From charlesreid1

Obtaining Remote Access Using SSH Keys

The basic idea behind this type of exploit is to copy your SSH keys into the remote machine's list of authorized keys. It requires write access to the remote filesystem.

On the attacker machine, the public key is located in ~/.ssh/

Using a remote shell on metasploitable, or by taking advantage of backdoors, or by mounting the remote filesystem using an exploit, gain write access to the victim's machine. Then copy the public key into </code>/root/.ssh/authorized_keys</code>, and you'll have passwordless root access.

If you have write access to a filesystem, this technique can turn that write access into remote shell access without cracking the root password.

Then you'll be able to log in like this:

# ssh root@
Last login: Tue Mar 22 20:26:16 EDT 2016 from :0.0 on pts/0
Linux metasploitable 2.6.24-16-server #1 SMP Thu Apr 10 13:58:00 UTC 2008 i686