Aztec Discloses a Critical Proving-System Flaw in Alpha V5 and Says It Cannot Tell Whether Anyone Exploited It

Aztec, the privacy-focused Ethereum layer-2 network, disclosed a critical vulnerability in the proving system of its Alpha V5 network. According to Aztec, an attacker could construct a proof that passes verification for a transaction the network should reject. If the network accepted that proof, it would apply a state change outside the rules V5 is meant to enforce. In practice that could mean balances or contract state that no legitimate transaction produced.

Core contributors found the flaw on July 27, 2026, through internal AI-assisted auditing, Aztec said. The disclosure arrives about three months later, and the fix is scheduled for V6. Aztec says it cannot determine whether the flaw was exploited before its contributors found it. The affected system does not record enough information to distinguish ordinary accepted transactions from transactions accepted through the flawed proving path. As a result, historical chain activity cannot show whether exploitation occurred or how large it was. Reviewers have found no other high-severity or critical V5 issues so far. Aztec nevertheless tells users to treat V5 funds, applications and contract state as exposed to a protocol-level failure until incident response is complete and operators take the required network actions.

This is not Aztec's first such disclosure. The team previously published a critical vulnerability in Alpha V4, and it has raised its total bug bounty to $2 million. The Alpha label is the team's stated defense. In its framing, testing networks exist to surface this kind of finding before mainnet.

The structural point concerns privacy rollups in particular. On a transparent rollup, a validator or watcher can re-execute public transactions and notice that a state transition is wrong. On a privacy network, the private inputs never reach validators, so the validity proof is the only check that a transaction followed the rules. When that proof system is unsound, no independent party can audit the damage afterward. Aztec's statement that it cannot quantify any impact follows directly from that design.