata`, `procesprocessMultiProof`, or `processMultiProofCalldat` 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 inadvertedly 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. The problem has been patched in version 4.9.2. Some workarounds are available. For those 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.