Despite our best attempts to prevent it, malware infections happen. When it does happen, we need to
respond appropriately to prevent the problem from becoming worse. In my experience, many IT personnel do not understand infections and compromises very well, and often employ very basic response techniques, such as relying on antivirus scans, or the ever popular Malwarebytes scan. Apparently nothing can evade Malwarebytes. (Side note: despite my cynical tone, I think Malwarebytes is very good, and I pay to run it on all of my and my family’s laptops, but it’s not perfect.)
Depending on the nature of the infection (A subsequent post will cover this), the only sure way to remove the infection is to wipe the drive and perform a reinstall. Malware authors and intruders can employ a wide range of techniques to maintain persistence, even if the malware itself is removed. These persistence mechanisms can reinfect the system with the same or new malware, provide other forms of access to an adversary, or destroy data.
For this reason, the only effective way to “clean” an infected system is wiping the drive and reinstalling the OS, applications, and data from a backup. It’s important for IT staff to understand this important nuance, and treat infections with the proper diligence. There are techniques emerging that can alter hardware components, such as UEFI and drive firmware, which may render even a wipe and reinstall ineffective, but fortunately these techniques are not yet common.
In summary: train your IT organization on the appropriate response to malware infections, which should start with disconnecting the system from the network, then may include making a forensic copy of the affected system and its memory, and finally should generally conclude with the affected system being wiped and reinstalled.