U2F reference implementations
U2F Authentication for Node.js
(OBSOLETE) U2F library in PHP
Python based U2F server library
FIDO U2F support for Safari.
FIDO U2F security token support for Django
Yubico Universal 2nd Factor (U2F) Host C Library
FIDO2 (WebAuthn) and FIDO U2F platform library for Linux written in Rust
Test suite for FIDO2, U2F, and other security key functions