A new sample came in today – an ad injector for Internet Explorer. I was analyzing it and noticed that the malware hid several of its key files. “Aha – a rootkit!” I thought and proceeded to find out how the trojan had hooked into the system to hide its traces. An SSDT hook perhaps, or maybe an injected user-mode DLL?
I looked and looked and couldn’t find a thing. No rootkit, no driver, no IAT modifications; nothing. Even stranger, the trojan seemed to have rootkitted the entire C:\Windows\system32 folder – it was invisible in Windows Explorer and couldn’t be seen when executing dir in a CMD prompt. That’s strange – why would a rootkit want to hide the system32 folder? If anything would tip you off that something is horribly wrong with your system, a missing system32 folder would be it (see figure 1 below).
After about an hour of looking for the rootkit and not finding it I started to get frustrated. So I decided to take another look at RegMon to see what the trojan was doing with the registry. That’s when I stumbled upon this:
The HKCU\Software\Microsoft\Windows\CurrentVersion\Explorer\Advanced\ShowSuperHidden determines whether or not Explorer shows files that have the hidden and system attribute set. It wasn’t a rootkit after all! The trojan simply disabled this setting and this caused all files with the system and hidden attribute to be invisible in Windows Explorer. And since the lab machine had the ShowSuperHidden setting enabled the trojan was hidden after performing the above registry tweak.
However, this didn’t explain why the files and folders were invisible in a Command Prompt as well. The explanation is obvious and simple: I had entered a simple “dir”. And since the system32 folder and the trojan files had attributes +h +s (hidden and system) set, they were hidden in the listing. Doing a “dir /ah” showed the missing files.
Moral of the story: Somtimes malware will use “old reliable” instead of messing about with a rootkit and drivers. So check the obvious stuff first before assuming it’s something more advanced.