Don’t Overlook Scheduled Tasks / AT when cleaning malware…

One of our clients picked up some sort of infection over the weekend. The sucker was persistent, and after running the usual battery of utilities — RootkitRevealer, SDFix, ComboFix, Stinger running inside a WinXP PE shell — we got rid of the thing.

When I checked the post-infection System Event Viewer log, however, I got an interesting message:

Event Type:Error
Event Source:Schedule
Event Category:None
Event ID:7901
Time:9:00:00 PM
The At46.job command failed to start due to the following error:
The system cannot find the file specified.

Huh? At46.job? I know the machine doesn’t use the AT scheduler… let’s see…

Microsoft Windows XP [Version 5.1.2600]
(C) Copyright 1985-2001 Microsoft Corp.
C:\Documents and Settings\administrator\>at
Status ID Day Time Command Line
 1 Each M T W Th F S Su 12:26 AM C:\WINDOWS\system32\Hi3TR1uq.exe
 10 Each M T W Th F S Su 9:00 AM C:\WINDOWS\system32\Hi3TR1uq.exe
 11 Each M T W Th F S Su 10:00 AM C:\WINDOWS\system32\Hi3TR1uq.exe
 12 Each M T W Th F S Su 11:00 AM C:\WINDOWS\system32\Hi3TR1uq.exe
 13 Each M T W Th F S Su 12:00 PM C:\WINDOWS\system32\Hi3TR1uq.exe
 14 Each M T W Th F S Su 1:00 PM C:\WINDOWS\system32\Hi3TR1uq.exe
 15 Each M T W Th F S Su 2:00 PM C:\WINDOWS\system32\Hi3TR1uq.exe
 16 Each M T W Th F S Su 3:00 PM C:\WINDOWS\system32\Hi3TR1uq.exe
 17 Each M T W Th F S Su 4:00 PM C:\WINDOWS\system32\Hi3TR1uq.exe
 18 Each M T W Th F S Su 5:00 PM C:\WINDOWS\system32\Hi3TR1uq.exe
 19 Each M T W Th F S Su 6:00 PM C:\WINDOWS\system32\Hi3TR1uq.exe
 2 Each M T W Th F S Su 1:00 AM C:\WINDOWS\system32\Hi3TR1uq.exe
 20 Each M T W Th F S Su 7:00 PM C:\WINDOWS\system32\Hi3TR1uq.exe
 21 Each M T W Th F S Su 8:00 PM C:\WINDOWS\system32\Hi3TR1uq.exe
Error 22 Each M T W Th F S Su 9:00 PM C:\WINDOWS\system32\Hi3TR1uq.exe

And so forth, with hourly jobs listed down thru job 72. (It kept adding duplicate schedules…)tasksched

You can see we got the infection eradicated before 9 PM, because the 9PM AT job show errors. 🙂

For those of you who prefer a GUI, you can see the same thing in the Scheduled Tasks pane in Control Panel

So, don’t overlook the AT scheduler as a place where infection might hide in an effort to replicate itself. This is the first time I’ve seen it there, and it will be a place I look at from here out…