U2F
also: Universal 2nd Factor, FIDO U2F
U2F (Universal 2nd Factor) is an open authentication standard that uses hardware security keys as a second factor for user verification, requiring both something you know (password) and something you have (physical key).
U2F is a cryptographic protocol that strengthens account security by requiring possession of a physical security key in addition to a password. The hardware key performs cryptographic operations to prove authentication without sending secrets over the network.
When logging in with U2F enabled, the user enters their password, then the system challenges the security key (often via USB, NFC, or Bluetooth). The key signs the challenge with a private key stored securely on the device, proving possession without exposing credentials. This makes phishing attacks ineffective since the attacker cannot satisfy the hardware requirement.
Example: A user logs into their Linux server or cloud account. After entering their password, they're prompted to tap their U2F key. The key authenticates and grants access. An attacker with stolen credentials still cannot log in without physical possession of that specific key.