I have two hosts (A and B) with the same public key stored in ~/.ssh/authorized_keys. Why am I able to ssh between them without a password. The keypair was generated on a third host (C), so I am confident the private key is not available on either A or B.
Update: Answered my own question, and prompting from heavyd was helpful. Turns out I had 'OpenSSH Agent Forwarding' turned on on my local SSH client. The local SSH client (machine C) does have access to the private key, and was acting as agent to authenticate machine A to machine B. This is not what I want, and I'm turning off Agent Forwarding in my client.
tl;dr: Agent Forwarding