http://www.reddit.com/r/GlobalOffensive ... s_related/
Basically, if you run a server or you put maps on the workshop, you can exploit the system to cause clients to download whatever the fuck you want, like a keylogger or even a vac-detected cheat. In other words, you could get a vac ban or lose all your items just by joining the wrong community server.
You can help this problem by doing cl_allowdownload 0, but this will still download workshop maps.
You can further help this problem by simply not joining any unknown community servers or servers running unheard-of workshop maps.