Trojan patches library code to avoid detection

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Researchers have discovered a dropper trojan that uses a new trick to stay active but hidden as it takes advantage of vulnerabilities in Windows code.

An analysis from BitDefender researchers revealed the approach of this new trojan. Rather than acting like traditional malware, Trojan.Dropper.UAJ differs in affecting computers by patching the library file, comres.dll, which forces all applications that rely on it to execute the threat.

Trojans typically add themselves to the start-up registry key, which makes them easy to detect by anti-virus solutions and astute users. However, this particular trojan makes a copy of the comres.dll file, patches it, then saves it to the Windows directory folder.

“DLL files are generally highly sensitive and need to have the right version and they have to be compiled either for 32- or 64-bit platforms in order to continue working after they have been compromised,” Bogdan Botezatu, e-threats analysis and communications specialist at BitDefender, told SCMagazine.com in an email. “This trojan simply patches the right file in order to ensure the DLL file stays compatible with the applications using it.”

The file then opens a backdoor to the infected system, giving the attacker access to carry out administrative actions on the PC. These include adding and removing users, changing user passwords, and running code.

“Although the modus operandi of this dropper is typical to all droppers, this particular take of patching a DLL file makes it rather unique,” Botezatu said. “We have seen just one similar approach, also targeting comres.dll, in 2010, in a variant of Trojan.PWS.OnlineGames.”

Dropper trojans traditionally infect systems through malicious email attachments that users click on, or via drive-by downloads, by which machines are infected simply by a user surfing to a tainted website, he said.

“A security solution that can proactively protect against emerging threats will stop the dropper right before patching the system file, thus minimizing the associated risks," Botezatu said.

This trojan is a classic example of malware designed to conduct long-term information siphoning.

“Cyber crooks are working hard to ensure that their creations stay undetected for as long as possible,” he said. “Although DLL patching is highly unlikely to be perfected further, malware creators will definitely upgrade the payloads.”
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