"Security companies are warning Linux users over a new
and dangerous Trojan that may have originated in the UK.
The Trojan contains self-replicating virus-like capabilities and
has similarities to the Windows-based Back Orifice tool, putting
Linux boxes at risk of remote control.
The so-called Remote Shell Trojan spreads through email as well
as replicating itself across the infected system. It installs a
backdoor which listens for incoming connections on UDP port 5503 or
higher, and allows remote attackers to connect to, and take control
of, an infected system."