The latest version of a distributed denial-of-service (DDoS) bot called Armageddon integrates a relatively new exploit known as Apache Killer, DDoS mitigation vendor Arbor Networks said on Tuesday.

The Apache Killer exploit was released in August 2011. It exploits a vulnerability in the Apache Web server by sending a specially crafted "Range" HTTP header to trigger a denial-of-service condition.

The attack is particularly dangerous because it can be successfully executed from a single computer and the entire targeted machine needs to be rebooted in order to recover from it.

"The Kill Apache attack abuses the HTTP protocol by requesting that the target web server return the requested URL content in a huge number of individual chunks, or byte ranges," said Arbor research analyst Jeff Edwards in a blog post on Tuesday. "This can cause a surprisingly heavy load on the target server."

The vulnerability exploited by Apache Killer is identified as CVE-2011-3192 and was patched in Apache HTTPD 2.2.20, a week after the exploit was publicly released. Apache 2.2.21 contains an improved fix.

This is the first time when Arbor researchers have seen this exploit being integrated into a DDoS botnet client that's actively being used by attackers, Edwards said.

Armageddon is a Russian malware family exclusively designed to launch DDoS attacks. Because it is sold as a toolkit on underground forums, there is more than one Armageddon-powered botnets on the Internet.

Aside from the Apache Killer exploit, the latest Armageddon version also incorporates other application-layer DDoS techniques that target popular Internet forum platforms like vBulletin or phpBB, however these are not particularly ground-breaking, Edwards said via email.

Arbor's researchers have cracked the encryption scheme used by the Armageddon botnets to communicate securely with their command and control servers and found that in at least one case, an Armageddon botnet was used to launch politically motivated DDoS attacks related to the elections in Russia.

Other denial-of-service exploits like Slowloris, started out as proof-of-concept examples and were later integrated into DDoS bots, so Apache Killer might see a similar adoption, Edwards said.

The security researcher couldn't estimate how many Apache Web servers are still vulnerable to the Apache Killer exploit at this time, but said that he wouldn't be surprised if it's a significant fraction of them.

System administrators should upgrade their Apache servers to the latest available version or should implement known work arounds. "There is an update to the Apache mod_security module that attempts to address this type of attack by filtering requests with 'Range' headers that are too large," Edwards said. "However, the difficulty lies in setting an acceptable threshold for 'too large'."