MSC4170: 403 error responses for profile APIs

proposals/4170-profile-403.md

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+# MSC4170: 403 error responses for profile APIs
+
+Matrix currently defines the following [client-server APIs] for profile look-ups:
+
+- [`GET /_matrix/client/v3/profile/{userId}`]
+- [`GET /_matrix/client/v3/profile/{userId}/avatar_url`]
+- [`GET /_matrix/client/v3/profile/{userId}/displayname`]
+
+These endpoints also support look-up over federation via the accompanying
+[server-server API]:
+
+- [`GET /_matrix/federation/v1/query/profile`]
+
+Each of these endpoints has a documented 404 response for the case that no profile
+information is available.
+
+> 404 	There is no profile information for this user or this user does not exist.
+>
+> 404 	There is no avatar URL for this user or this user does not exist.
+>
+> 404 	There is no display name for this user or this user does not exist.
+>
+> 404 	The user does not exist or does not have a profile.
+
+However, `GET /_matrix/client/v3/profile/{userId}` additionally reserves a 403
+status code that is not available on the other endpoints and can be used to deny
+profile look-ups.
+
+> 403 	The server is unwilling to disclose whether the user exists and/or has profile information.
+
+Unfortunately, the concrete semantics of when to respond with 403 are not fully
+spelled out in the spec and understanding prior proposals' intention requires some
+archeology.
+
+In [2017], the user directory API, [`POST /_matrix/client/v3/user_directory/search`],
+which closely relates to the profile APIs, was introduced into the spec. Since its
+inception, it contained the requirement for servers to consider (at least) users from
+shared and public rooms in the search.
+
+Later, [MSC1301] proposed a 403 `M_USER_NOT_PUBLIC` response on all four profile
+endpoints to optionally disallow profile look-up for users that the requester does
+not share a room with. This MSC was never accepted, but in 2019
+was partially implemented by Synapse[^1]: it was only implemented for the client-server
+endpoints, and an error code of `M_FORBIDDEN` was used rather than `M_USER_NOT_PUBLIC`.
+
+In 2021, Synapse implemented[^2] a switchable feature to disable profile look-up
+over federation via a 403 `M_FORBIDDEN` response. [MSC3550] picked up on this
+feature and introduced this response type in the spec though only on the
+`GET /_matrix/client/v3/profile/{userId}` endpoint in the client-server API.
+
+The current proposal aims to restore consistency among the profile endpoints
+by standardizing their 403 error response format and behaviour.
+
+
+## Proposal
+
+For the endpoints in the client-server API
+
+- [`GET /_matrix/client/v3/profile/{userId}`]
+- [`GET /_matrix/client/v3/profile/{userId}/avatar_url`]
+- [`GET /_matrix/client/v3/profile/{userId}/displayname`]
+
+homeservers MUST at a minimum allow profile look-up for users that either share a room
+with the requester or reside in a public room known to the homeserver (i.e, the same
+requirements as [`POST /_matrix/client/v3/user_directory/search`])[^3]. In all other
+cases, homeservers MAY deny profile look-up by responding with 403 `M_FORBIDDEN`.
+
+If a remote user is queried through the client-server endpoints and the query is not
+denied per the preceding paragraph, homeservers SHOULD query the remote server for the
+user's profile information.
+
+Homeservers MAY deny profile look-up over federation by responding with 403 `M_FORBIDDEN`
+to [`GET /_matrix/federation/v1/query/profile`]. To be clear: there is no requirement to return
+profiles of users in public or shared rooms over the federation API.
+
+Whenever profile look-up is disabled on any of the four endpoints, the server's
+response MUST be 403 `M_FORBIDDEN` regardless of whether the user exists or not.
+
+
+## Potential issues
+
+Synapse already complies with this proposal in its default configuration. However,
+its `limit_profile_requests_to_users_who_share_rooms` config setting is only partially
+compatible with this proposal because it disallows profile look-up for users in public
+rooms that the requester does not share a room with. This inconsistency would need to
+be fixed if this proposal is accepted into the spec.
+
+
+## Alternatives
+
+None.
+
+
+## Security considerations
+
+None.
+
+
+## Unstable prefix
+
+None.
+
+
+## Dependencies
+
+None.
+
+
+[^1]: https://github.com/element-hq/synapse/commit/c0e0740bef0db661abce352afaf6c958e276f11d
+[^2]: https://github.com/matrix-org/synapse/pull/9203/files#diff-2f70c35b9dd342bfdaaed445847e0ccabbad63aa9a208d80d38fb248cbf57602L311
+[^3]: As stated in https://github.com/matrix-org/matrix-spec/issues/633, the spec currently
+      doesn't clearly define what a public room is. This proposal does not aim to solve this
+      problem and instead only requires that the user directory and profile APIs use the same
+      definition.
+
+[`GET /_matrix/client/v3/profile/{userId}`]: https://spec.matrix.org/v1.11/client-server-api/#get_matrixclientv3profileuserid
+[`GET /_matrix/client/v3/profile/{userId}/avatar_url`]: https://spec.matrix.org/v1.11/client-server-api/#get_matrixclientv3profileuseridavatar_url
+[`GET /_matrix/client/v3/profile/{userId}/displayname`]: https://spec.matrix.org/v1.11/client-server-api/#get_matrixclientv3profileuseriddisplayname
+[`GET /_matrix/federation/v1/query/profile`]: https://spec.matrix.org/v1.11/server-server-api/#get_matrixfederationv1queryprofile
+[`POST /_matrix/client/v3/user_directory/search`]: https://spec.matrix.org/v1.11/client-server-api/#post_matrixclientv3user_directorysearch
+[2017]: https://github.com/matrix-org/matrix-spec-proposals/pull/1096/files#diff-332ce28a7277b9375050644632f99c0e606acb751adc54c64c5faabf981ac7edR35
+[MSC1301]: https://github.com/matrix-org/matrix-spec-proposals/issues/1301
+[MSC3550]: https://github.com/matrix-org/matrix-spec-proposals/pull/3550
+[client-server APIs]: https://spec.matrix.org/v1.11/client-server-api/#profiles
+[server-server API]: https://spec.matrix.org/v1.11/server-server-api/#get_matrixfederationv1queryprofile