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Hundreds of servers running Apache HTTP server software have been infected with a new malicious Linux backdoor known as "Cdorked." The malware appears to be connected to the so-called Darkleech attack campaign that's been using compromised servers and malicious Apache modules to launch drive-by attacks that target known browser vulnerabilities.
While Darkleech has been running for at least two months, attackers appear to still be upping their game. "Linux/Cdorked is one of the most sophisticated Apache backdoors we have seen so far," said Pierre-Marc Bureau, security intelligence program manager for security firm ESET, in a blog post that details how to identify and remediate servers infected by the malware.
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Unfortunately, detecting servers that are infected with Cdorked isn't straightforward. "The backdoor leaves no traces of compromised hosts on the hard drive other than its modified httpd binary, thereby complicating forensics analysis," Bureau explained, noting that the malware stores no data on a server's hard drive. "All of the information related to the backdoor is stored in shared memory. The configuration is pushed by the attacker through obfuscated HTTP requests that aren't logged in normal Apache logs. This means that no command and control information is stored anywhere on the system."
Attackers access a "backdoored server" either by using a reverse shell or by using HTTP requests to relay commands. The reverse shell -- or connect-back-shellcode -- requests, however, leave traces that can help administrators identify servers that have been compromised by attackers. "[When] the shell is used by the attacker, the HTTP connection creating it is hung [the backdoor code does not implement forking]," said Bureau. "This implies that malicious shells can be found if one has access to the server and checks for long-running HTTP connections. On the other hand, the HTTP request does not appear in Apache's log file due to the way the malicious code is hooked into Apache."
But the best way to identify infected servers, Bureau said, is to scan servers for the presence of shared memory created by the malware, which will comprise about 6 MB and store the malware's state and configuration information.
Early this month, meanwhile, Cisco security researcher Mary Landesman warned that an estimated 20,000 legitimate websites that use Apache HTTP server software had been compromised as part of Darkleech. Those attacks -- as with Cdorked -- have focused on infecting vulnerable Apache installations with an SSHD backdoor. Attackers were able to load malicious modules onto the servers, which then served up drive-by attacks against website visitors.
Which Apache vulnerabilities are attackers exploiting? Cisco last week reported that Darkleech attackers may be exploiting a Horde/IMP Plesk Webmail bug that's present in unpatched versions of the Parallels Plesk control panel software used by many Web hosting providers. "By injecting malicious PHP code in the username field, successful attackers are able to bypass authentication and upload files to the targeted server," said Craig Williams, who works in Cisco's Security Intelligence Operations threat research group for (SIO), in a blog post.
To help block Darkleech attacks, Williams recommended that website administrators keep their Apache server software fully patched and updated.
Update: A Parallels spokeswoman said via email that a patch is available for the Plesk vulnerability identified by Cisco. "The exploit warned about by a Cisco researcher was in the third-party Horde webmail for Plesk 9.3 and earlier (products circa 2009 and earlier), not in the Plesk control panel itself," she said. "These Plesk versions are end-of-lifed now, but a patch was promptly issued in February 2012.
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