zkLink attack exploits transient storage vulnerability, losses reach $783,000

ChainCatcher report: According to analysis by SlowMist Security Team, the cross-chain protocol zkLink was compromised, resulting in losses of approximately $783,000. The attacker exploited residual validation values in transient storage to forge batch roots required for withdrawal verification, subsequently completing 12 withdrawals—including 1 ETH withdrawal and 11 ERC-20 token withdrawals. The attacker first triggered a legitimate cross-chain message to retrieve and write the validation value, then reused this value in subsequent calls within the same transaction, leveraging the relay and gateway contracts to inject a forged batch root. The relevant contracts allowed relayers to specify external call targets and parameters, but the validation value was neither cleared nor bound to the current call; batch root updates only verified the gateway’s identity without requiring independent proof of validation.