Ongoing exploits infecting tens of thousands of reputable sites running the Apache Web server have only grown more powerful and stealthy since Ars first reported on them four weeks ago. Researchers have now documented highly sophisticated features that make these exploits invisible without the use of special forensic detection methods.

Linux/Cdorked.A, as the backdoor has been dubbed, turns Apache-run websites into platforms that surreptitiously expose visitors to powerful malware attacks. According to a blog post published Friday by researchers from antivirus provider Eset, virtually all traces of the backdoor are stored in the shared memory of an infected server, making it extremely hard for administrators to know their machine has been hacked. This gives attackers a new and stealthy launchpad for client-side attacks included in Blackhole, a popular toolkit in the underground that exploits security bugs in Oracle's Java, Adobe's Flash and Reader, and dozens of other programs used by end users. There may be no way for typical server admins to know they're infected.

"Unless a person really has some deep-dive knowledge on the incident response team, the first thing they're going to do is kill the evidence," Cameron Camp, a security researcher at Eset North America, told Ars. "If you run a large hosting company you're not going to send a guy in who's going to do memory dumps, you're going to go on there with your standard tool sets and destroy the evidence."

Linux/Cdorked.A leaves no traces of compromised hosts on the hard drive other than its modified HTTP daemon binary. Its configuration is delivered by the attacker through obfuscated HTTP commands that aren't logged by normal Apache systems. All attacker-controlled data is encrypted. Those measures make it all but impossible for administrators to know anything is amiss unless they employ special methods to peer deep inside an infected machine. The backdoor analyzed by Eset was programmed to receive 70 different encrypted commands, a number that could give attackers fairly granular control. Attackers can invoke the commands by manipulating the URLs sent to an infected website.

"The thing is receiving commands," Camp said. "That means that suddenly you have a new vector that is difficult to detect but is receiving commands. Blackhole is a tricky piece of malware anyway. Now suddenly you have a slick delivery method."

In addition to hiding evidence in memory, the backdoor is programmed to mask its malicious behavior in other ways. End users who request addresses that contain "adm," "webmaster," "support," and similar words often used to denote special administrator webpages aren't exposed to the client exploits. Also, to make detection harder, users who have previously been attacked are not exposed in the future.

It remains unclear what the precise relationship is between Linux/Cdorked.A and Darkleech, the Apache plug-in module conservatively estimated to have hijacked at least 20,000 sites. It's possible they're the same module, different versions of the same module, or different modules that both expose end users to Blackhole exploits. It also remains unclear exactly how legitimate websites are coming under the spell of the malicious plugins. While researchers from Sucuri speculate it takes hold after attackers brute-force the secure-shell access used by administrators, a researcher from Cisco Systems said he found evidence that vulnerable configurations of the Plesk control panel are being could be exploited to spread Darkleech. Other researchers who have investigated the ongoing attack in the past six months include AV provider Sophos and those from the Malware Must Die blog.

The malicious Apache modules are proving difficult to disinfect. Many of the modules take control of the secure shell mechanism that legitimate administrators use to make technical changes and update content to a site. That means attackers often regain control of machines that are only partially disinfected. The larger problem, of course, is that the highly sophisticated behavior of the infections makes them extremely hard to detect.

Eset researchers have released a tool that can be used by administrators who suspect their machine is infected with Linux/Cdorked.A. The free python script examines the shared memory of a sever running Apache and looks for commands issued by the stealthy backdoor. Eset's cloud-based Livegrid system has already detected hundreds of servers that are infected. Because Livegrid works only with a small percentage of machines on the Internet, the number of compromised Apache servers is presumed to be much higher.

Article updated to add detail about http daemon in the fourth paragraph.