Workarounds Issued For 'Apache Killer' Attack

A working proof-of-concept christened the "Apache Killer" released this week uses an as-yet unpatched flaw in the server software that pounds Apache servers with a DDoS attack -- and all it would take is one machine to bring the server to its knees.

The Apache development team late yesterday issued an alert and workarounds in advance of rolling out a patch for the flaw in Apache HTTPD Web Server 1.3 and 2.X. The Apache Killer lets an attacker use a single PC to wage a denial of service attack against an Apache server.

"By sending specially crafted HTTP requests which include malformed range HTTP header, an attacker can disrupt the normal function of the web server, thus disallowing legitimate users to receive responses from the web server," the team's advisory says. "This issue affects all Apache software versions and a patch has not been released yet."

The underlying flaw was apparently first reported on bugtraq in 2007. "It appears due to its lack of sophistication, that it did not get much attention by Apache developers and it has remained unpatched all of this time," wrote Kevin Shortt of the SANS Internet Storm Center, who noted that he had not yet tested the PoC, but planned to.

In an updated advisory posted this morning, the Apache team revealed further exposure of the platform to the attack, and noted that "active use of this tool has been observed."

Meanwhile, vendors were stepping forward today announcing their protections against the Apache Killer attack.

Sourcefire says its IPS and open-source Snort technology have been able to detect this flaw for several years and that its Vulnerability Research Team today beefed up that protection with a new rule specific to the Apache Killer. "A lot of people have been freaking out about the "Apache Killer" tool released on Full-Disclosure last Friday. While it's an effective way to cause a Denial of Service (DoS) against an Apache web server, and readily accessible to your average malfeasant, the good news is you don't need to let your hair catch fire over it, because the VRT had it covered before the tool was even released," blogged Alex Kirk, a member of the Sourcefire VRT.

Trustwave's SpiderLabs earlier this week added protection for the attack to its ModSecurity Web application firewall.

And Imperva subsidiary Incapsula says its Web security service users are shielded from the Apache Killer. "Web sites that are using Incapsula and configured to block illegal resource access attempts are protected from such exploit attempts," according to an Incapsula advisory issued today.

The Apache team promised that a patch or new Apache version for Apache 2.0 and 2.2 would be available this week. "Note that, while popular, Apache 1.3 is deprecated," the advisory says. Meantime, the team offered several workarounds, including limiting the size of the HTTP request field to "a few hundred bytes."

Have a comment on this story? Please click "Add Your Comment" below. If you'd like to contact Dark Reading's editors directly, send us a message.