Worked example · 2026-10

X7 Finance — claimRewards() with an empty array added the same ETH balance to total rewards again and again — X7 Finance (X7Pioneer) / Ethereum — 2026-10-08

Loss

~6.78 ETH (~$16.5K), per SlowMist.

OAK Techniques observed

OAK-T9.016 (Contract Accounting / State-Invariant Flaw — primary, per SlowMist. In X7Pioneer.claimRewards(), totalRewards grows by address(this).balance - lastETHBalance on every call, but lastETHBalance is only updated when claimable > 0. Calling with an empty array leaves claimable at 0, so the same unaccounted balance is re-added each time and the per-token reward inflates. The attacker donated ETH first to enlarge the delta. See techniques/T9.016-contract-accounting-state-invariant-flaw.md).

Attribution

pseudonymous.

Key teaching point

Update the checkpoint on every path that reads it. A "new income since last time" delta is only safe if last time moves whenever the delta is counted, not only when someone is paid.