# Aztec Says a Flaw in Its V5 Proving System Could Let an Invalid Transaction Pass Verification

Core contributors found the defect on 27 July through internal artificial-intelligence-assisted auditing and now tell users to treat funds and applications on the privacy network as exposed until operators complete the required actions.

- Published: 2026-09-06T06:01:25.324Z
- Canonical: https://polylog.news/crypto/2026-09-06/aztec-says-a-flaw-in-its-v5-proving-system-could-let-an-inva
- Publisher: Polylog (Crypto desk)
- Section: crypto
- Sources: [Aztec Network](https://aztec.network/blog/alpha-v5-proving-system-vulnerability), [Aztec Network (Alpha V5)](https://aztec.network/alpha-v5), [The Defiant](https://thedefiant.io/news/defi/aztec-v4-withdraw-june-25-v5-upgrade-security-vulnerability)

Aztec, the privacy-focused Ethereum layer-2 network, [disclosed a critical vulnerability](https://aztec.network/blog/alpha-v5-proving-system-vulnerability) in the proving system behind its Alpha V5 release. According to the project, an attacker may be able to construct a proof that passes verification for a transaction the network should reject. If the network accepted such a proof, it would produce a state transition outside the rules V5 is meant to enforce. Aztec says core contributors identified the defect on 27 July 2026 through internal auditing assisted by artificial intelligence, after both internal and external human audits had already finished.

The project told users to treat funds, applications and contract state on [Alpha V5](https://aztec.network/alpha-v5) as exposed to a protocol-level failure until contributors finish incident response and network operators carry out the required actions. Reviewers have not identified other high-severity or critical defects in V5 at this time.

This is the second proving-system failure Aztec has disclosed this year. It reported a critical vulnerability in Alpha V4 in March, saying the flaw affected the proving system as a whole and could lead to theft of user funds, and it [told V4 users to withdraw before 25 June](https://thedefiant.io/news/defi/aztec-v4-withdraw-june-25-v5-upgrade-security-vulnerability) because the V5 governance proposal would make the underlying V4 bugs public.

The category matters more than the individual bug. A soundness defect is not a smart-contract bug that a careful reader of Solidity can spot. It exists within the mathematics and the circuit implementation that determine which state transitions are valid at all. Most optimistic and hybrid rollups rely on validators re-executing transactions, but on a network where transaction contents are encrypted by design, that fallback does not exist. The safety of user balances depends on the correctness of the verifier and on the operators who can pause or upgrade the chain, which is the opposite of the credible-neutrality claim privacy rollups make in their marketing.

## What this means

Soundness bugs move risk from application developers to the chain itself, and the people exposed are every user and application holding value on that chain, because a forged proof can rewrite balances without breaking any contract. Aztec's disclosure also shows who is finding these defects now: automated review caught what earlier human audits had missed. Two outcomes are in play. Either proof-system diversity and fast emergency-override mechanisms become standard for zero-knowledge rollups, which raises engineering cost and slows launches, or teams keep shipping single-verifier designs and each disclosure forces another emergency upgrade window during which operators hold effectively total control.

## What to watch

- Whether Aztec publishes the technical detail of the V5 defect after operators upgrade, since the delay between discovery on 27 July and public disclosure sets a precedent other rollups will follow.
- Whether other zero-knowledge rollups respond by adding a second, independently written verifier, which would be a real reduction in single-implementation risk rather than a governance promise.
- Whether users move value off privacy networks during the remediation window, which would show how much confidence the disclosure cost and how quickly it returns.