DarkLeech is believed to have compromised thousands of Web servers around the globe running Apache 2.2.2 and above. The attackers infected the servers with an SSHD backdoor that enables them to upload and configure malicious Apache modules that are, in turn, used to inject malicious iFrames onto legitimate sites.
According to Craig Williams, engineering technical leader at Cisco Systems, the malicious script in question may be playing a role in the DarkLeech campaign by providing an avenue of attack. The script was spotted targeting a Horde/IMP Plesk Webmail vulnerability that exists in unpatched versions of the Parallels Plesk control panel software commonly used by Web hosting providers. By injecting malicious PHP code in the username field, attackers can bypass authentication and upload files to the targeted server.
In a blog post, Williams called the situation a reminder to Web operators and administrators to keep their systems up-to-date.
"In order to upload the malicious DarkLeech modules, attackers must first gain root access to the server," he tells Dark Reading. "We know that both cPanel and Plesk have been vectors for compromise in the DarkLeech attacks. Given that the Horde/IMP Plesk Webmail vulnerability is being actively exploited, it is a possible avenue for gaining the needed access. However, as we noted in our blog post, this particular vector is not as common as the Plesk remote access vulnerability [CVE-2012-1557] described in the MalwareMustDie report."
In this case, the attackers were using an IRC-based botnet as a payload that included basic flooding capabilities. According to Williams, the Perl script used by the attackers has been around for a number of years, and the bot is openly discussed in PHP exploit groups.
"Upon successful compromise, the script joins the compromised server to an IRC channel for command and control," he says. "It then begins a series of port scans and Google searches to find other vulnerable servers. Information collected is then relayed to the aforementioned IRC channel, where it can be acted upon by the attacker(s).
"As for the exploit itself, since this is command injection vulnerability, there are no complicating factors to limit the number of times it can be exploited on a single server. Contrast this with a buffer overflow, for example, where the memory would likely become corrupted after repeated exploitation attempts, resulting in unpredictable behavior. That resulting instability would help inhibit future compromises -- a constraint not found with the Horde/IMP Plesk Webmail vulnerability."
A Parallels spokesperson says that the vulnerability Williams is warning about was in the third-party Horde webmail for Plesk 9.3 and earlier, and that while those Plesk versions are "end-of-lifed now," a patch for the issue was released in February 2012.
While the exact number of servers compromised by DarkLeech may not be readily known by researchers, Cisco TRAC analyzed several thousand compromised websites and tracked them back to their associated hosts, says Williams. Ultimately, the company identified infected hosts in more than 48 countries.
"The highest concentration of compromised Web servers were in the U.S., U.K., and Germany," he says. "Given that the average Web server hosts multiple websites, it's not at all speculative to state that tens of thousands of websites have been impacted by this."
* This story was updated to add a statement from Parallels.
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