Aztec Discloses Proving-System Defect That Could Let Invalid Transactions Pass Verification

Aztec, the privacy-focused Ethereum layer-2 network, has disclosed a critical vulnerability in the proving system behind its Alpha V5 network. According to the project's core contributors, an attacker may be able to construct a proof that passes verification for a transaction the network should reject. Contributors say the finding places funds, applications and contract state on Alpha V5 at risk of a protocol-level failure until incident response finishes and network operators carry out the required actions, with the correction planned for V6.

This is a soundness defect, not a smart-contract bug. In a zero-knowledge system, the verifier accepts a short proof in place of re-executing the underlying computation, so if the proof system can be made to accept an invalid statement, every guarantee built above it fails at once. Validator re-execution, the fallback most rollups rely on, does not catch this class of error, because the network never re-runs the computation in the first place.

The disclosure follows the same pattern Aztec used for a critical vulnerability in Alpha V4, where the project urged users to withdraw funds before the flaw was publicly described alongside the V5 upgrade. Withholding disclosure of a soundness bug until a replacement network is ready protects users while the flaw is open, but it also means the public record of a network's security lags behind the network itself.

Contributors say internal and external human audits had already been completed and that the defect surfaced through artificial-intelligence-assisted review that continues alongside human work. That cuts against the argument that advancing model capability is purely a threat to cryptographic systems.

Aztec has been building out the surrounding stack in parallel, publishing an explanation of how gas and fees work on the network and pointing to applications going live. This disclosure measures the distance between an alpha network carrying real balances and a network whose proof system has been shown, in theory, to be defeatable.