| Package | Affected versions | Patched versions |
|---|---|---|
| contracts | ≥ 4.7.0, < 4.9.2 | 4.9.2 |
| contracts upgradeable | ≥ 4.7.0, < 4.9.2 | 4.9.2 |
| @openzeppelin/contracts | ≥ 4.7.0, < 4.9.2 | 4.9.2 |
| @openzeppelin/contracts-upgradeable | ≥ 4.7.0, < 4.9.2 | 4.9.2 |
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.
The problem has been patched in 4.9.2.
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.
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