How to disable CBC Mode Ciphers in RHEL 8 or Rocky Linux 8


This writeup is reference from The Geek Diary

Edit /etc/sysconfig/sshd and uncomment CRYPTO_POLICY line:

CRYPTO_POLICY=

Edit /etc/ssh/sshd_config file. Add Ciphers, MACs and KexAlgorithms have been added

KexAlgorithms curve25519-sha256@libssh.org,ecdh-sha2-nistp521,ecdh-sha2-nistp384,ecdh-sha2-nistp256,diffie-hellman-group-exchange-sha256
Ciphers chacha20-poly1305@openssh.com,aes256-gcm@openssh.com,aes128-gcm@openssh.com,aes256-ctr,aes192-ctr,aes128-ctr
MACs hmac-sha2-512-etm@openssh.com,hmac-sha2-256-etm@openssh.com,umac-128-etm@openssh.com,hmac-sha2-512,hmac-sha2-256,umac-128@openssh.com

After making changes to the configuration file, you may want to do a sanity check on the configuration file

# sshd -t

Restart sshd services

# systemctl restart sshd

To test if weak CBC Ciphers are enabled

$ ssh -vv -oCiphers=3des-cbc,aes128-cbc,aes192-cbc,aes256-cbc [youruserid@IP of your Server]

References:

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