Devise-Two-Factor does not throttle or otherwise restrict login attempts at the server by default. When combined with the Time-based One Time Password algorithm's (TOTP) inherent entropy limitations, it's possible for an attacker to bypass the 2FA mechanism through brute-force attacks.

Impact

If a user's username and password have already been compromised an attacker would be able to try possible TOTP codes and see if they can hit a lucky collision to log in as that user. The user under attack would not necessarily know that their account has been compromised.

Patches

Devise-Two-Factor has not released any fixes for this vulnerability. This library is open-ended by design and cannot solve this for all applications natively. It's recommended that any application leveraging Devise-Two-Factor implement controls at the application level to mitigate this threat. A non-exhaustive list of possible mitigations can be found below.

Mitigations

1. Use the lockable strategy from Devise to lock a user after a certain number of failed login attempts. See https://www.rubydoc.info/github/heartcombo/devise/main/Devise/Models/Lockable for more information.

2. Configure a rate limit for your application, especially on the endpoints used to log in. One such library to accomplish this is rack-attack.

3. When displaying authentication errors hide whether validating a username/password combination failed or a two-factor code failed behind a more generic error message.

Acknowledgements

Christian Reitter (Radically Open Security) and Chris MacNaughton (Centauri Solutions)