According to a blog posted Sunday by researchers at Malcovery Security, the new approach could enable the malware to escape conventional security defenses.
Gary Warner, co-founder and CTO at Malcovery, writes that GameOver Zeus' authors are "encrypting their EXE file so that as it passes through your firewall, Web filters, network intrusion detection systems, and any other defenses you may have in place, it is doing so as a non-executable '.ENC' file."
While antivirus tools and other products have recently improved their ability to detect and block the dangerous Zeus variant, the encrypted attack was not detected by any of the 50 products used by VirusTotal, according to the blog.
"Why? Well, because technically, [the encrypted ENC file] isn't malware," Warner writes. "It doesn't actually execute! All Windows EXE files start with the bytes 'MZ.' These files start with 'ZZP.' They aren't executable, so how could they be malware? Except they are."
In the blog, Warner gives many examples that show how the campaign works and how the malware is delivered. He also offers a bit of advice.
"If you are in charge of network security for your enterprise, you may want to check your logs to see how many .ENC files have been downloaded recently," the blog warns.
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