If you cannot upgrade your OpenSSH client and/or server to fix CVE-2023-48795, also known as the Terrapin attack, the way to mitigate it is to disable the vulnerable ciphers as Red Hat explains very well.
If you have a recent OpenSSH version, you can disable the the ciphers by adding “-” before them in the Ciphers and MACs options. This works for both the ssh client config (
/etc/ssh/ssh_config by default) and the ssh server config (
If you have an older OpenSSH version, you may not be able to use the “-“. Then you must explicitly list all the allowed ciphers. Simply remove the vulnerable ciphers and MACs from the respective lists.
Ciphers email@example.com,firstname.lastname@example.org,aes256-ctr,aes192-ctr,aes128-ctr<br>MACs hmac-sha2-512,hmac-sha2-256<br>KexAlgorithms diffie-hellman-group-exchange-sha256,diffie-hellman-group16-sha512,ecdh-sha2-nistp521,ecdh-sha2-nistp384,ecdh-sha2-nistp256<br>HostKeyAlgorithms ecdsa-sha2-nistp521,ecdsa-sha2-nistp384,ecdsa-sha2-nistp256
A few things to be aware of:
- be sure to check if the mentioned options are available to your systems before you restart your ssh daemons
- make sure you have host keys matching the ciphers
- verify you can connect to your servers after restarting sshd and before you disconnect
- monitor for connection failures from your clients.
Be careful, Mozilla OpenSSH guidelines have not been updated for a long time and they still recommend vulnerable algorithms.
The team behind Terrapin published a scanner to check if your servers are vulnerable on GitHub.