Microsoft has released a Security Update for Microsoft .NET Framework 3.5.1

A security issue has been identified that could allow an attacker to compromise your Windows-based system that is running the Microsoft .NET Framework and gain complete control over it. You can help protect your computer by installing this update from Microsoft. After you install this item, you may have to restart your computer.

Update for Windows Server 2008 x64 Edition (KB2545698)
Update for Windows Server 2008 R2 for Itanium-based Systems (KB2545698)
Update for Windows Vista for x64-based Systems (KB2545698)
Update for Windows Server 2008 (KB2545698)
Update for Windows 7 for x64-based Systems (KB2545698)
Update for Windows 7 (KB2545698)
Update for Windows Server 2008 for Itanium-based Systems (KB2545698)
Update for Windows Server 2008 R2 x64 Edition (KB2545698)
Update for Windows Vista (KB2545698)