CVE-2023-34459

What it is

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.

At a glance

Exploitability

Can be exploited over the network with a low-privilege account. High risk of data tampering.

Exploit odds

0.4% chance of exploitation in the next 30 days (EPSS).

Fix

A patch, advisory or fixed version is referenced. Upgrade or apply it.

Timeline

1. 2023-06-16 23:15 UTC PublishedDisclosed publicly on CVE.org and NVD
2. 2026-06-17 06:03 UTC Last updatedMost recent change to the record

Why it ranks here

• Exploited in the wild: no, CISA does not list it.
• Exploit odds: 0.4% in the next 30 days (EPSS).
• Severity: 5.3 MEDIUM (CVSS).
• Fix: a patch or advisory is linked.

Details

Published

2023-06-16 23:15 UTC

Last modified

2026-06-17 06:03 UTC

Exploit probability

0.4% · EPSS, next 30 days

Weakness

CWE-354

Record status

Modified

Affected

• Openzeppelin Contracts < 4.9.2
• Openzeppelin Contracts Upgradeable < 4.9.2

Attack profile CVSS 3.1 · 5.3

Attack vector

Network

Complexity

High

Privileges needed

Low

User interaction

None

Scope

Unchanged

Confidentiality

None

Integrity

High

Availability

None

Is it in your code?

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