Universal 2nd Factor

Universal 2nd Factor (U2F) is an open standard that strengthens and simplifies two-factor authentication (2FA) using specialized Universal Serial Bus (USB), near-field communication (NFC), or Bluetooth Low Energy (BLE) devices based on similar security technology found in smart cards.

[6] While initially developed by Google and Yubico, with contribution from NXP Semiconductors, the standard is now hosted by the FIDO Alliance.

Specifically: In terms of disadvantages, one significant difference and potential drawback to be considered regarding hardware-based U2F solutions is that unlike with TOTP shared-secret methods, there is no possibility of "backing up" recovery codes or shared secrets.

[9][failed verification – see discussion] This avoids the need for the user to install special hardware driver software in the host computer and permits application software (such as a browser) to directly access the security features of the device without user effort other than possessing and inserting the device.

U2F security keys can be used as an additional method of two-step verification on online services that support the U2F protocol, including Google,[2] Azure,[17] Dropbox,[18] GitHub,[19] GitLab,[20] Bitbucket,[21] Nextcloud,[22] Facebook,[23] and others.

The U2F standard has undergone two major revisions: Additional specification documents may be obtained from the FIDO web site.

WebAuthn and CTAP provide a complete replacement for U2F, which has been renamed "CTAP1" in the latest version of the FIDO2 standard.