Aztec Says an Attacker Could Forge Valid Proofs on Its Live Privacy Network

Aztec, the privacy-focused Ethereum layer-2 network, has disclosed a critical vulnerability in the proving system underpinning its live Alpha V5 network. According to the project, an attacker may be able to construct a zero-knowledge proof (a cryptographic argument that a computation was performed correctly, without revealing its inputs) that passes verification for a transaction the network is supposed to reject. If the network accepted such a proof, the resulting state transition would fall outside the rules V5 is designed to enforce, placing funds, applications and contract state at risk.

Core contributors say they identified the defect on 27 July 2026 through internal auditing assisted by artificial intelligence, after internal and external human audits had already been completed. They also state plainly that they cannot determine whether anyone exploited the flaw before the finding. Reviewers have not identified other high-severity or critical issues in V5 Alpha, and the fix is planned for V6, expected later in 2026.

The disclosure follows the same pattern Aztec used earlier this year, when it withheld details of a critical V4 proving bug until a governance upgrade moved users off the affected version. That sequencing is deliberate. A soundness defect cannot be quietly patched around, because publishing it before users can exit would give any attacker the technical detail needed to replicate the exploit.

The class of failure matters more than this one network. Most rollups treat validator re-execution as the backstop that catches bad state. Re-execution catches an invalid transaction. It does not catch a valid-looking proof of an invalid transaction, because the proof itself is the thing being trusted. Aztec's Alpha V5 release and the applications now deployed on it run on software the team itself labels experimental, an honest framing that most production zero-knowledge systems do not offer.