MSC4323: User suspension & locking endpoints (#4323)

Author: nexy7574

proposals/4323-agnostic-suspend-and-lock.md

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+# MSC4323: User suspension & locking endpoints
+
+Currently the specification outlines error codes for suspended and locked users,
+even going as far as to suggest which endpoints can and cannot be executed by suspended users.
+However, it does not currently define an endpoint which server administrators can call to suspend
+and unsuspend users on their server.
+As a result, moderation tooling such as Draupnir and Meowlnir have to implement
+implementation-specific calls, which is not sustainable as more implementations other than Synapse
+integrate this feature.
+
+This proposal will outline new endpoints that will allow server administrators to
+suspend, unsuspend, lock, and unlock given users.
+
+## Proposal
+
+> [!IMPORTANT]
+> What defines a "server administrator" is left up to the implementation itself as most already have
+> their own systems for defining administrators (e.g. Synapse has a database flag, Conduit has room
+> membership) which rarely has a reason to be exposed outside of their respective management
+> interfaces.
+
+Complementing [section 10.22 (Server Administration)][p1] of the client-to-server specification,
+four new endpoints are introduced:
+
+- `GET /_matrix/client/v1/admin/suspend/{userId}`
+- `PUT /_matrix/client/v1/admin/suspend/{userId}`
+- `GET /_matrix/client/v1/admin/lock/{userId}`
+- `PUT /_matrix/client/v1/admin/lock/{userId}`
+
+These new endpoints are similar to [`GET /_matrix/client/v3/admin/whois/{userId}`][p2] in that they
+are clearly defined as administration endpoints, however are restricted to only permitting
+execution on local server users.
+
+### New endpoint definitions
+
+The response body of both the `GET` and `PUT` endpoints, as well as the request body of the
+`PUT` endpoints, are defined below. Custom properties may be used provided they utilise
+[proper namespacing][p3] in their fields.
+
+**Suspend**:
+
+A single key, `suspended`, with a boolean indicating if the target account is suspended:
+
+```json
+{"suspended": true}
+```
+
+**Lock**:
+
+A single key, `locked`, with a boolean indicating if the target account is locked:
+
+```json
+{"locked": false}
+```
+
+### New capability definition
+
+The server should advertise that these new endpoints are available for the authenticated user
+to use by including the following new capability:
+
+```json5
+{
+  "capabilities": {
+    "m.account_moderation": {
+      "suspend": true, // or false if unavailable/unimplemented
+      "lock": true // or false if unavailable/unimplemented
+    }
+  }
+}
+```
+
+This allows clients to determine whether they are able to suspend/lock users on this homeserver,
+allowing them to do things like dynamically show or hide a "suspend user" button, etc.
+
+The capability should not be advertised at all if both `suspend` and `lock` would be `false`.
+Omitting a key is equivalent to it being `false`.
+
+### Errors and restrictions
+
+Sending a request to the respective endpoints returns:
+
+- `400 / M_INVALID_PARAM`: The user ID does not belong to the local server.
+- `403 / M_FORBIDDEN`: The requesting user is not a server administrator, is trying to suspend/lock
+  their own account, or the target user is another administrator.
+- `404 / M_NOT_FOUND`: The user ID is not found, or is deactivated.
+
+In order to prevent user enumeration, implementations have to ensure that authorization is checked
+prior to trying to do account lookups.
+As this endpoint requires authentication with an administrator account, guest access is not
+permitted. Additionally, no rate-limiting is imposed.
+
+[p1]: https://spec.matrix.org/v1.15/client-server-api/#server-administration
+[p2]: https://spec.matrix.org/v1.15/client-server-api/#get_matrixclientv3adminwhoisuserid
+[p3]: https://spec.matrix.org/v1.15/appendices/#common-namespaced-identifier-grammar
+
+## Potential issues
+
+This proposal does not outline any metadata fields for management, such as action reasons,
+temporary actions, authors, etc, which implementation-specific methods may currently have.
+This is for the sake of brevity and simplicity, and may be expanded upon by a future proposal.
+
+This proposal is also written under the assumption that all server administrators are equal, and
+cannot be suspended or locked. If a server permits such actions against a privileged user, without
+stripping their privileges, conflicting behaviours may be encountered.
+
+## Alternatives
+
+A full "user-info" endpoint has been suggested, which would include more information about a user's
+account that could interest server administrators. This proposal focuses solely on providing
+tooling with the capability to suspend and lock without needing to maintain several
+implementation-specific versions of their code, and adding a full-fleged user-info endpoint is
+almost entirely out of scope.
+
+## Security considerations
+
+Adding these new endpoints may provide a path to circumvent restrictions previously imposed on
+implementation-specific versions of the suspend and lock endpoints (such as them being blocked off
+at the reverse proxy) - server developers will likely want to ensure that their users are made
+appropriate aware of this (via release notes or some other similar high-visibility channel) so that
+they can apply those same restrictions to these new endpoints if necessary.
+
+These endpoints also increase the compromise value of an administrator account, as an attacker who
+breaches one of these accounts will be able to suspend or lock other users on the server. While
+the attacker would not be able to lock out other administrators, a single-administrator homeserver
+may be vulnerable to a temporary takeover if the sole administrator account is breached and
+the deployment cannot be secured & have the change reverted quickly.
+It may possibly also allow an attacker to disable other room-level moderation tooling, such as
+moderation bots, assuming those tools are not also server administrators.
+
+## Unstable prefix
+
+| Stable | Unstable |
+| ------ | -------- |
+| `/_matrix/client/v1/admin/...` | `/_matrix/client/unstable/uk.timedout.msc4323/admin/...` |
+| `m.account_moderation` | `uk.timedout.msc4323` |
+
+`locked` and `suspended` in the request/response bodies do not require an unstable prefix
+as the entire body is new.
+
+Servers should advertise `"uk.timedout.msc4323":true` in their `/versions` response while this
+proposal is unstable in order to advertise support for this new feature.
+It is left as an implementation detail whether to require authentication to view this version flag
+or not, as the capabilities endpoint requires authentication regardless.
+
+## Dependencies
+
+None