As the MS10-070 update points out, the vulnerability could be exploited to allow information disclosure and data tampering. The update also notes:
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.
According to PCWorld.com, the update will not be immediately available through Automatic Updates, but rather through the Download Center. As Microsoft told IDG News Service:
This is the first time we've released [an] update this way, but due to the nature of the active attacks and the severity of the potential loss of data, we are releasing the security update to the Microsoft Download Center first so customers (specifically large enterprises, hosting providers, and ISVs) can begin updating their systems.