Multiple serious vulnerabilities have been found in .NET Framework. Malicious users can exploit these vulnerabilities to cause denial of service, bypass security restrictions or gain privileges.
.NET Framework 2.0 Service Pack 2
.NET Framework 3.5, 3.5.1
.NET Framework 4, 4.5, 4.5.1, 4.5.2, 4.6
Install necessary updates from the KB section, that are listed in your Windows Update (Windows Update usually can be accessed from the Control Panel)