Eset researchers, together with their counterparts at Web security firm Sucuri, have been analysing a new threat affecting Apache Web servers, the most well-known and widely-used Web server in the world. 
The threat is a highly advanced and stealthy backdoor being used to drive traffic to malicious Web sites carrying Blackhole exploit packs. Researchers have named the backdoor Linux/Cdorked.A and it is the most sophisticated Apache backdoor seen so far.
To date, Eset researchers have identified hundreds of compromised Web servers, thanks to the EsetLiveGrid threat telemetry.
“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. All information related to the backdoor is stored in shared memory on the server, making detection difficult and hampering analysis,” says Pierre-Marc Bureau, Eset security intelligence programme manager.
In addition, Linux/Cdorked.A takes other steps to avoid detection, both on the compromised Web server and Web browsers of computers visiting it.
“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. The configuration is stored in memory, meaning no command and control information for the backdoor is visible, making forensic analysis complex,” adds Righard Zwienenberg, senior research fellow at Eset.
The Blackhole exploit kit is a popular and prevalent exploit kit using new zero day and known exploits.