Revision Note: V1.2 (September 24, 2010): Added an entry to the FAQ to announce a revision to the workaround, "Enable a UrlScan or Request Filtering rule, enable ASP.NET custom errors, and map all error codes to the same error page." Customers who have already applied the workaround should reapply all listed steps. Advisory Summary:Microsoft is investigating a new public report of a vulnerability in ASP.NET. An attacker who exploited this vulnerability could view data, such as the View State, which was encrypted by the target server, or read data from files on the target server, such as web.config. This would allow the attacker to tamper with the contents of the data. By sending back the altered contents to an affected server, the attacker could observe the error codes returned by the server. Microsoft is aware of limited, active attacks at this time.