MSC2209: Alter auth rules to check notifications in m.room.power_levels

proposals/2209-auth-rules-other-keys-in-m.room.power.levels.md

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+# Update auth rules to check notifications key in m.room.power_levels
+
+## Introduction
+
+The key `notifications` was added to the `m.room.power_levels` event after the
+finalisation of the auth rules specified in room version 1. This leads to the
+fact, that this dictionary is not subject to the same validation as other
+dictionaries in the event, such as `users` or `events`. This especially means
+that Alice is able to alter any entry within the dictionary including ones,
+that are above her own power level, which is inconsistent with the behaviour
+for the other two dictionaries.
+
+[m.room.power_levels](https://matrix.org/docs/spec/client_server/r0.5.0#m-room-power-levels)
+[room version 1](https://matrix.org/docs/spec/rooms/v1)
+
+## Proposal
+
+The auth rules regarding `m.room.power_levels` have been established in room
+version 1. The general idea here was that creators of a new `m.room.power_levels`
+event are only able to make changes to something that is equal to or lower than
+their own power level.
+So, assuming a room with Alice (50), Bob(100), `m.room.power_levels` at 50,
+`m.room.name` at 75 and `m.room.topic` at 25 would mean the following:
+
+* Alice CAN alter `m.room.topic` to any power level up to her own, in this case 50
+* Alice is NOT able to alter `m.room.name` since the current value is higher than
+her own (75 > 50)
+* Alice is NOT able to alter the power level of Bob, since his current value is
+higher than her own (100 > 50)
+* Alice is free to set the level for anything that has not been defined such as
+`org.alice.message` up to a maximum of 50
+
+Later on the key `notifications` was added to the `m.room.power_levels` event.
+It contains a mapping of notification keys to a power level, that is required
+for a user to trigger the specific notification. The most famous notification
+type is the `@room` notification.
+
+Going back to the original example because this key was added later on, the auth
+rules make no mention of it, which enables the following behaviour. *It is assumed
+that `@room` is at 75*
+
+* Alice can add any key to the dictionary and set the value to any value she wants,
+including ones higher than her own.
+* Alice can alter the value for `@room` to any value she wants, including ones that
+are higher than her own, even though her own value is lower.
+
+The proposed solution is to alter the auth rules and add the `notifications` dictionary
+to the same rules that handle `events` and `users`.
+
+So the rule [10.d](https://matrix.org/docs/spec/rooms/v1.html) of the auth rules in
+room version 1 would be updated in a new room version to:
+
+
+For each entry being added, changed or removed in events, users __and notifications__
+keys:
+
+
+## Tradeoffs
+
+The proposed solution would be a breaking change with current room versions and
+the alternative would be to leave the `notifications` key without any checks.
+
+## Security considerations
+
+This is likely to improve security because it prevents malicious users that were
+only given the right to emit `m.room.power_levels` so that they could alter a very
+specific key, such as `invite`, from altering the rules established for triggering
+notifications.
+
+## Conclusion
+
+The current spec conform behaviour regarding `notifications` is inconsistent with
+behaviour shown by the other dictionaries and room administrators are very likely
+expecting the `notifications` to work the same as them. The required change is minimal
+is and also in line with the general spirit of the auth rules regarding the
+`m.room.power_levels` event. A new room version is, however, required. This can be
+done with other pending changes.