MSC3905: Application services should only be interested in local users

proposals/3905-appservice-only-interested-in-local-users.md

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+# MSC3905: Application services should only be interested in local users
+
+Application services receive events that they are "interested" in.
+
+The current language in the spec describes it like this:
+
+> An application service is said to be "interested" in a given event if one of the IDs
+> in the event match the regular expression provided by the application service
+> [registration]. [...] the application service is said to be interested in a given
+> event if one of the application service's namespaced users is the target of the event,
+> or is a joined member of the room where the event occurred.
+
+This language is ambiguous around which users it should match against though. Should it
+be all members of the room including remote users from other homeservers or just the
+local users where the application service lives? It could be assumed either way and
+naively applied to all members of the room which is still valid with the current
+language and even what Synapse does.
+
+But matching against remote users is merely a footgun because an application service may
+assume that it'll receive all events sent by that remote user, even though it will only
+receive events in rooms that are shared with a local user. This leaves us with a
+behavior mismatch between remote and local users.
+
+
+## Proposal
+
+Therefore the proposal is that the `users` namespace regex should only be applied
+against local users of the homeserver.
+
+A basic implementation of this would look like:
+
+```js
+const isLocalUser = sender.endsWith(":" + homeserver.domain);
+const isInterestingUser = isLocalUser && sender.matches(regex);
+```
+
+```js
+const localRoomMembers = getLocalRoomMembers(roomId);
+const interestingUsers = localRoomMembers.filter((localRoomMember) => localRoomMember.matches(regex));
+```
+
+---
+
+To avoid confusion, please note that the `rooms` and `aliases` namesapces are not
+affected. You can still match whatever rooms and aliases to listen to all events
+that occur in them.
+
+
+## Potential issues
+
+There are use cases like moderation where an application service wants to hear all
+messages from remote users in rooms but these are are also covered by the `rooms`
+namespace where all events in a matched room are considered "interesting".
+
+
+
+## Alternatives
+
+The alternative is clarify that the `users` namespace should be matched against all
+users (local and remote). This still leaves us with the behavior-difference footgun.
+
+
+
+## Security considerations
+
+Since we're reducing the surface area, there doesn't seem to be any additional security
+considerations introduced.
+
+With this MSC, an application service will be receiving less events than before.
+
+
+## Historical context
+
+According to @turt2live (SCT member), "the spec intended to [originally] say the
+namespace can make an appservice interested in remote users, though there's obviously no
+ability for the server to call `/user` on remote users (it's not like the appservice can
+create them)." (https://github.com/matrix-org/synapse/pull/13958#discussion_r988369446)
+
+This intention goes back further than `r0` (or `v1.0` in marketing versions speak) but
+this history is lost to time since there isn't really anything concrete to point to
+beyond the original spec
+[issue](https://github.com/matrix-org/matrix-spec-proposals/issues/1307) and
+[PR](https://github.com/matrix-org/matrix-spec-proposals/pull/1533) which don't mention
+these details.
+
+Since we're unable to come up with any valid use cases nowadays, it's unclear to
+outsiders from that time whether the original intention is actually true. In any case,
+we're clarifying it here and making an MSC to change it explicitly.
+
+
+## Unstable prefix
+
+While this MSC is not considered stable, appservices can opt-in to this behaviour in
+either of two ways:
+
+ 1. Specifying a regex which includes the local homeserver's domain in the regex (eg:
+    `@_irc.*:example.org`)
+ 1. Adding `"org.matrix.msc3905": true` to the root level of their registration file