Last Updated: 2008-02-28 00:21:54 UTC
by Bojan Zdrnja (Version: 1)
As a frequent reader of ISC, I have no doubt that you are aware of malware that was distributed on digital frames and other devices (if you haven't read those diaries, see http://isc.sans.org/diary.html?storyid=3817).
After we received some samples from our readers (thank you!) I decided to analyze one of them just for fun. According to VirusTotal, all AV programs (except for one) detected this sample, so at least all users running an up to date AV program are safe.
Most of the activities by the trojan were more or less standard until I saw that it creates a high number of new registry keys. I dig a bit further and found that it uses one relatively old technique that I haven't seen abused for quite some time: the trojan used the Image File Execution Options section of the registry.
Disassembly of the trojan showed that it cycles through a loop and creates a Debugger value for a lot of keys:
The question now was: what is this doing? I had to dig through MSDN to find what exactly this section of the registry does (HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows NT\CurrentVersion\Image File Execution Options). Basically, the Debugger value allows a programmer to specify a debugger for any executable on the system. This will make Windows start that program (the debugger) instead of the executable you wanted to start in order to allow you to debug that program (it actually makes it pretty difficult to start the real executable and not the specified debugger).
One really cool usage for this feature is to replace the default Task Manager with Sysinternal's Process Explorer – one has to create this key for the taskmgr.exe application and point to Process Explorer and voila, it'll get started instead of Task Manager.
The trojan abused this feature – it had a list of almost dozens of well know anti-virus and other security tools executables. Then it created these registry keys for every single application so the trojan would get executed instead – pretty sneaky. You can see part of the registry of an infected machine below:
You can see the trojan trying to disable the NOD32 AV program on the screenshot above. Since Windows don't really check if it's a real debugger that is being started, I hope that all AV vendors are aware of this (old) technique and that they check for their own entries in this section of the registry. By the way, this feature can be used for some nasty pranks so don't abuse it please.