Stealthy Server Malware Spreading

Friday, May 10, 2013 @ 05:05 PM gHale

Malware is taking hold in some of the most popular Web servers, and researchers are still looking for answers.

Security companies Eset and Sucuri found Apache servers infected with Linux/Cdorked. If that malware is running on a Web server, victims end up redirected to another website that tries to compromise their computer.

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In addition, Eset said it found versions of Linux/Cdorked engineered for the Lighttpd and Nginx Web servers, both widely used across the Internet.

Marc-Etienne M. Leveille of Eset said the company found 400 Web servers infected so far, of which 50 rank in Web analytics company Alexa’s top 100,000 websites.

“We still don’t know for sure how this malicious software was deployed on the web servers,” Leveille said. “One thing is clear, this malware does not propagate by itself and it does not exploit a vulnerability in a specific software.”

Linux/Cdorked has been active since December and it redirects visitors to another compromised website hosting the Blackhole exploit kit, which is a malicious program that tests computers for software vulnerabilities.

The redirect only goes to computers using Internet Explorer or Firefox on Microsoft’s XP, Vista or 7 operating systems, Leveille said. People using an iPad or iPhone do not go to the exploit kit but instead to pornography sites.

The pattern of the domain names where people end up redirected suggests the attackers have also compromised some DNS (Domain Name System) servers, Leveille said.

The malware also will not serve up the attack if a person is in certain IP ranges or if “the victim’s Internet browser’s language is set to Japanese, Finnish, Russian and Ukrainian, Kazakh or Belarusian,” Leveille said.

“We believe the operators behind this malware campaign are making significant efforts to keep their operation under the radar and to hinder monitoring efforts as much as possible,” Leveille said. “For them, not being detected seems to be a priority over infecting as many victims as possible.”

Linux/Cdorked is stealthy but is not impossible to detect. It leaves a modified httpd binary on the hard drive, which is detectable.

But commands sent by the attackers to Linux/Cdorked do not log in the normal Apache logs, and the redirect — which sends people to a malicious website — runs only in memory and not on the hard drive, Eset officials said.

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