Microsoft is warning of an attack that could be used to divert an organisation's Web traffic through a malicious proxy server.
Applications such as Internet Explorer use the Web Proxy Automatic Discovery (WPAD) protocol to find a file that enables a browser to configure its proxy settings. However, it's possible to plant a configuration file that would route traffic through a malicious proxy, the company said.
A malicious WPAD.dat file could be placed in the Domain Name System (DNS) or the Windows Internet Naming Service (WINS), Microsoft said. The client application looks in DNS or WINS to resolve the name of the hosting that has the proxy configuration file.
Once the bad file is there, WPAD clients "may be able to route their Internet traffic through a malicious proxy server," Microsoft said.
Microsoft details on its support site how administrators can configure DNS and WINS on their servers to help prevent what it calls "malicious registrations" of WPAD files. The fix is for Windows Server 2003 and Windows 2000 Service Pack 4.
Microsoft wasn't immediately able to comment.