Detecting Viral Persistence

Persistence is something that malware strives to achieve. If malware cannot survive the monthly reboot due to the Microsoft patch cycle or the usual Windows troubleshooting process (reboot first!), then it's going to have a short lifetime and little effectiveness. There are a few exceptions to the rule in terms of persistence.Things like "downloaders," whose sole function is to grab additional pieces of malware and run them, don't need to stay resident after they've done their job. However, the malware retrieved by the downloader almost always includes functionality for itself to stay resident on a system. A recent experience with a very persistent malware that I'd read about reminded me of a recent blog that listed startup locations in Windows.

"Windows Startup Locations" at Immutable Security shows a list of locations in the Registry that can be used by malware to start upon boot. The list was pulled from the Microsoft Autoruns utility and is pretty comprehensive. There are still a few locations missing like "autoexec.bat" but the location used by the malware I saw last week is something that just about every "startup" checking tool with miss.

Remember the Master Boot Record (MBR)? Years ago, in the days of DOS, we had boot sector viruses that targeted the MBR because it's the first code to be loaded after the firmware boots. It was a trend that died off, but now we're seeing it again where modern malware leveraging the MBR to stay stealthy and avoid detection by antivirus and tools looking for changes in the common startup locations.

Last week, I helped a friend with an infected system that had a new variant of Mebroot. It was a multi-user system similar to what you'd find in a public library where all changes to the system were wiped out when the computer system was rebooted. There are several solutions that do this -- Microsoft Steady State and Faronics Deep Freeze come to mind. What's cool (or maybe I should really say "evil") about this infection is that because Mebroot infects the boot sector, Steady State and Deep Freeze are ineffective at reversing the infection.

This was one of those incidents where it would have gone undetected for a long time had my friend not had multiple layers of defense in place. In this particular case, activity in the intrusion detection system (IDS) led to the investigation; scans with multiple AV products detected nothing; and it was through carving out the MBR with FTK Imager and uploading it to VirusTotal that we confirmed it was indeed infected.

As many times as you will hear the terms "defense in depth" and "layered security" throughout your career, it's something we must practice and live by as a security professional. It also helps to know where to look, which is why tools like Autoruns can be handy at determining if a system is infected (unless it's Mebroot).

John H. Sawyer is a senior security engineer on the IT Security Team at the University of Florida. The views and opinions expressed in this blog are his own and do not represent the views and opinions of the UF IT Security Team or the University of Florida. When John's not fighting flaming, malware-infested machines or performing autopsies on blitzed boxes, he can usually be found hanging with his family, bouncing a baby on one knee and balancing a laptop on the other. Special to Dark Reading.