U2F or Universal 2-Factor Authentication is a form of 2-factor authentication, in which the user completes a login process using a physical device as one form of verification to prove their identity and be granted access. U2F devices are physical security keys in and are usually combined with one of the other two major authentication factors: 1) something a user knows (such as a password or the answer to a question) or something the user is (such as a unique biometric marker like a fingerprint) – in order to grant access to a system. The benefit of a physical key over its counterparts, usually software-based keys, is that software keys, which generate one-time passwords delivered by phone or email, are known to be vulnerable to phishing attacks.