A severe security hole discovered in the NET framework recently now has a patch available. It affects versions of the net framework from 1.1 to 4.0. It affects current versions of windows from XP onward. The patch is discussed in Scott Guthrie's (aka Scott Gu) blog here:
http://weblogs.asp.net/scottgu/archive/2010/09/28/asp-net-security-update-now-available.aspx
The patch its self is available here:
http://www.microsoft.com/technet/security/bulletin/ms10-070.mspx
Please apply to all public facing web servers immediately.