Windows is not the only operating system to suffer from viruses, worms and distributed denial of service (DDoS) attacks, according to experts at the Virus Bulletin conference in Prague.

Jakub Kaminski, of Computer Associates Australia, said that Unix, Solaris and Linux systems have seen some serious threats in the wild for the first time this year.

The amount and variety of Linux worms has risen sharply over the last 12 months, and viruses such as Ramen have spread widely.

"The last 12 months have proved that Linux is here to stay. Unfortunately, there is also a steady increase in the variety and occurrence of malicious software written for Linux. There are a number of new Linux viruses, but the main work is being done in the area of Trojans, especially backdoor ones, and vulnerability exploits such as DDoS attacks," said Kaminski.

He predicted that the increasing popularity of Linux systems will bring more viruses that use a cocktail of binary code and shell scripts to cross platforms.

Some BSD systems, such as FreeBSD, NetBSD and OpenBSD, are also at risk because they have the ability to emulate Linux a.out and ELF binaries. Kaminski also said that Linux viruses were able to replicate - a problem that could result from not installing Linux libraries properly.

He expects the success of DDoS worms such as Code Red and Nimda to encourage virus writers to launch attacks on more machines, hitting Linux and Solaris by exploiting unpatched systems.

Joe Hartmann, director of antivirus research for North America at Trend Micro, said that most operating systems are at risk from buffer overflows because the C++ programming language they are based on has no buffer clean-up capability.