MS09-020  Vulnerabilities in Internet Information Services (IIS) Could Allow Elevation of Privilege (970483)

Important - Elevation of Privilege This security update resolves one publicly disclosed vulnerability and one privately reported vulnerability in Microsoft Internet Information Services (IIS). The vulnerabilities could allow elevation of privilege if an attacker sent a specially crafted HTTP request to a Web site that requires authentication. These vulnerabilities allow an attacker to bypass the IIS configuration that specifies which type of authentication is allowed, but not the file system-based access control list (ACL) check that verifies whether a file is accessible by a given user. Successful exploitation of these vulnerabilities would still restrict the attacker to the permissions granted to the anonymous user account by the file system ACLs.
Bulletin details at Microsoft.com