Terrapin: an SSH bug to fix calmly
A new attack on the SSH protocol sounds scary. In practice it needs a man in the middle and does limited damage. Update anyway.
This week researchers published Terrapin (CVE-2023-48795), an attack on the SSH protocol itself. Headlines make it sound like all SSH is broken. It isn’t.
What it does: an attacker who sits in the middle of the connection can quietly remove a few messages at the start, which can switch off some security extensions. It only works with certain cipher modes (ChaCha20-Poly1305, and CBC with encrypt-then-MAC). It doesn’t let anyone read your session or steal your password.
What to do, without panic:
- Update servers and clients as your distribution releases fixes. OpenSSH 9.6 adds “strict key exchange”, and it needs both sides updated to work.
- Don’t forget other SSH software: PuTTY, libraries in your code, network devices.
- If you can’t update soon, you can disable the affected cipher modes in the server config, after checking your clients still connect.
Good security news has a sense of proportion. This one is “update in your normal cycle”, not “stop everything”.