Aztec Discloses an Alpha V5 Flaw That Could Let an Invalid Transaction Pass Proof Verification

Aztec, the privacy-focused Ethereum layer-2 network, disclosed a critical vulnerability in the proving system behind its Alpha V5 release. According to the project, an attacker could construct a proof that passes verification for a transaction the network should reject, producing a state transition outside the rules Alpha V5 is meant to enforce.

Two details make this a soundness problem rather than an ordinary bug. First, the project says it cannot determine whether anyone exploited the flaw, because the affected system does not log enough information to distinguish ordinary accepted transactions from transactions accepted through the defective proving path. Historical chain data cannot settle the question or measure the impact. Second, contributors say they found the defect through internal AI-assisted auditing and are shipping the fix in V6 rather than patching V5. They are advising teams planning a V5 deployment to pause, and teams with live contracts on the Alpha V5 network to review how they would isolate affected functionality and migrate users to new deployments.

This is the second critical finding in Aztec's alpha testing cycle, after the V4 disclosure that led users to withdraw ahead of the V5 upgrade. The project describes both findings as the intended result of alpha testing on a network it deliberately labels pre-production, and it has raised its bug bounty to 2 million dollars. Application teams building on the network, including the consumer and game deployments Aztec has promoted, such as the on-chain game Dark Forest, now face a migration cost they did not plan for.

Proof-system defects differ from contract exploits in one respect that matters for every rollup, not just this one. An optimistic system lets honest parties re-execute transactions and challenge a bad result. A validity-proof system instead treats the verifier as the final authority. If the verifier accepts a false statement, there is no second check, and a chain that does not log which proving path accepted a transaction cannot audit itself afterward.