MSC4263: Preventing MXID enumeration via key queries

Author: Johennes

proposals/4263-prevent-mxid-enumeration-via-key-queries.md

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+# MSC4263: Preventing MXID enumeration via key queries
+
+The client-server API allows searching users and querying their profiles via
+[`/_matrix/client/v3/user_directory/search`] and
+[`/_matrix/client/v3/profile/{userId}`], respectively. Both of these APIs can
+among others be abused to enumerate MXIDs. Servers are, therefore, only required
+to return results for users who either share a room with the requesting user or
+are a member of a public room known to the server. In all other cases,
+homeservers can respond with 403 or simply omit the user from the response.
+
+Similarly, on the server-server API, servers are generally permitted to deny
+requests to [`/_matrix/federation/v1/query/profile`] with 403.
+
+The [`/_matrix/client/v3/keys/query`] and
+[`/_matrix/federation/v1/user/keys/query`] endpoints have a similar problem but
+do not currently permit server admins to restrict their responses to conceal
+users.
+
+This proposal carries the behaviour of the user directory and profile APIs over
+to the key query APIs.
+
+## Proposal
+
+When processing [`/_matrix/client/v3/keys/query`] requests, homeservers MUST at
+a minimum consider users who share a room with the requesting user or are a
+member of a public room. This is regardless of the concrete membership value of
+the queried users in those room.
+
+In all other cases, homeservers MAY ignore the queried MXIDs and omit them from
+the response.
+
+Servers MAY deny key queries over federation by responding with 403 and an error
+code of `M_FORBIDDEN` on [`/_matrix/federation/v1/user/keys/query`].
+
+## Potential issues
+
+None.
+
+## Alternatives
+
+None.
+
+## Security considerations
+
+None.
+
+## Unstable prefix
+
+None.
+
+## Dependencies
+
+None.
+
+  [`/_matrix/client/v3/user_directory/search`]: https://spec.matrix.org/v1.13/client-server-api/#post_matrixclientv3user_directorysearch
+  [`/_matrix/client/v3/profile/{userId}`]: https://spec.matrix.org/v1.13/client-server-api/#get_matrixclientv3profileuserid
+  [`/_matrix/federation/v1/query/profile`]: https://spec.matrix.org/v1.13/server-server-api/#get_matrixfederationv1queryprofile
+  [`/_matrix/client/v3/keys/query`]: https://spec.matrix.org/v1.13/client-server-api/#post_matrixclientv3keysquery
+  [`/_matrix/federation/v1/user/keys/query`]: https://spec.matrix.org/v1.13/server-server-api/#post_matrixfederationv1userkeysquery