Retroactive auditing.

Retroactive auditing is a new approach for detecting past intrusions and vulnerability exploits based on security patches. It works by spawning two copies of the code that was patched, one with and one without the patch, and running both of them on the same inputs observed during the system's original execution. If the resulting outputs differ, an alarm is raised, since the input may have triggered the patched vulnerability. Unlike prior tools, retroactive auditing does not require developers to write predicates for each vulnerability.