Rocket.Chat is an open-source, secure, fully customizable communications platform. Prior to 8.5.0, 8.4.1, 8.3.3, 8.2.3, 8.1.4, 8.0.5, 7.13.7, and 7.10.11, an unauthenticated network attacker obtains a valid Rocket.Chat OAuth access token for an arbitrary user by sending a single HTTP POST with MongoDB query operators to /oauth/token. The Rocket.Chat OAuth2 server does not validate that grant parameters are strings before forwarding them to findOne({...}) against the oauth_apps and oauth_access_tokens collections, so an attacker substitutes {"$ne": null} for client_id, client_secret, and refresh_token and receives a freshly minted {access_token, refresh_token} pair bound to whichever user's refresh token Mongo returned first. The resulting access token is a first-class bearer credential against the full /api/v1/* surface as that user. By iterating with $nin / $regex operators the attacker walks the entire oauth_access_tokens collection, collecting one fresh access token per user per request. If any matched token belongs to an admin, the stolen bearer gives full admin API access (including Apps-Engine app installation, i.e. server-side code execution). No account, credentials, userId, or prior interaction with the instance are required. This vulnerability is fixed in 8.5.0, 8.4.1, 8.3.3, 8.2.3, 8.1.4, 8.0.5, 7.13.7, and 7.10.11.