Fedify is a TypeScript library for building federated server apps powered by ActivityPub and other standards. At present, when Fedify needs to retrieve an object or activity from a remote activitypub server, it makes a HTTP request to the @id or other resources present within the activity it has received from the web. This activity could reference an @id that points to an internal IP address, allowing an attacker to send request to resources internal to the fedify servers network. This applies to not just resolution of documents containing activities or objects, but also to media URLs as well. Specifically this is a Server Side Request Forgery attack. Users should upgrade to Fedify version 0.9.2, 0.10.1, or 0.11.1 to receive a patch for this issue.
The web server receives a URL or similar request from an upstream component and retrieves the contents of this URL, but it does not sufficiently ensure that the request is being sent to the expected destination.