Serious new worm targets Linux/Apache servers

A new worm with distributed denial of service (DDoS) capabilities and the ability to grant hackers backdoor access to remote systems is being actively propagated across the Internet, according to Internet Security Systems Inc.

The Slapper worm is a modified version of the earlier Scalper worm and targets systems running the Linux OS and the open-source Apache Web server. The worms exploits a previously disclosed flaw in the Secure Sockets Layer 2.0 (SSLv2) handshake process, ISS said in a security advisory Saturday.

The worm – which ISS said has forced two ISPs in the U.S. to close down – specifically targets servers running Apache with mod_ssl. Mod_ssl is the Apache Web server interface to OpenSSL, an open source implementation of the Secure Sockets Layer (SSL) and Transport Layer Security (TLS) protocols. The worm targets OpenSSL versions up to and including 0.9.6d and 0.9.7 beta1, ISS said.

The Slapper worm exploits a buffer overflow in the SSLv2 handshake process using a malformed client master key. Once an Apache/mod_ssl server with affected OpenSSL versions is infected, the backdoor can be accessed without any authentication. This may lead to third parties using infected hosts to launch future DDoS attacks.

According to ISS, the DDoS capabilities associated with this worm are very powerful and have already been used to attack and disable high-profile targets.

The worm affects most major Linux distributions, including Red Hat, Debian, Mandrake, SuSE and Slackware. Other Unix platforms, as well as Apache with OpenSSL for Windows may also be vulnerable to the OpenSSL vulnerability, ISS said.

Binary and source code versions of the worm are available and are being actively circulated, which may lead to the development of more powerful variants, ISS said

To prevent attacks by this worm, network administrators should upgrade to the latest 0.9.6g version of OpenSSL, available from, which was created after the SSL vulnerability was first detected in July, ISS said.