All Switches Installed from the Same Cumulus Linux 4.0.0 and 4.1.0 Image Have the Same SSH Host Keys

Issue

Due to a packaging error, all switches installed from the same Cumulus Linux image have the same SSH host keys. This affects switches originally installed with Cumulus Linux 4.0.0 and 4.1.0 from a disk image only (including those that you upgraded by apt to a later release).

As a result, this issue allows an attacker to more easily bypass remote host verification when a user connects by SSH to what he or she believes to be a previously used remote host but is actually the attacker’s host. For example, someone can exploit this issue by a spoofing or man-in-the-middle attack.

Environment

• Switches originally installed with Cumulus Linux 4.0.0 and 4.1.0 from a disk image only, including those that you upgraded by apt to a later release

Resolution

To resolve this issue, generate new SSH host keys for any switch that has Cumulus Linux 4.0.0 or 4.1.0 installed on it:

cumulus@switch:~$ sudo rm /etc/ssh/ssh_host*
cumulus@switch:~$ sudo dpkg-reconfigure openssh-server
cumulus@switch:~$ sudo systemctl restart ssh

After generating new SSH host keys, SSH clients that have previously logged into that switch display a warning that the switch’s SSH host key changed; this is normal behavior. Be sure to inform anyone who might log in to the switch that you generated new SSH host keys. These users must log in to the affected switches with their SSH clients, where they receive instructions on how to remove the old SSH host keys from the known hosts files to avoid a spoofing or man-in-the-middle attack directed at their SSH clients.

Notes

• Cumulus Linux 4.1.1 has the fix for this issue. However, NVIDIA recommends you generate new SSH host keys as this is the most reliable solution.
• If you upgrade from Cumulus Linux 4.0.0 or 4.1.0 to version 4.1.1 or later using apt-get and you did not generate new SSH host keys, you must generate new SSH host keys after the upgrade.
• If you perform a fresh install of Cumulus Linux 4.1.1 or later using a disk image, you lose your existing local configuration.