Definitive guide to writing a Linux virus

Following on from this week's discovery of the Jac Linux virus, an open source hacker from the University of Linz, Austria, has released a definitive guide to writing Linux viruses.

The Jac virus was discovered on Tuesday and quickly dismissed by the Linux community for its inability to spread effectively - a common occurrence in Linux viruses.

But yesterday, Alexander Bartolich released "the Linux virus writing howto".

"This document describes how to write parasitic file viruses infecting ELF executables on Linux/i386. Though it contains a lot of source code, no actual virus is included," reads the abstract for the site.

Bartolich said that "an astonishing number of people think that viruses require secret black magic."

This may not be entirely the case, but "writing a program that inserts code into another program file is one thing. Writing that program so that it can be inserted itself is a very different art."

In his White Paper, Bartolich shows code and techniques for constructing ELF (Executable and Linking Format) infecting code, which he claims is not a "Construction Kit for Dummies".

"Since regular users can't overwrite system files (we are talking about serious operating systems here) that is not even half the journey," he said.

"To make any impact you need root permissions. Either by tricking the super user to run your virus, or combining it with a root-exploit. And since all popular distributions come with checksum mechanisms, a single command can detect any modification. Unless you implement kernel-level stealth functionality."

Bartolich then goes on to describe and demonstrate how to build said virus. "You may have heard that Linux is a difficult target for malware because there are so many different distributions," he said.

"Well, basically they all use the same compiler, producing the same idiosyncrasies. This allows us to cheat in big style," he said.

Although Bartolich identifies techniques and tools for Linux virus writing, the task itself is considerable. Bartolich acknowledges that he wrote the document to combat FUD (fear, uncertainty, doubt) and not to give script kiddies a leg up.

The full White Paper can be found here.