GitHub reported an incident in which the RSA private key used as the host key when accessing GitHub repositories via SSH was mistakenly published to a publicly accessible repository. The leak affected only the RSA key, the ECDSA and Ed25519 host SSH keys continue to be secure. A publicly exposed host SSH key does not allow access to the GitHub infrastructure or user data, but can be used to intercept Git operations performed via SSH.
To prevent possible hijacking of SSH sessions to GitHub if the RSA key falls into the wrong hands, GitHub has initiated a key replacement process. On the user side, deletion of the old GitHub public key (ssh-keygen -R github.com) or manual replacement of the key in the ~/.ssh/known_hosts file is required, which can break auto-executed scripts.
Source: opennet.ru