CVE-2023-34459

OpenZeppelin Contracts using MerkleProof multiproofs may allow proving arbitrary leaves for specific trees

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.

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.

CVE-2023-34459 has a CVSS score of 5.3 (Medium). The vector is network-reachable, low privileges required, and no user interaction. A CVSS score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether this affects your application depends on whether the vulnerable code is present and reachable in your environment. A fixed version is available (4.9.2); upgrading removes the vulnerable code path.

Kodem intelligence

Already deployed Kodem?

The problem has been patched in 4.9.2.

1. What is CVE-2023-34459? CVE-2023-34459 is a medium-severity security vulnerability in @openzeppelin/contracts (npm), affecting versions >= 4.7.0, < 4.9.2. It is fixed in 4.9.2.
2. How severe is CVE-2023-34459? CVE-2023-34459 has a CVSS score of 5.3 (Medium). This score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether it represents real risk in your environment depends on whether the vulnerable code is present and reachable.
3. Which packages are affected by CVE-2023-34459?
   • @openzeppelin/contracts (npm) (versions >= 4.7.0, < 4.9.2)
   • @openzeppelin/contracts-upgradeable (npm) (versions >= 4.7.0, < 4.9.2)
4. Is there a fix for CVE-2023-34459? Yes. CVE-2023-34459 is fixed in 4.9.2. Upgrade to this version or later.
5. Is CVE-2023-34459 exploitable, and should I be worried? Whether CVE-2023-34459 is exploitable in your environment depends on whether the vulnerable code is present and reachable. A CVSS score is a worst-case rating; it does not account for your specific deployment, configuration, or usage patterns. Kodem, an Intelligent Application Security platform, uses runtime intelligence to show which vulnerabilities actually execute in production, so you can focus on the ones that represent real risk. Get a demo
6. What actually determines whether CVE-2023-34459 is exploitable, and how bad it is? Exploitability and impact are not fixed properties of a CVE. They depend on runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A high CVSS score on a dependency that never runs is not the same as real risk. Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter.
7. How do I fix CVE-2023-34459?
   • Upgrade @openzeppelin/contracts to 4.9.2 or later
   • Upgrade @openzeppelin/contracts-upgradeable to 4.9.2 or later

Other vulnerabilities in @openzeppelin/contracts