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proposals/3843-federated-reports.md

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+# MSC3843: Reporting content over federation
+
+Clients can currently [report content](https://spec.matrix.org/v1.3/client-server-api/#reporting-content)
+to their homeserver, however these reports don't currently translate over federation to affected
+servers.
+
+Some content might want to be reported directly to the server which generated it, or tooling on a
+given server could find it desirable to generate a report to be sent to a specific server. For example,
+if a homeserver plugin detected spam then it might wish to report that directly to the server which
+generated the content, bypassing any local report handling which might be attached to the local server's
+client-server API endpoint.
+
+## Proposal
+
+Mirroring the client-server API a bit, we introduce a new endpoint for reporting events in rooms over
+federation: `POST /_matrix/federation/v1/rooms/{roomId}/report/{eventId}`. To report a whole room the
+caller would reference the `m.room.create` event. To report a user within the context of that room the
+caller would use the `m.room.member` event for that user. Otherwise, the potentially objectionable
+content's event ID would be used to report that content.
+
+The new endpoint takes very similar body parameters to the client-server API, though with the `score`
+notably missing and `reason` being explicitly required. For example:
+
+```
+POST /_matrix/federation/v1/rooms/!bad:example.org/report/$spammyspam
+{"reason":"This message is spam"}
+```
+
+`reason` cannot be blank, unlike the client-server API.
+
+It is intentional that the original reporter is not sent over federation, however given the endpoint
+is authenticated using the server's name it may still be possible to identify the original reporter.
+If the homeserver holds exactly one known user then there's a pretty good chance they reported the
+content.
+
+How the report is handled is deliberately left as an implementation detail. Homeservers might wish to
+add the record to their existing reports table/system, or create a new handling pipeline for it.
+Similarly, when to call this endpoint is also left as an implementation detail. It may not be desirable
+for all client-server `/report` calls to be proxied over federation, for example.
+
+The content being reported *should* originate from the server it is being reported to, but that is
+not a requirement. It may be valid to notify other homeservers that there is spam in a room to help
+all affected users, for example. It is up to the implementation if it wants to handle content generated
+from other servers - if it doesn't, it can respond with an `M_UNACTIONABLE` error code (new with this
+proposal) and an HTTP `400` status. Further, if the server just doesn't want to deal with federated
+reports at all it can return a `400` `M_UNACTIONABLE` error for everything.
+
+Homeservers are strongly encouraged to rate limit the number of reports which come in. A possible
+approach might be to limit servers to 10 reports per minute, or similar.
+
+## Potential issues
+
+This could open up servers to receiving spammy or abusive reports, however given most servers have
+a federation-capable reporting mechanism already (email, community room, etc) this is not expected
+to have much of an impact. The added recommendation that things be rate limited also helps a lot.
+
+## Alternatives
+
+Briefly while writing this proposal it was considered to expose a shared policy room between the servers,
+where somehow Server A and Server B negotiate a room where they can publish
+[policy rules](https://spec.matrix.org/v1.3/client-server-api/#moderation-policy-lists) that the other
+side picks up and actions, if needed. This feels overly complex for the time being, but if proven useful
+could be explored in a different MSC.
+
+## Security considerations
+
+Servers should protect themselves from abusive or spammy reports. Rate limiting and rejecting content
+that didn't originate from itself will help a lot on that.
+
+Like with the client-server API, servers should be cautious about hooking up automated responses to
+reported content. Without the added context of *who* made the report, it's very possible that the content
+is being reported 40 times by the same person rather than 40 different people - this could easily trip
+an automated response of spam handling or deactivation if not careful.
+
+## Unstable prefix
+
+While this MSC is not considered stable, implementations should use `/_matrix/federation/unstable/org.matrix.msc3843/rooms/{roomId}/report/{eventId}`
+as the endpoint. The body stays as-proposed.
+
+## Dependencies
+
+None relevant.