You may have already read this somewhere out there, it is making the rounds across the Twitter-phere, blogs, and news. But I felt it would be important to post, or I should say re-post the issue.
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. We are actively working with partners in our Microsoft Active Protections Program (MAPP) to provide information that they can use to provide broader protections to customers. Upon completion of this investigation, Microsoft will take the appropriate action to help protect our customers. This may include providing a security update through our monthly release process or providing an out-of-cycle security update, depending on customer needs.
Scott Guthrie has an excellent post up regarding this vulnerability at http://weblogs.asp.net/scottgu/archive/2010/09/18/important-asp-net-security-vulnerability.aspx. As normal, he has gone through with code snippets to explain the issue and what you can due to protect your systems.
Good luck keeping your systems safe!