How to disable cipher block chaining encryption

Introduction

Disabling the Cipher Block Chaining (CBC) encryption is done by modifying the cipher configuration within your SSH configuration. You may be asked to do this to comply with certain compliance scans.

Please note that the cPanel software does not have any tools to manage the SSH configuration directly. Changes made to SSH can alter your ability to connect to the server. If you have any concerns about applying these changes, we recommend you consult with your System Administrator. If you lose access to your server due to a misconfiguration in SSH, you may need to contact your hosting provider to restore it.

Procedure

1. Connect to your server through SSH as the root user
2. Make a backup of your SSHd configuration to use in case of unexpected results:

   cp -v /etc/ssh/sshd_config{,.$( date +%Y%m%d )}

3. Using your preferred text editor, open your SSH configuration:

   /etc/ssh/sshd_config

4. Locate the "Ciphers" line. It may look something like this:

   Ciphers aes128-ctr,aes192-ctr,aes256-ctr,arcfour256,arcfour128,aes128-cbc,3des-cbc

5. Remove the following ciphers from the list if they are present:

   arcfour
   arcfour128
   arcfour25
   aes128-cbc
   3des-cbc
   blowfish-cbc
   cast128-cbc
   aes192-cbc
   aes256-cbc

6. Save your changes and restart the SSHd service:

   /scripts/restartsrv_sshd

7. Your changes should be applied. You can use the following command to verify the currently active ciphers in your SSH configuration:

   sshd -T | grep ciphers