SSH Persistence

Adversaries may add an attacker-controlled SSH public key to a target account's authorized_keys file or, on Google Compute Engine VMs using metadata-based SSH keys, to instance metadata. Possession of the matching private key can enable SSH login as that account; adding a key for a more privileged account can enable privilege escalation. The Compute Engine metadata method is ineffective when OS Login is enabled because the guest agent ignores metadata SSH keys.