Big Galoot Diatribe - The Trojan Defence…the sleeping giant for computer forensics?

June 10th, 2007 Drazen Drazic Posted in Big Galoot Diatribe, Forensics |

The rantings of Craig Chapman, Computer Forensics Geek.

A couple of recent cases, including http://www.securityfocus.com/news/11469 in the US have highlighted malware and trojans as an emerging problem for the computer forensics community - testing the validity of the expert evidence and calling into question the practise as a whole.

In this most recent case, problems emerged after a teacher was wrongly convicted following an
incident where her classroom PCs became infected with pop-up ads displaying pornographic images. The prosecution alleged that the pop-ups were caused by the teacher’s activity on her PC following expert testimony from a computer forensics detective.

Problems in the case emerged after the defence’s computer forensics expert successfully argued that a harmless hairstyling web site had actually re-directed the PC’s browser to pornographic sites, setting off a chain of offensive pop up ads (a sub-argument was also presented about access control).

With the benefit of hindsight, this case was perhaps more about poor forensics practises - the investigating detective was apparently not thorough enough.

But it raised a bigger issue: What about really hard-core trojans & malware? How do we prove that malware didn’t exist on a suspect’s system? Recent studies into the potential problems facing computer forensics community of malware\trojans\viruses suggests this problem is not going to go away any time soon.

Highlighting this problem, some conceptual tools developed by Security-Assessment.com and Joanna Rutowska from www.invisiblethings.org have shown the ability already exists for
malware to defeat ‘volatile’ memory forensics. Make no mistake, this is a big threat facing computer forensics practises and its ability to withstand rigorous cross-examination in the witness box.

The really big questions facing the computer forensics community right now
must be:

- How can the trojan defence be negated? and;
- What practises can be put into place by the corporate world to assist computer forensics ?

The nitty-gritty of ‘The Trojan Defence’ is that we don’t know what we don’t know. In other words, how do we prove that something (a trojan) didn’t exist?……The mere possibility of the existence of a trojan may itself be enough for a case to be thrown out, in the absence of any corroborating evidence.

The solution? (Is there any?)

In terms of hard-drive forensics, (and even perhaps volatile memory?) the ability exists to make a ‘known good’ copy of a system prior to it’s deployment & have it locked away in a safe. In an attempt to negate the trojan or malware defence argument, the ‘known good’ copy could be dragged out of the safe & compared to the original, and forensically examined for changes to that system. Operating system active processes, dlls etc could all be mapped & compared against those of the ‘known good’ system. This practise could also be a really good tool for very quickly detecting what is going wrong with a particular system when the IT Security guys are called in following an ‘incident’, say, an intrusion where their system became owned or whatever.

In reality though, this practise is unlikely to be adopted in the short term. But I’d be very interested to learn if some companies out there are already adopting the practises of having a secured, ‘known good’ copy for forensics or IT Security purposes. Has anyone heard of this being done ?

Or, perhaps someone has some other ideas about how ‘The Trojan Defence’ argument can be (relatively expeditiously) negated in a forensic manner ?