A stealthy malicious software program is taking hold in some of the most popular Web servers, and researchers still don't know why.
Last week, security companies Eset and Sucuri found Apache servers infected with Linux/Cdorked. If that malware is running on a Web server, victims are redirected to another website that tries to compromise their computer.
Eset said on Tuesday it has now 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 wrote that the company has found 400 Web servers infected so far, of which 50 are ranked 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 wrote. "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 at least December. 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 is only served to computers using Internet Explorer or Firefox on Microsoft's XP, Vista or 7 operating systems, Leveille wrote. People using an iPad or iPhone are not directed to the exploit kit but instead to pornography sites.
The pattern of the domain names where people are redirected suggests the attackers have also compromised some DNS (Domain Name System) servers, Leveille wrote.
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 wrote.
"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 wrote. "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 can be detected.
But commands sent by the attackers to Linux/Cdorked are not logged 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 wrote last week.