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TLS Configuration Check

We check and validate Exchange servers TLS 1.0 - 1.3 configuration. We can detect mismatches in TLS versions for client and server. This is important because Exchange can be both a client and a server.

We will also show a yellow warning, if TLS 1.0 and/or TLS 1.1 is enabled. Microsoft's TLS 1.0 implementation is free of known security vulnerabilities. Due to the potential for future protocol downgrade attacks and other TLS 1.0 vulnerabilities not specific to Microsoft's implementation, it is recommended that dependencies on all security protocols older than TLS 1.2 be removed where possible (TLS 1.1/1.0/ SSLv3/SSLv2).

At this time TLS 1.3 is not supported by Exchange and has been known to cause issues if enabled. If detected to be anything but disabled on Exchange, it will be thrown as an error and needs to be addressed right away.

We also check for the SystemDefaultTlsVersions registry value which controls if .NET Framework will inherit its defaults from the Windows Schannel DisabledByDefault registry values or not.

An invalid TLS configuration can cause issues within Exchange for communication.

Only the values 0 or 1 are accepted and determined to be properly configured. The reason being is this is how our documentation provides to configure the value only and it then depends on how the code reads the value from the registry interpret the value.

By not having the registry value defined, different versions of .NET Frameworks for what the code is compiled for will treat TLS options differently. Therefore, we throw an error if the key isn't defined and action should be taken to correct this as soon as possible. To correct this, you create the missing DWORD registry key with the value you wish to have.

The Configuration result can provide a value of Enabled, Disabled, Half Disabled, or Misconfigured. They are defined by the following conditions:

Value Definition
Enabled Client and Server Enabled values are set to 1 and DisabledByDefault is set to 0 on the TLS Version.
Disabled Client and Server Enabled values are set to 0 and DisabledByDefault is set to 1 on the TLS Version.
Half Disabled Client and Server Enabled values are set to either 0 or 1 and DisabledByDefault is set to the opposite where the value doesn't equal Enabled or Disabled.
This is not a supported configuration as it doesn't follow the documentation that we have provided.
Misconfigured When either the Enabled or the DisabledByDefault values do not match between the Client and Server of that TLS Version.
Exchange can be a Client and a Server and this will cause problems and needs to be addressed ASAP.

The location where we are checking for the TLS values are here:

SYSTEM\CurrentControlSet\Control\SecurityProviders\SCHANNEL\Protocols\TLS 1.0\Client SYSTEM\CurrentControlSet\Control\SecurityProviders\SCHANNEL\Protocols\TLS 1.0\Server SYSTEM\CurrentControlSet\Control\SecurityProviders\SCHANNEL\Protocols\TLS 1.1\Client SYSTEM\CurrentControlSet\Control\SecurityProviders\SCHANNEL\Protocols\TLS 1.1\Server SYSTEM\CurrentControlSet\Control\SecurityProviders\SCHANNEL\Protocols\TLS 1.2\Client SYSTEM\CurrentControlSet\Control\SecurityProviders\SCHANNEL\Protocols\TLS 1.2\Server SYSTEM\CurrentControlSet\Control\SecurityProviders\SCHANNEL\Protocols\TLS 1.3\Client SYSTEM\CurrentControlSet\Control\SecurityProviders\SCHANNEL\Protocols\TLS 1.3\Server

At each location, we are looking at the value of Enabled and DisabledByDefault. If the key isn't present, Enabled is set to true and DisabledByDefault is set to false. With the exception of TLS 1.3, if that isn't present it is disabled by default.

The location for the .NET Framework TLS related settings are located here:

SOFTWARE\Wow6432Node\Microsoft\.NETFramework\v4.0.30319 SOFTWARE\Microsoft\.NETFramework\v4.0.30319 SOFTWARE\Wow6432Node\Microsoft\.NETFramework\v2.0.50727 SOFTWARE\Microsoft\.NETFramework\v2.0.50727

At each location, we are looking at the value of SystemDefaultTlsVersions and SchUseStrongCrypto. If the key isn't present, both are set to false.

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Additional resources:

Exchange Server TLS configuration best practices

Solving the TLS 1.0 Problem, 2nd Edition

TLS 1.2 support at Microsoft

Exchange Server TLS guidance, part 1: Getting Ready for TLS 1.2

Exchange Server TLS guidance Part 2: Enabling TLS 1.2 and Identifying Clients Not Using It

Exchange Server TLS guidance Part 3: Turning Off TLS 1.0/1.1