After installing system updates, you may find that you're suddenly unable to SSH into systems that you could once reach without issue.
After updating to Ubuntu Jammy Jellyfish (22.04), I found I was no longer able to SSH onto my router
ssh 192.168.4.254 Unable to negotiate with 192.168.4.254 port 22: no matching host key type found. Their offer: ssh-rsa
I also found that I was no longer able to authenticate with a different box
ssh root@mikasa root@mikasa: Permission denied (publickey).
Both of these issues occured because OpenSSH had been updated to a version >= 8.2:
ii openssh-client 1:8.9p1-3 amd64 secure shell (SSH) client, for secure access to remote machines
SHA-1 signatures were deprecated in OpenSSH 8.2.
This post discusses how to verify and resolve this.
Getting More Information
Verify that the
permission denied error is caused by the upgrade rather than a problem with the key (permissions etc).
-v when calling ssh
ssh -v root@mikasa
At the bottom of the output, you should see the following
debug1: send_pubkey_test: no mutual signature algorithm debug1: No more authentication methods to try. root@mikasa: Permission denied (publickey).
The line with
no mutual signature algorithm tells us what we need to know, and confirms that this is related to the SHA-1 deprecation. In essence, it's telling us the same thing as
no matching host key type found, just about a different stage of the authentication flow.
Although we're pretty sure we know the cause, we can verify it by re-enabling SHA-1 in the client
ssh -oHostkeyAlgorithms="+ssh-rsa" -oPubkeyAcceptedAlgorithms="+ssh-rsa" 192.168.4.254 ssh -oHostkeyAlgorithms="+ssh-rsa" -oPubkeyAcceptedAlgorithms="+ssh-rsa" mikasa
If access is granted then the issues are because the server is trying to use
The correct fix is to upgrade OpenSSH on the systems that you're trying to SSH to:
ssh-rsa wasn't disabled just for the fun of it:
It is now possible to perform chosen-prefix attacks against the SHA-1 hash algorithm for less than USD$50K. For this reason, we will be disabling the "ssh-rsa" public key signature algorithm that depends on SHA-1 by default in a near-future release. This algorithm is unfortunately still used widely despite the existence of better alternatives, being the only remaining public key signature algorithm specified by the original SSH RFCs. The better alternatives include: * The RFC8332 RSA SHA-2 signature algorithms rsa-sha2-256/512. These algorithms have the advantage of using the same key type as "ssh-rsa" but use the safe SHA-2 hash algorithms. These have been supported since OpenSSH 7.2 and are already used by default if the client and server support them. * The ssh-ed25519 signature algorithm. It has been supported in OpenSSH since release 6.5. * The RFC5656 ECDSA algorithms: ecdsa-sha2-nistp256/384/521. These have been supported by OpenSSH since release 5.7. To check whether a server is using the weak ssh-rsa public key algorithm for host authentication, try to connect to it after removing the ssh-rsa algorithm from ssh(1)'s allowed list: ssh -oHostKeyAlgorithms=-ssh-rsa user@host If the host key verification fails and no other supported host key types are available, the server software on that host should be upgraded. A future release of OpenSSH will enable UpdateHostKeys by default to allow the client to automatically migrate to better algorithms. Users may consider enabling this option manually.  "SHA-1 is a Shambles: First Chosen-Prefix Collision on SHA-1 and Application to the PGP Web of Trust" Leurent, G and Peyrin, T (2020) https://eprint.iacr.org/2020/014.pdf
However, if you do not currently have the ability to upgrade those systems, you can work around this by enabling
ssh-rsa for these hosts in your SSH config file
For each affected host, add a block like the following
Host 192.168.4.254 HostkeyAlgorithms +ssh-rsa PubkeyAcceptedAlgorithms +ssh-rsa
But, you absolutely should view this only as a short-term fix and plan to get the target systems upgraded as soon as possible.
You'll likely run into this issue if you've still got systems running older SSH daemons. The issue won't appear when SSHing to newer installs because the client and server will be able to negotiate use of
rsa-sha2-256 (or better).
It is possible to work around this in the client as a short-term solution, but you should plan to upgrade affected systems as soon as possible.