Aztec Discloses a Soundness Flaw That Let a Valid Proof Authorize an Invalid Transaction

Aztec, the privacy-focused layer-2 network on Ethereum, has published a disclosure describing a critical defect in the proving system behind its live Alpha V5 release. According to the team's write-up, an attacker could construct a proof that passes verification for a transaction the network is supposed to reject, producing a state transition outside the rules the protocol intends to enforce. Core contributors identified the flaw on 27 July through internal auditing assisted by artificial intelligence, and the repair is scheduled for the V6 release.

This is a soundness failure, not a bug in an application contract. In a zero-knowledge rollup, the proof is the only check between a submitted transaction and the network's accounting of who owns what. Optimistic rollups can fall back on other participants re-executing transactions and challenging a bad result. A proof-based system cannot, because there is nothing to re-execute if the verifier accepts a fraudulent proof as valid.

The disclosure follows the same pattern as Aztec's March disclosure of a critical vulnerability in Alpha V4, which the team said affected the proving system as a whole and could enable theft of user funds. In both cases Aztec withheld details until a fixed version was ready, and told V4 users to withdraw before the disclosure went public.

Aztec has been explicit that its Alpha releases are test-phase software, with applications such as an on-chain game running on the network and a bug bounty raised to $2 million. The straightforward reading is that the disclosure process worked as designed. The less comfortable reading is that two consecutive versions of a production-labeled privacy network shipped with proof defects that only internal review caught.