Aztec Discloses Critical Proving-System Flaw That Could Validate Invalid Transactions

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's disclosure, an attacker could construct a proof that passes verification for a transaction the network should reject, producing a state transition outside the rules the protocol intends to enforce. Core contributors said they found the defect through internal auditing assisted by artificial intelligence (AI) tools. They advised users to treat funds, applications and contract state on V5 as exposed until incident response is complete and network operators carry out the required actions.

The failure class matters more than this particular network. In an optimistic rollup, validators re-execute transactions and can catch a bad state transition. In a zero-knowledge system, the proof is the only thing standing between a submitted transaction and accepted state, so a soundness defect in the proving stack is not caught by anyone re-running the computation. It is caught by auditing, formal verification, or by an attacker.

This is the second such episode for Aztec within months. When the project moved from V4 to V5, it told users to withdraw before June 25 because the upgrade had exposed critical flaws in the earlier release. Aztec has raised its bug bounty to $2 million and says more than 234 researchers have engaged with the program on Cantina. The network remains in alpha, which limits the value at risk, and the contributors said further bugs should be expected as usage grows.

The disclosure can be read two ways. Disclosing a soundness bug found in-house, before an attacker used it, is the behavior a serious engineering organization should show. At the same time, each disclosure confirms that privacy rollups are shipping systems whose security nobody, including their authors, can yet fully verify, and that users who bridge assets into them are accepting that residual risk in exchange for confidentiality.