Debian / Ubuntu Linux Public key Blacklisted (see ssh-vulnkey(1)) Error and Solution

If you are trying to log in to a Debian or Ubuntu Linux system using SSH and receive an error message that says "Public key blacklisted (see ssh-vulnkey(1))," it means that the public key you are using has been added to a list of known vulnerabilities and is not allowed to be used for authentication.

This error message is usually displayed when you try to log in using a public key that has been identified as vulnerable to certain attacks, such as the "Debian OpenSSL Predictable PRNG" vulnerability (also known as DSA-1024). This vulnerability allows an attacker to predict the private key that corresponds to a given public key, which could allow them to gain unauthorized access to the system.

To fix this problem, you will need to generate a new, secure public key and use it to replace the blacklisted key. Here are the steps you can follow:

1. Generate a new SSH key pair on your local system using the ssh-keygen command. For example:

ssh-keygen -t rsa -b 4096

This will generate a new RSA key pair with a length of 4096 bits. You can use a different type of key or a different key length if you prefer.

2. Copy the public key to the remote system using the ssh-copy-id command. For example:

ssh-copy-id user@remote-system

Replace user with your username on the remote system and remote-system with the hostname or IP address of the system.

3. Try logging in to the remote system again using your new public key. If the login is successful, it means that the new key has been accepted and the old, blacklisted key has been replaced.

If you are still having trouble logging in after generating a new key, you may need to check the permissions on your .ssh directory and authorized keys file on the remote system. Make sure that these files are only readable by you, and that the permissions are set correctly.