About two weeks ago, I registered the wpad.nu and wpad.fm domains. WPAD stands for Web Proxy Autodiscovery Protocol and is used by browsers to automatically find the correct proxy server for a URL. The protocol is quite simple: basically if your hostname is host-27.us.company.local, the browser will try to download a wpad.dat configuration file from wpad.us.company.local and if that doesn’t exist wpad.company.local and in some cases finally wpad.local. This doesn’t sound too bad, unless your hostname is wpad.us.company.com (notice the .com tld instead of .local). In this case your browser may be trying to download a proxy configuration file from wpad.com, which could be owned by anyone!
Fortunately wpad.com, wpad.net and wpad.org are owned by Duane Wessels, one of the good guys. He provides some statistics on his site, which show his domain serves about 25 404’s each second (he returns a 404 for the wpad.dat configuration file)!
I registered wpad.nu and wpad.fm thinking I would get a couple of requests every day, however in the last two weeks, nearly 600 unique IP addresses have been asking for the wpad.dat file! About 2/3 seem to access the file through wpad.fm, the rest through wpad.nu. The .nu TLD belongs to some small island in the South Pacific Ocean, but is quite popular as ‘nu’ is the word for 'now' in several languages, including Dutch.
As I am one of the good guys as well, I also do not serve a wpad.dat file. If I did, I could tell the victims browsers to use my server as a proxy, which would allow me to eavesdrop on or modify their web traffic (replacing all downloaded .exe files would be an easy attack). Also, as Chris Paget points out, Internet Explorer will happily NTLM authenticate against any proxy, so stealing domain credentials is also a possibility (which then could potentially be abused for VPN or Outlook Web Access).