WARNING: DO NOT JOIN ANY SERVERS WITH !!! IN THE TITLE!
As of 4/18/2014 about an hour before this post is uploaded, some sort of LUA virus has been attacking servers with rcon enabled and infecting the players which visit these servers. On the next server they visit, this player then distributes the infected LUA file to the other users on the server.
Please follow this tutorial if you wish to temporarily remove the infected files from your garrysmod folder (or if you own one, server):
http://pastebin.com/eS20xfqj1) Copy code in the pastebin above
2) go to your steamapps/*username*/garrysmod folder
3) create a new text file
4) paste in the code from the pastebin
5) File -> save as.. -> name it fix.bat (don't forget to change save .txt to ALL FILES)
6)save the file
7)run the bat in your garrysmod folder and follow the instructions printed in the terminal.
Note that supposedly joining a server again after running this fix with a person with the infection will give it to you again.
I will leave this thread open for discussion of this event, problems you are facing, or any solutions and fixes you might have.
Also, keep an eye on this facepunch thread for details as they evolve.
http://facepunch.com/showthread.php?t=1386818