This is the first time in years I have ever heard a problem like this in a Blizzard game, but considering it is the first rare instance, it is outstanding. Yet it is a dire issue that might affect players in unpredictable ways. Beware and listen.  Malicious software can be installed in your computer while playing Warcraft III UMS maps by other players. This is what Karune reported today.

Blizzard Quote:
Warcraft III Custom Map Security Warning
We have identified an exploit that could allow malicious software to be spread through Warcraft III maps. We have applied a temporary fix to address this issue when playing on, and we are working on a patch to permanently address the issue when playing on a LAN or playing single-player custom maps. In the meantime, we recommend that players avoid downloading maps from unofficial sources or websites they do not trust—be aware that corrupted maps may share the same name as other popular maps. If you encounter custom maps that no longer function or other issues related to this fix, please post details below.