Aztec discloses a critical flaw in its Alpha V5 proving system that could let a forged proof pass verification

Aztec's core contributors have disclosed a critical vulnerability in the proving system of Alpha V5, the current version of the Ethereum layer-2 network built for private smart contracts. They say an attacker may be able to construct a proof that passes verification for a transaction the network should reject. If the network accepted such a transaction, it could produce a change in state outside the rules V5 is meant to enforce.

That class of bug breaks the guarantee a zero-knowledge (ZK) system exists to provide: a valid proof means a valid computation. Aztec says it found the flaw on July 27 through internal AI-assisted auditing. It also says it cannot tell whether anyone used the flaw before then. In its words, the affected system lacks the information needed to distinguish ordinary accepted transactions from those accepted through the flawed proving path. Historical chain activity therefore cannot establish whether exploitation occurred or measure its impact.

The privacy design explains this gap. On a transparent chain, investigators can re-execute history and look for impossible balances. On a network where users generate proofs of private execution and the chain checks only the proof, a forged proof leaves no different trace from an honest one.

Aztec has told teams planning V5 deployments to pause until contributors publish further guidance. Teams with live contracts should review how to limit user exposure, isolate affected functions and, if needed, move users to fresh deployments. The fix will come in V6, which will include circuit updates so that the network rejects proofs tied to the affected proving system. Aztec says reviewers have found no other high-severity or critical V5 issues. It also says internal and external human audits are complete, and that AI-assisted auditing continues.

Aztec's track record adds to the picture. Earlier this year Aztec told Alpha V4 users to withdraw funds before June 25, when the V5 governance vote would publicly disclose a critical proving-system vulnerability in V4. The project has raised its total bug bounty to $2 million. Two consecutive versions with soundness flaws show that the proving layer, not only the application contracts, is a major unresolved risk for privacy rollups.