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A virulent Linux worm is creating an attack network on the Internet, security clearing house CERT warned this weekend.

Slapper exploits a previously-disclosed OpenSSL vulnerability, to create an attack platform for distributed denial-of-service (DDoS) attacks against other sites. The worm also has backdoor functionality, according to ISS. The security tools vendor describes the malicious code as a variation of the much less virulent Apache "Scalper" BSD worm.

The OpenSSL server vulnerability exploit exists on a wide variety of platforms, but Slapper appears to work only on Linux systems running Apache with the OpenSSL module (mod_ssl) on Intel architectures.

The Slapper worm was first seen on Friday the 13th. Since then it has infected thousands of web servers around the world and continues to spread. By late last night 6,000 servers were infected with the worm, according to AV vendors F-Secure.

The worm scans for potentially vulnerable systems on 80/tcp using an invalid HTTP GET request (GET /mod_ssl:error:HTTP-request HTTP/1.0).

When an Apache system is detected, it attempts to send exploit code to the SSL service via 443/tcp. If successful, a copy of the malicious source code is then placed on the victim server, where the attacking system tries to compile and run it. Once infected, the victim server begins scanning for additional hosts to continue the worm's propagation.

During the infection process, the attacking host instructs the newly infected victim to initiate traffic on 2002/udp back to the attacker. Once this communications channel has been established, the infected system becomes part of the worm's DDoS network.

For this reason blocking port 2002/UDP at enterprise firewalls may be a good idea.

While the Windows-affecting Nimda nor Code Red worms attacked nearby subnets indiscriminately, Slapper creates a peer-to-peer network which an attacker can harness for attacks. This troubling development sets Slapper apart from other worms.

Binary and source code versions of the worm are available and are being actively circulated - and access to the source code might lead to the development of more powerful variants.

The vulnerability exploited by the Slapper (Apache/mod_ssl) worm was fixed beginning with OpenSSL version 0.9.6e. Administrators may want to upgrade to the latest version as of this writing the latest version of OpenSSL is 0.9.6g.

Users should also update their AV software to detect the worm. ®

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