Security Response

In light of the recent CSRSS double free bug, I wanted to providesome information on the exploitation of double frees on Windows on XPSP2 and later. Prior to XP SP2, double frees were trivial to exploit,but now the security cookie (in each heap chunk) and safe unlinkingchecks make it more difficult to exploit. So this blog entry willdiscuss the exploitability on XP SP2 and later heap implements.

Note: If you're not familiar with Windows heap terminology, pleasereview the slides from our CanSecWest 2004 heap presentation, archivedhere: http://www.cybertech.net/~sh0ksh0k/projects/winheap.

Oded Horovitz and I did not look into this topic much in ouroriginal presentation on Reliable Windows Heap Exploitation atCanSecWest 2004. Later that same year, I discussed how to defeat thesafe unlinking check at SyScan 2004, but I did not consider itsrelevance to double free vulnerabilities. In other...