ISS Warns of Critical Hole in Snort IDS

Security experts from Internet Security Systems Inc. and the U.S. Computer Emergency Readiness Team are warning companies...

Security experts from Internet Security Systems Inc. and the U.S. Computer Emergency Readiness Team are warning companies that the popular open-source Snort intrusion detection system technology contains a critical and easily exploitable hole that could be used to compromise vulnerable systems.

ISS and U.S. CERT both issued advisories last week concerning a vulnerability in a Snort component called the Back Orifice preprocessor. Remote attackers could use the hole to take control of systems running some versions of Snort or software by Sourcefire Inc., which sells a commercialized version of Snort. The hole is ideally suited to adoption by a self-replicating worm, ISS said. A Sourcefire spokesperson said the Columbia, Md., company was made aware of the hole earlier this month and issued a software patch on Oct. 18 to fix it.

The Snort IDS is free and widely deployed across the Internet to look for network attacks.

Versions 2.4.0, 2.4.1 and 2.4.2 of Snort create a stack-based overflow when processing Back Orifice back-door packets. Other commercial IDS and IPS (intrusion prevention system) products that use Snort may also be affected.

Back Orifice is a well-known hacking and remote system administration tool designed to give remote users total control over Windows systems.

Attackers can trigger the stack overflow by sending a single UDP (User Datagram Protocol) packet to any port monitored by Snort. Because of the way IDS sensors work, attackers do not need to know the specific address of a system running Snort to trigger the hole. They need only to send an attack packet to a network monitored by Snort. That makes the hole ideally suited to use by Internet worms and other self-replicating attacks, ISS officials in Atlanta said.

To patch the hole, Sourcefire released Snort Version 2.4.3 and advised customers to upgrade.

Customers can also disable the Back Orifice preprocessor to mitigate the threat posed by the hole, Sourcefire said.

The SANS Institutes Internet Storm Center changed its rating of Internet security from "Green" to "Yellow" late last week because of the vulnerability and urged IT managers to upgrade vulnerable Snort sensors or disable the Back Orifice preprocessor.

The Back Orifice preprocessor vulnerability could become a "big problem," given the widespread use of Snort and the relative ease with which the hole can be exploited, according to a blog post by ISC Chief Technology Officer Johannes Ullrich.

As of Oct. 20, security discussion lists such as Full-Disclosure contained example exploit code for the hole.

Bad pork Details of the new flaw in Snort IDS
  • Remotely exploitable
  • Triggers a stack overflow
  • Affects versions 2.4.0, 2.4.1, 2.4.2 of Snort
  • Requires only a single UDP packet