MSC4202: Reporting User Profiles

proposals/4202-user-reporting.md

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+# MSC4202: Reporting User Profiles over Federation
+
+*This proposal extends [MSC3843](https://github.com/matrix-org/matrix-spec-proposals/pull/3843) to
+allow reporting user profiles over federation. Clients can offer a user interface feature, such as
+a button on a user's profile, enabling users to report another user's profile to the administrators
+of their own homeserver, which can then forward the report over federation if necessary.*
+
+## Proposal
+
+Building upon the reporting mechanism proposed in
+[MSC3843](https://github.com/matrix-org/matrix-spec-proposals/pull/3843), this MSC proposes that
+clients offer an option in user profiles to report the most recent `m.room.member` event of the
+user in a shared room. This leverages the existing client-server and federation APIs for reporting
+events, allowing servers to handle profile reports without any changes to the APIs.
+
+### Client Behaviour
+
+Clients SHOULD offer a user interface element in user profiles (e.g., a "Report User" button) that
+allows users to report problematic profiles. When reporting a profile, clients SHOULD:
+
+- Identify a room (`room_id`) that both the reporting user and the reported user share. This could
+  be any mutual room, including a direct message room. If no mutual room exists, clients MAY
+  display an error to the user indicating that the profile cannot be reported through this mechanism.
+
+- Obtain the most recent `m.room.member` event (`event_id`) for the reported user in that room. This
+  event contains the profile information (display name, avatar URL) set by the user in that room.
+
+- Use the existing client-server endpoint `POST /_matrix/client/v3/rooms/{roomId}/report/{eventId}`
+  to report the `m.room.member` event. The request body SHOULD include:
+
+  - A `reason` provided by the user, explaining why the profile is being reported.
+
+  - A `score` indicating the severity, as per the existing specification, though servers MAY ignore
+    this parameter.
+
+  - If reporting content in the user's profile, it is recommended for the client include an MXC URI
+    for a screenshot of the profile (including MXID and offending content) to aid investigation.
+
+#### Example Request
+
+Reporting a user's profile:
+
+```
+POST /_matrix/client/v3/rooms/!roomid:example.org/report/$eventid
+Content-Type: application/json
+
+{
+  "reason": "Inappropriate profile content: mxc://example.org/abcd1234",
+  "score": 0
+}
+```
+
+### Homeserver Behaviour
+
+Upon receiving the report, the homeserver SHOULD process it according to its moderation policies.
+If the event being reported originated from a remote homeserver, and if the homeserver supports
+federation reporting as per MSC3843, it MAY forward the report to the remote homeserver using the
+federation endpoint:
+
+```
+POST /_matrix/federation/v1/rooms/{roomId}/report/{eventId}
+Content-Type: application/json
+
+{
+  "reason": "Inappropriate profile content"
+}
+```
+
+The homeserver SHOULD ensure that the `event_id` corresponds to an event in the room and SHOULD
+verify that the event was issued by the reported user's homeserver.
+
+### Federation Considerations
+
+When a homeserver receives a report over federation for an `m.room.member` event, it SHOULD handle
+it similarly to how it handles other event reports:
+
+- If the reported event was created by the receiving homeserver, it SHOULD process the report and
+  take appropriate action as per its moderation policies.
+
+- If the reported event was not created by the receiving homeserver, it MAY choose to ignore the
+  report and respond with an `M_UNACTIONABLE` error code and an HTTP `400` status.
+
+#### Example Error Response
+
+```json
+{
+  "errcode": "M_UNACTIONABLE",
+  "error": "Cannot act on the reported event."
+}
+```
+
+### Considerations
+
+- **Selecting the Appropriate Event**: Clients need to ensure they report the correct
+  `m.room.member` event. In rooms where the reported user has updated their profile multiple times,
+  the most recent membership event SHOULD be used.
+
+- Clients may offer to report previous member events, if the user has changed their profile to hide
+  the original content that caused offence.
+
+- **Shared Rooms**: Reporting a user in a room requires that both users share a room. If no mutual
+  room exists, the client cannot report the profile via this mechanism.
+
+- **Event Context**: By reporting the `m.room.member` event, servers have the necessary context to
+  investigate the profile as it appeared in that room. The event includes the display name and
+  avatar URL used by the user in that room.
+
+## Security Considerations
+
+- **Anonymity**: The original reporter's identity is not included in the report sent over
+  federation. However, since the report is sent from the reporter's homeserver, it may still be
+  possible to infer their identity, especially if the homeserver has a small user base.
+
+- **Abuse**: This mechanism could be abused to send spam reports. Homeservers SHOULD implement rate
+  limiting and MAY require additional verification steps before acting on reports.
+
+- **Privacy**: Reporting an `m.room.member` event includes the profile information set by the user
+  in that room. Servers SHOULD handle this data appropriately and respect user privacy.
+
+## Potential Issues
+
+- **Lack of Mutual Rooms**: If the reporting user and the reported user do not share any rooms, the
+  client cannot report the profile using this method.
+
+- **Profile Changes**: If the user changes their profile after the report is made, the reported
+  `m.room.member` event may no longer reflect the current profile. Servers may need to consider
+  this when handling reports.
+
+- **Reporting Outside Rooms**: There is currently no method to report a user/profile without a room,
+  so clients that allow viewing a user profile elsewhere (e.g. links to users outside a room, or
+  while searching the user directory) may require a non-room-based endpoint for these scenarios.
+
+## Alternatives
+
+- **Reporting by User ID**: Previous versions of this MSC proposed allowing reports to specify a
+  user ID directly. However, reusing the existing event reporting mechanism simplifies
+  implementation and leverages existing APIs.
+
+- **New Federation Endpoint**: Define a new federation endpoint specifically for reporting user
+  profiles. This adds complexity and is unnecessary if the existing mechanisms can be used, however
+  some clients may offer the ability to view user profiles outside of a room context and therefore
+  require a reporting method not tied to a specific room event. This could be facilitated with a
+  variant of [MSC4151](https://github.com/matrix-org/matrix-spec-proposals/pull/4151) that adds an
+  endpoint like `/_matrix/client/v3/profile/{userId}/report` to report a user profile specifically
+  to the homeserver without optionally notifying room admins.
+
+## Dependencies
+
+- [MSC3843](https://github.com/matrix-org/matrix-spec-proposals/pull/3843): Reporting content over
+  federation