Efficient Patch-based Auditing for Web Application Vulnerabilities.

POIROT is a system that, given a patch for a newly discovered security vulnerability in a web application, helps administrators detect past intrusions that exploited the vulnerability. POIROT records all requests to the server during normal operation, and given a patch, re-executes requests using both patched and unpatched software, and reports to the administrator any request that executes differently in the two cases. A key challenge with this approach is the cost of re-executing all requests, and POIROT introduces several techniques to reduce the time required to audit past requests, including filtering requests based on their control flow and memoization of intermediate results across different requests. A prototype of POIROT for PHP accurately detects attacks on older versions of MediaWiki and HotCRP, given subsequently released patches. POIROT's techniques allow it to audit past requests 12-51× faster than the time it took to originally execute the same requests, for patches to code executed by every request, under a realistic Media-Wiki workload.