This either means that the host has a new key and the fingerprint has changed or that you are the victim of a man-in-the-middle exploit. In order to protect you, the Mac OS X SSH client will not let you complete the connection to this host until the old key is removed from your computer. If you determine that the host's key has been changed (see connection instructions for current keys), follow these steps to remove the old key:
- Open Terminal in the Applications > Utilities folder.
- Type open .ssh in the terminal window then press the Return key. This opens the .ssh folder which contains the file known_hosts.
- Drag known_hosts into the trash then close the .ssh folder.
- Close Terminal.
Deleting known_hosts removes all host keys from your computer so all hosts you connect to will be treated as though you are connecting to them for the first time.