The Web Proxy Auto-Discovery (WPAD) feature in Microsoft Internet Explorer 6 and 7, when a primary DNS suffix with three or more components is configured, resolves an unqualified wpad hostname in a second-level domain outside this configured DNS domain, which allows remote WPAD servers to conduct man-in-the-middle (MITM) attacks.

More... (http://nvd.nist.gov/nvd.cfm?cvename=CVE-2007-5355)