The threat is a highly advanced and stealthy backdoor being used to drive traffic to malicious websites carrying Blackhole exploit packs.
“The Linux/Cdorked.A backdoor does not leave traces on the hard-disk other than a modified httpd file, the daemon (or service) used by Apache,” said Pierre-Marc Bureau, ESET security intelligence program manager.
“All information related to the backdoor is stored in shared memory on the server, making detection difficult and hampering analysis."
In addition, Linux/Cdorked.A takes other steps to avoid detection, both on the compromised webserver and web browsers of computers visiting it.
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“The backdoor’s configuration is sent by the attacker using HTTP requests that are not only obfuscated, but also not logged by Apache, reducing the likelihood of detection by conventional monitoring tools,” said Righard Zwienenberg, ESET senior researcher.
“The configuration is stored in memory, meaning no command and control information for the backdoor is visible, making forensic analysis complex,” he said.
The Blackhole exploit kit is a popular and prevalent exploit kit using zero-day and known exploits, to take control of systems when users visit a site that is comprised and infected by the Blackhole kit.
When someone visits a compromised webserver, they are not simply redirected to a malicious website and a web cookie is set in the browser so the backdoor will not send them there a second time.
The web cookie is not set on the administrator pages. The backdoor checks the visitor’s referrer field and if they are redirected to the webpage from a URL that has certain key words in it, like "admin" or "cpanel", no malicious content is served.
ESET has called on system administrators to check their servers and verify that they are not affected by this threat.
A free detection tool, detailed instructions on how to check for the backdoor and a full technical analysis of Linux/Cdorked.A are available on ESET’s WeLiveSecurity.com site in the Linux/Cdorked blog post.