Severity Rating: Important - Revision Note: Bulletin published.Summary: 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.

Read more at: http://www.microsoft.com/technet/security/bulletin/MS09-020.mspx?pubDate=2009-06-09