Lien Finance — exchangeEquivalentBonds multiset integrity flaw

Timeline: On July 24, 2026, an attacker exploited a logic flaw in the Lien Finance protocol on Ethereum, draining approximately $542,144.63 USDC. The attacker first deployed an orchestration contract (0xe74d17c1...) from EOA 0x0D7d9023..., which had been funded by CCE.Cash Exchange with 0.18854407 ETH. The orchestration contract permissionlessly registered a crafted bond group on BondMakerCollateralizedEth with a malicious payoff function. The root cause (per SlowMist TI Alert): the exchangeEquivalentBonds function checked total exception occurrences rather than per-bondID presence within each group. By repeating a single bondID in the output exception list, the attacker consumed the exception counter twice, concealing a missing input bond and allowing the minting of unbacked BondTokens without burning corresponding input bonds. These unbacked tokens were then swapped via three pre-authorized GeneralizedDotc OTC pool endpoints, which overvalued the crafted bonds and released USDC liquidity. Four sequential exploit calls were executed in block 25599302. The attacker converted proceeds to 287.65576801 ETH (sent to 0xcb2bFAe...) and used EIP-7702 delegation. SlowMist issued a TI Alert the same day. No response from Lien Finance was published.