Unreal IRCD is an open source server which has been infected with a trojan horse since November 2009. Unreal IRcd Forum has say:
"This is embarrassing, we have found the file Unreal126.96.36.199.tar.gz has been replaced long time ago with a another file with the Trojan inside. That backdoor allow anyone to execute any commands with the user privilages. The substitution may occur on November 2009 but nobody know this until today"
We didn't know, but we should know this, say the Unreal IRcd Forum. We didn't know exactly how many downloads has been made since November 2009 (I don't believe this) but we know there are a lot Linux machine infected.
If you download this file from November 2009 to today, maybe you have to check your Pc, just to be sure.
This incident doesn't mean Linux is less safer, in fact, thanks to this the Trojan is unable to put on risk the entire system (just part of it) or in the words of Unreal IRCD:
"On an unrelated side note, I find the claims in various media that this security incident indicates that Linux and Open Source cannot be trusted and that Microsoft and closed-software is better really silly. It lacks any foundation. A hacker, once in, could just as easily have inserted the backdoor in Windows software. In fact, it is *THANKS* to it being Open Source that this backdoor got noticed, though - I fully agree - much too late."