Resolving SSH Permission Denied Errors Due to OpenSSH Version Vulnerabilities

If you have encountered errors while using SSH, such as the ones below:

Example 1:

foo@12.34.56.789: Permission denied (publickey).

Example 2:

sign_and_send_pubkey: no mutual signature supported
foo@12.34.56.789: Permission denied (publickey).

It's possible that you may be using an older version of OpenSSH on your server that is vulnerable to the SHA-1 algorithm, which was disabled in OpenSSH version 8.8 (2021-09-26). If you're certain that you've correctly added the public key to your server, this could be the cause of the error messages.

To determine your local OpenSSH version, enter the following command:

ssh -vv local

The result should look like this:

To resolve the issue, add PubkeyAcceptedKeyTypes +ssh-rsa to your Host entry in the ~/.ssh/config file. It should look like this:

Host fooServer
PubkeyAcceptedKeyTypes +ssh-rsa
Hostname 12.34.56.789
User forge
IdentityFile=~/.ssh/id_rsa

After making these changes, try again and the issue should be resolved.