Sui's Mysticeti consensus has ordering bias, researchers show 89% win rate

A new paper from researchers Iliya Mirzaei and Mohammad Javad Amiri, titled "Fair on the Surface: Transaction-Ordering Bias and MEV in Mysticeti DAG-based BFT Protocol," exposes critical fairness flaws in Mysticeti, the consensus protocol powering the Sui blockchain (roughly $3B market cap). Mysticeti is a state-of-the-art DAG-based Byzantine fault-tolerant protocol where many validators propose blocks in parallel, with the total order derived from the resulting DAG. It was widely believed to order transactions fairly because committed transactions are re-sorted by gas price before execution. However, the researchers demonstrate this assumption breaks down in practice—and the effects are already present on Sui's live network.

Three specific exploitations were identified. First, when merging blocks from the same round into a single total order, validators are sorted by index, giving lower-indexed validators a permanent head start. On a 13-validator network with no attacker, lower-indexed validators win same-round ordering about 89% of the time. Second, the gas-price re-sort uses a stable sort, so transactions paying equal fees (common at the reference gas price) retain the original biased order—letting an attacker profit without paying extra. Third, a validator can amplify this advantage by choosing when to stay silent, a fully legitimate action that violates no protocol rule, raising its ordering win rate above 94%. The paper confirms Mysticeti remains resilient below the standard Byzantine fault threshold and proposes a simple fix: replace the validator-index tiebreaker with an unpredictable, per-commit random key.