WebAuthn/FIDO2 is a W3C standard that defines a cryptographic protocol between a relying party (your Flask application) and an authenticator. In simple terms this allows connecting your Flask application to a variety of authenticators including dedicated hardware (e.g. YubiKey) and devices that have cryptographic capabilities (e.g. a mobile phone with fingerprint or face id).

This protocol is supported by all major browsers, however as of Spring 2022, few web application actually support it, and those that do normally just support using a WebAuthn key as an additional, optional, second factor.

Note that a WebAuthn key can possibly satisfy a complete 2-factor authentication requirement - something you have and something you are (think a mobile device with face-Id). Flask-Security supports this use case.

Key Concepts

While the spec is quite complex - there are 2 important concepts to know. WebAuthn keys can be classified as ‘platform’/’cross-platform’ and ‘resident’/or not. If a key is a platform key that means it is tied to a particular device. If you set up a second factor WebAuthn key that is a platform key you will ONLY BE ABLE TO AUTHENTICATE using that device. For that reason it is a best practice to make sure there is at least one second-factor authentication method setup that is NOT platform specific (such as SMS, an authenticator app, etc.).

Flask-Security requires that when registering a WebAuthn key, the user must specify whether the key will be used for first/primary authentication or for multi-factor/second authentication.

It should be noted the the current spec REQUIRES javascript to communicate from your front-end to the browser. Flask-Security ships with the basic required JS (static/{webauthn.js,base64.js}). An application should be able to simply wire those into their templates or javascript.


As with many features in Flask-Security, configuration is a combination of config variables, constructor parameters, and a sub-classable utility class. The WebAuthn spec offers a lot of flexibility in supporting a wide range of authenticators. The default configuration is:

The bundled WebauthnUtil class implements the following defaults:

  • The AuthenticatorSelectionCriteria is set to CROSS_PLATFORM for webauthn keys being registered for first/primary authentication.

  • The UserVerificationRequirement is set to DISCOURAGED for keys used for secondary authentication, and PREFERRED for keys used for first/primary or multi-factor.