# Aztec discloses a critical V5 flaw that let forged zero-knowledge proofs pass verification, and builds the fix into its V6 upgrade

Core contributors found the soundness bug on July 27 with internal AI-assisted auditing and warned that funds on V5 should be treated as exposed until migration is complete. V6 also carries 30 days of support for the old network.

- Published: 2026-10-27T05:37:50.880Z
- Canonical: https://polylog.news/crypto/2026-10-27/aztec-discloses-a-critical-v5-flaw-that-let-forged-zero-know
- Publisher: Polylog (Crypto desk)
- Section: crypto
- Sources: [Aztec Network](https://aztec.network/blog/alpha-v5-proving-system-vulnerability), [Aztec Network](https://aztec.network/alpha-v5), [Aztec Network](https://aztec.network/blog/post-quantum-aztec)

Aztec, the privacy-focused layer-2 (L2) network on Ethereum, has published a disclosure of a [critical vulnerability in the V5 Alpha proving system](https://aztec.network/blog/alpha-v5-proving-system-vulnerability). According to the disclosure, an attacker could build a proof that passes verification for a transaction the network should reject. If the network accepted such a proof, the transaction would produce a state change outside the rules V5 is meant to enforce. Core contributors say they found the flaw on July 27, 2026, through internal AI-assisted auditing.

Aztec's wording is blunt. The team says V5 funds, applications and contract state "should be treated as exposed to a protocol-level failure" until contributors finish incident response and operators complete the required network actions. The fix is part of the V6 release. V6 adds circuit updates that stop the network from accepting proofs tied to the affected proving system. Aztec says reviewers have found no other high-severity or critical V5 issues. Internal and external human audits are complete, and AI-assisted review is still running.

V6 is more than a patch. Reports on the governance process say the upgrade was [approved and scheduled to execute on October 26](https://www.kucoin.com/news/flash/aztec-announces-v6-upgrade-approval-voting-still-open). It is designed to stream Ethereum-to-Aztec messages into blocks as nodes see them. Aztec puts deposit and bridge latency at about 12 to 30 seconds under V6, down from the current 24 to 108 seconds. It also funds the V5 network for 30 days during migration. The Defiant noted that Aztec [announced approval while its governance dashboard still showed voting as open](https://thedefiant.io/news/blockchains/aztec-announces-v6-approval-while-governance-dashboard-still-shows-voting-open). That timing matters for a network that markets itself as decentralized.

This is Aztec's second critical disclosure in one cycle. In June, the team told V4 users to [withdraw before June 25](https://thedefiant.io/news/defi/aztec-v4-withdraw-june-25-v5-upgrade-security-vulnerability) after separate flaws surfaced during the move to V5. In both cases, contributors found a defect under embargo, disclosed it only when the fix shipped in an upgrade, and followed up with a funded migration period. Aztec has also published its [post-quantum design work](https://aztec.network/blog/post-quantum-aztec) and continues to market its [Alpha network](https://aztec.network/alpha-v5) to builders. The V5 finding places Aztec's main security exposure in the proving system itself, not in the application code built on top of it.

## What this means

On a privacy rollup, validators cannot re-execute private transactions, because those transactions run on users' devices and only the proof reaches the network. A soundness bug therefore cannot be caught by a second check, and V5 depositors and app teams bear that exposure directly. The closest precedent is Zcash's 2018 counterfeiting flaw in its original proving parameters. Zcash fixed it quietly in the Sapling upgrade and disclosed it months later. Its "turnstile" accounting, which tracks the total value in each shielded pool, became the only public evidence that nobody had exploited it. Aztec faces the same proof problem. Users can check deposits and withdrawals at the Ethereum bridge, but they cannot directly confirm that no forged state existed. The desk's view is that, over the next several weeks, capital will move off V5 within the 30-day support window. The likely next step after that is pressure on Aztec to run more than one proof system or to add a forced-exit path. That depends on V6's circuit changes holding up under outside review. The desk's view would be wrong if bridge accounting showed withdrawals from V5 above legitimate deposits, which would mean the flaw was exploited before the fix.

## What to watch

- Whether bridge balances on the Ethereum side reconcile with V5's recorded deposits after migration. A gap would be the first sign that someone used the flaw.
- Whether outside auditors or L2Beat publish an independent review of V6's circuit changes. Without one, the fix depends on the same team that shipped the bug.
- Whether Aztec's governance records reconcile the approval announcement with the open vote shown on its dashboard. This tests whether upgrade control rests with token holders or with core contributors.