Airbender Assessment Found Three Critical Soundness Issues, All Fixed

A scoped security assessment of ZKsync’s Airbender proving system found three critical flaws affecting proof soundness, all fixed during the review. Formal checks verified 21 of 26 targets, leaving five unresolved.

Conducted from Feb. 16 to April 15, 2026, the work spanned 21 person-weeks and covered selected circuits and proving components. Reviewers used tools and targeted manual checks on those components rather than examining every part of the repository.

The assessment identified six issues: three critical soundness flaws, one low-severity issue and two warnings. Developers fixed the critical flaws during the engagement. The low-severity issue and warnings remained open because the affected paths were unused or outside Airbender’s intended trusted-bytecode model.

Matter Labs security engineer Antonio Locascio said, “Under-constrained soundness bugs worry me most, because you cannot test your way to confidence about them.” A soundness flaw can weaken the assurance that a proof establishes a block followed Ethereum’s rules.

The findings add a security dimension to Ethereum co-founder Vitalik Buterin’s call for zkEVM teams to prioritize security, which followed Airbender Prime’s Tuesday, Oct. 13 speed benchmark. The assessment examined Airbender components; the benchmark tested Airbender Prime, including one NVIDIA RTX PRO 6000 Blackwell GPU processing a sample of 7,200 consecutive Ethereum mainnet blocks.

In an L1 zkEVM verification design, validators could check a cryptographic proof of a block’s execution instead of re-running its transactions. That approach remains in research and has not been integrated into production Ethereum clients.

Ethereum’s end-of-2026 security targets include 128-bit provable security, a final proof size of 300 KiB or less, and a formal security argument for the soundness of the recursion architecture.