The most recent Netcraft Web Server Survey found more than 62 million websites running Microsoft IIS 6.0, but many of these are unlikely to be affected by the latest WebDAV remote authentication bypass vulnerability.

A new WebDAV vulnerability was published by Nikolaos Rangos on Friday, and details how attackers can bypass access restrictions using a flaw in the WebDAV functionality on IIS 6.0. By failing to handle Unicode tokens properly, the bug gives attackers access to password protected folders and, in some cases, the ability to upload files to the affected web servers.

Although IIS 6.0 accounts for more than 90% of the Microsoft sites on the Internet, the total number of vulnerable sites is likely to be substantially less than 62 million because WebDAV is not a default component of IIS 6.0 when a Windows Server 2003 machine is given the role of Application Server. Nonetheless, some people may install and enable WebDAV to provide a convenient means of publishing and managing web server content through firewalls – because WebDAV is an extension to the HTTP protocol, it can operate over the same port number as HTTP.

Microsoft issued a security advisory on Monday, which also lists IIS 5.0 as vulnerable. This issue may affect a much larger proportion of the 2.8 million IIS 5.0 websites as, unlike its successor, Windows 2000 Server automatically installs WebDAV alongside IIS 5.0.