By Andrew Brandt
When the threat research analysts here at Webroot recently started seeing malware swapping out legitimate components of Windows and replacing them with malware payloads, I couldn’t help but wonder what these malware authors were thinking.
After all, cybercriminals with a lick of sense know very well that messing with system files is dangerous juju. Such an act could, in the right (or should I say wrong) circumstances, render a PC inoperable, or at the very least, bogged down in crashes and instability. And for the authors of phishing malware, it would be incredibly thick-headed to do something to an infected system which might alert the user that something is wrong. After all, when it comes to stealing passwords, flying under the radar is the goal, otherwise the owner of the infected machine might hunt down the problem and remove the Trojan before it has a chance to do its work.
Well, it’s probably a good idea never to underestimate the stupidity of some malware authors. In the past four months, we’ve created new definitions for two phishing Trojans – Trojan-PWS-Mockworthy and Trojan-Phisher-Cassicant — that routinely replace system files with their own malicious payload. Removal is incredibly easy, but generates error messages on the system. That’s just annoying. The best news is, you don’t even need an antivirus product to restore a system file that’s been replaced in this way: A system sweep will remove the malicious components, and a service called Windows File Protection will find the correct system file on your Windows CD and replace it for you. Read on for some step-by-step instructions on just how to do that.