The GebProxyActions contract was exploited due to missing caller access control in the quitSystem function. Victims had previously called it directly instead of via DSProxy delegatecall, setting ownsSAFE[safe] to the GebProxyActions contract. The attacker called GebProxyActions.quitSystem(manager, safe, dst) directly, bypassing GebSafeManager’s safeAllowed check and transferring collateral to themselves.