Microsoft .NET Framework 1.1 SP1, 2.0 SP1 and SP2, 3.5, 3.5 SP1, 3.5.1, and 4.0, as used for ASP.NET in Microsoft Internet Information Services (IIS), provides detailed error codes during decryption attempts, which allows remote attackers to decrypt and modify encrypted View State (aka __VIEWSTATE) form data, and possibly forge cookies or read application files, via a padding oracle attack, aka "ASP.NET Padding Oracle Vulnerability."