IIS 5 and 5.1 supporting WebDAV methods allows remote attackers to determine the internal IP address of the system (which may be obscured by NAT) via (1) a PROPFIND HTTP request with a blank Host header, which leaks the address in an HREF property in a 207 Multi-Status response, or (2) via the WRITE or MKCOL method, which leaks the IP in the Location server header.