This writeup is reference from The Geek Diary

• How To Disable Weak Cipher And Insecure HMAC Algorithms In SSH Services In CentOS/RHEL 8
• How To Disable Weak Cipher And Insecure HMAC Algorithms in SSH services for CentOS/RHEL 6 and 7

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]

You should receive a aimilar message message

Unable to negotiate with 172.21.33.13 port 22: no matching cipher found. Their offer: chacha20-poly1305@openssh.com,aes256-gcm@openssh.com,aes128-gcm@openssh.com,aes256-ctr,aes192-ctr,aes128-ctr

References:

• How To Disable Weak Cipher And Insecure HMAC Algorithms In SSH Services In CentOS/RHEL 8
• How To Disable Weak Cipher And Insecure HMAC Algorithms in SSH services for CentOS/RHEL 6 and 7
• OpenSSH Tip: Check Syntax Errors before Restarting SSHD Server