Microsoft has just released patch for all versions of ASP.NET and flagged important the update.

Executive Summary
This security update resolves a publicly disclosed vulnerability in ASP.NET. The vulnerability could allow information disclosure. An attacker who successfully exploited this vulnerability could read data, such as the view state, which was encrypted by the server. This vulnerability can also be used for data tampering, which, if successfully exploited, could be used to decrypt and tamper with the data encrypted by the server. Microsoft .NET Framework versions prior to Microsoft .NET Framework 3.5 Service Pack 1 are not affected by the file content disclosure portion of this vulnerability.

This security update is rated Important for all supported editions of ASP.NET except Microsoft .NET Framework 1.0 Service Pack 3. For more information, see the subsection, Affected and Non-Affected Software, in this section.

The security update addresses the vulnerability by additionally signing all data that is encrypted by ASP.NET. For more information about the vulnerability, see the Frequently Asked Questions (FAQ) subsection for the specific vulnerability entry under the next section, Vulnerability Information.

This security update also addresses the vulnerability first described in Microsoft Security Advisory 2416728.

Recommendation. Microsoft recommends that customers apply the update at the earliest opportunity.

See also the section, Detection and Deployment Tools and Guidance, later in this bulletin.

Known Issues. Microsoft Knowledge Base Article 2418042 documents the currently known issues that customers may experience when installing this security update. The article also documents recommended solutions for these issues.

More info at this link: http://www.microsoft.com/technet/security/bulletin/MS10-070.mspx