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Disable TLS RC4 cipher in .Net


Finding ID Version Rule ID IA Controls Severity
V-225238 APPNET0075 SV-225238r615940_rule Medium
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.
Microsoft DotNet Framework 4.0 Security Technical Implementation Guide 2020-12-11


Check Text ( C-26937r612666_chk )
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:

If the “SchUseStrongCrypto” value name does not exist, or is not a REG_DWORD type set to “1”, this is a finding.

Fix Text (F-26925r612667_fix)
Use regedit to access the following registry key.

For 32-bit systems:

For 64-bit systems:

Modify or create the following Windows registry value: SchUseStrongCrypto

Set SchUseStrongCrypto to a REG_DWORD value of “1”.