Rallly is an open-source scheduling and collaboration tool. Versions up to and including 3.22.1 of the application features token based authentication. When a user attempts to login to the application, they insert their email and a 6 digit code is sent to their email address to complete the authentication. A token that consists of 6 digits only presents weak entropy however and when coupled with no token brute force protection, makes it possible for an unauthenticated attacker with knowledge of a valid email address to successfully brute force the token within 15 minutes (token expiration time) and take over the account associated with the targeted email address. All users on the Rallly applications are impacted. As long as an attacker knows the user's email address they used to register on the app, they can systematically take over any user account. For the authentication mechanism to be safe, the token would need to be assigned a complex high entropy value that cannot be bruteforced within reasonable time, and ideally rate limiting the /api/auth/callback/email endpoint to further make brute force attempts unreasonable within the 15 minutes time. As of time of publication, no patched versions are available.