Update and configure the .NET Framework to support TLS.

Severity	Group ID	Group Title	Version	Rule ID	Date	STIG Version
medium	V-225238	SRG-APP-000383	APPNET0075	SV-225238r1069480_rule	2025-02-20	2

Description
Use of the RC4 cipher in TLS could allow an attacker to perform man-in-the-middle attacks and recover plaintext from encrypted sessions. Applications that target .Net version 4.x running on multiple Windows versions could be vulnerable to these types of attacks. The registry settings in this requirement will prevent .Net applications that target the 4.x framework from selecting and utilizing the Schannel.dll RC4 cipher for TLS connections. Applications that use TLS when connecting to remote systems will perform a handshake and negotiate the TLS version and cipher that is to be used between the client and the server. This is standard protocol for all TLS connections. If the server and client are not configured to use the same TLS version and cipher, the TLS connection may fail. Applications should be tested with these registry settings prior to production implementation of the fix in order to avoid application outages.

Description

Use of the RC4 cipher in TLS could allow an attacker to perform man-in-the-middle attacks and recover plaintext from encrypted sessions. Applications that target .Net version 4.x running on multiple Windows versions could be vulnerable to these types of attacks. The registry settings in this requirement will prevent .Net applications that target the 4.x framework from selecting and utilizing the Schannel.dll RC4 cipher for TLS connections. Applications that use TLS when connecting to remote systems will perform a handshake and negotiate the TLS version and cipher that is to be used between the client and the server. This is standard protocol for all TLS connections. If the server and client are not configured to use the same TLS version and cipher, the TLS connection may fail. Applications should be tested with these registry settings prior to production implementation of the fix in order to avoid application outages.

ℹ️ Check
In older Windows systems (Windows Server 2012 or earlier), TLS 1.2 must be enabled systemwide by setting "SchUseStrongCrypto". SystemDefaultTlsVersions is a configuration switch in .NET Framework (starting from 4.6) that allows the application to use the default TLS version supported by the underlying Windows operating system instead of hardcoding a specific TLS version (like TLS 1.2). Check Registry: Use regedit to review the following Windows registry keys: For 32-bit systems: HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\.NETFramework\v4.0.30319\ For 64 bit systems: HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\.NETFramework\v4.0.30319\ HKEY_LOCAL_MACHINE\SOFTWARE\Wow6432Node\Microsoft\.NETFramework\v4.0.30319\ 1. If the "SchUseStrongCrypto" value name does not exist, or is not a REG_DWORD type set to "1", this is a finding. 2. For .NET Framework >4.6, use the default TLS version supported by the underlying Windows operating system. If the "SystemDefaultTlsVersions" value name does not exist, or is not a REG_DWORD type set to "1", this is a finding. Note: The SchUseStrongCrypto setting allows .NET to use TLS 1.1 and TLS 1.2. The SystemDefaultTlsVersions setting allows .NET to use the OS configuration.

ℹ️ Check

In older Windows systems (Windows Server 2012 or earlier), TLS 1.2 must be enabled systemwide by setting "SchUseStrongCrypto". SystemDefaultTlsVersions is a configuration switch in .NET Framework (starting from 4.6) that allows the application to use the default TLS version supported by the underlying Windows operating system instead of hardcoding a specific TLS version (like TLS 1.2). Check Registry: Use regedit to review the following Windows registry keys: For 32-bit systems: HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\.NETFramework\v4.0.30319\ For 64 bit systems: HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\.NETFramework\v4.0.30319\ HKEY_LOCAL_MACHINE\SOFTWARE\Wow6432Node\Microsoft\.NETFramework\v4.0.30319\ 1. If the "SchUseStrongCrypto" value name does not exist, or is not a REG_DWORD type set to "1", this is a finding. 2. For .NET Framework >4.6, use the default TLS version supported by the underlying Windows operating system. If the "SystemDefaultTlsVersions" value name does not exist, or is not a REG_DWORD type set to "1", this is a finding. Note: The SchUseStrongCrypto setting allows .NET to use TLS 1.1 and TLS 1.2. The SystemDefaultTlsVersions setting allows .NET to use the OS configuration.

✔️ Fix
1. SchUseStrongCrypto enabled: Use regedit to access the following registry key. For 32-bit systems: HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\.NETFramework\v4.0.30319\ For 64-bit systems: HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\.NETFramework\v4.0.30319\ HKEY_LOCAL_MACHINE\SOFTWARE\Wow6432Node\Microsoft\.NETFramework\v4.0.30319\ Modify or create the following Windows registry value: SchUseStrongCrypto. Set SchUseStrongCrypto to a REG_DWORD value of "1". 2. SystemDefaultTlsVersions enabled (.NET Framework >4.6): For 64-bit Windows, create a .reg file with the following content and apply it: Windows Registry Editor Version 5.00 [HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\.NETFramework\v4.0.30319] "SystemDefaultTlsVersions"=dword:00000001 [HKEY_LOCAL_MACHINE\SOFTWARE\Wow6432Node\Microsoft\.NETFramework\v4.0.30319] "SystemDefaultTlsVersions"=dword:00000001 3. Restart the system for changes to take effect.

✔️ Fix

1. SchUseStrongCrypto enabled: Use regedit to access the following registry key. For 32-bit systems: HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\.NETFramework\v4.0.30319\ For 64-bit systems: HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\.NETFramework\v4.0.30319\ HKEY_LOCAL_MACHINE\SOFTWARE\Wow6432Node\Microsoft\.NETFramework\v4.0.30319\ Modify or create the following Windows registry value: SchUseStrongCrypto. Set SchUseStrongCrypto to a REG_DWORD value of "1". 2. SystemDefaultTlsVersions enabled (.NET Framework >4.6): For 64-bit Windows, create a .reg file with the following content and apply it: Windows Registry Editor Version 5.00 [HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\.NETFramework\v4.0.30319] "SystemDefaultTlsVersions"=dword:00000001 [HKEY_LOCAL_MACHINE\SOFTWARE\Wow6432Node\Microsoft\.NETFramework\v4.0.30319] "SystemDefaultTlsVersions"=dword:00000001 3. Restart the system for changes to take effect.