ASP.NET 3.5 is getting pretty old and the ideal solution would be to update to the latest ASP.NET version but let’s face it, there are some legacy systems already running 3.5 that are not easy to upgrade and most external services are closing support for the depreciated TLS 1.0 and TLS 1.1 protocols.
I searched for solutions online and most replies were indicating that there was no TLS 1.2 support in ASP.NET 3.5 and it would not be supported which is not true.
Microsoft released upgrade a while ago which added TLS 1.2 support for ASP.NET 3.5. Chances are that you already have it installed on the server but if not, you can download it here. If you get the error: “The update is not applicable to your computer.” it’s probably already installed.
To make ASP.NET use TLS 1.2 you just have to make some changes to the registry and tell it to use the system default SSL/TLS settings by adding these DWORD keys to the system using regedit:
For 64-bit operating systems:
[HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\.NETFramework\v2.0.50727]
"SystemDefaultTlsVersions"=dword:00000001
[HKEY_LOCAL_MACHINE\SOFTWARE\Wow6432Node\Microsoft\.NETFramework\v2.0.50727]
"SystemDefaultTlsVersions"=dword:00000001
For 32-bit operating systems:
[HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\.NETFramework\v2.0.50727]
"SystemDefaultTlsVersions"=dword:00000001
No restart is required and your applications should now be using the TLS 1.2 protocol.