"Microsoft is investigating new public reports of a vulnerability in the way Windows resolves hostnames that do not include a fully-qualified domain name (FQDN)," the company said in a security advisory. "The technology that the vulnerability affects is Web Proxy Auto-Discovery (WPAD)."
Microsoft said it is not aware of any attacks on its customers that exploit this flaw.
The vulnerability was patched in 1999 but has since resurfaced in subsequent versions of Windows, a resurrection reminiscent of a security flaw in Apple Mail that was fixed last year and was subsequently reintroduced. It was discussed recently at Kiwicon '07, a computer security and hacking conference held in New Zealand.
"Customers whose domain name begins in a third-level or deeper domain, such as 'contoso.co.us,' or for whom the following mitigating factors do not apply, are at risk from this vulnerability," Microsoft warns.
The problem arises when affected versions of Windows attempt to resolve a third-level domain, or subdomain, by querying a nonexistent or inaccessible WPAD server on the network. Failing to find the information it needs to resolve the subdomain inside the network, affected versions of Windows will automatically expand such a search, possible to untrusted WPAD servers, which could point DNS requests to a malicious source.
Microsoft customers who have a trusted WPAD server in their organizations, have a manually specified proxy server in Internet Explorer, or who have disabled "Automatically Detect Settings" in Internet Explorer are not at risk from this issue.