Hackers have released code for a cross-platform virus that could infect Linux and Windows computers.

The virus, which has been given the names Virus.Linux.Bi.a/ Virus.Win32.Bi.a, was reported at the end of last week by security firm Kaspersky Lab. Researchers worry that the malicious code may be part of a disturbing new trend of viruses that can run on Windows, as well as other operating systems that have been largely ignored by hackers.

"The virus is written in assembler and is relatively simple," Kaspersky wrote in a posting to the company's Viruslist.com website. "However, it is interesting in that it is capable of infecting the different file formats used by Linux and Windows - ELF and PE format
files respectively." )

The ELF (Executable and Linking Format) and PE (Portable Executable) file formats are used to format certain types of binary files in Linux and Windows, including the .exe and .dll files used in Windows.

The virus appears to have no practical application, Kasperky said. "It's a classic proof-of-concept code, written to show that it is possible to create a cross-platform virus," Kaspersky said. "However, our experience shows that once proof-of-concept code is released, virus writers are usually quick to take the code, and adapt it for their own use."

Security training organisation The SANS Institute agreed with Kaspersky's assessment, saying that the software should come as a warning to Linux and Mac OS X users who may think their computers are "invulnerable" to virus threats.

"As the developers of viruses continue to research this, we will see [more] cross platform malware come about in the future," wrote SANS Internet Storm Center contributor Swa Frantzen.

"Protecting the Linux, Unix and Mac OS X, machines with anti-virus measures is a good thing to start on now if you haven't done so already," he added.