MSC3757: Restricting who can overwrite a state event

proposals/3757-restricting-who-can-overwrite-a-state-event.md

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+# MSC3757: Restricting who can overwrite a state event.
+
+## Problem
+
+Currently there are three main ways to limit who can overwrite a state event:
+
+ * If a user's PL is greater than the `m.room.power_levels` `state_default` field
+ * If a user's PL is greater than the `m.room.power_levels` `events` field for that event type
+ * If a state event has a state key which begins with an `@`, then the sender's mxid must match that state key.
+
+This is problematic if an unprivileged user needs to publish multiple state
+events of the same type in a room, but would like to set access control so
+that only they can subsequently update the event. An example of this is if a
+user wishes to publish multiple live location share beacons as per [MSC3489](https://github.com/matrix-org/matrix-spec-proposals/pull/3489)
+and [MSC3672](https://github.com/matrix-org/matrix-spec-proposals/pull/3672), for instance one per device.  They will typically not want
+other users in the room to be able to overwrite the state event,
+so we need a mechanism to prevent other peers from doing so.
+
+[MSC3489](https://github.com/matrix-org/matrix-spec-proposals/pull/3489) originally proposed that the event type could be made variable,
+appending an ID to each separately posted event so that each one could
+separately be locked to the same mxid in the state_key.  However, this is
+problematic because you can't proactively refer to these event types in the
+`events` field of the `m.room.power_levels` event to allow users to post
+them - and they also are awkward for some client implementations to
+manipulate.
+
+## Proposal
+
+Therefore, we need a different way to state that a given state event may only
+be written by its owner. **We propose that if a state event's state_key *starts with* a matrix ID (followed by an underscore), only the sender with that matrix ID (or higher PL users) can set the state event.**  This is an extension of the current behaviour where state events may be overwritten only if the sender's mxid *exactly equals* the state_key.
+
+We also allow users with higher PL than the original sender to overwrite state 
+events even if their mxid doesn't match the event's state_key. This fixes an abuse
+vector where an unprivileged user can immutably graffiti the state within a room
+by sending state events whose state_key is their matrix ID.
+
+Practically speaking, this means modifying the [authorization rules](https://spec.matrix.org/v1.2/rooms/v9/#authorization-rules) such that rule 8:
+
+> If the event has a `state_key` that starts with an `@` and does not match the `sender`, reject.
+
+becomes:
+
+> If the event has a `state_key` that starts with an `@`, and the substring before the first `_` that follows the first `:` (or end of string) does not match the `sender`, reject - unless the sender's powerlevel is greater than the event type's *required power level*.
+
+For example, to post a live location sharing beacon from [MSC3672](https://github.com/matrix-org/matrix-spec-proposals/pull/3672):
+
+```json=
+{
+    "type": "m.beacon_info",
+    "state_key": "@stefan:matrix.org_assetid1", // Ensures only the sender or higher PL users can update
+    "content": {
+        "m.beacon_info": {
+            "description": "Stefan's live location",
+            "timeout": 600000,
+            "live": true
+        },
+        "m.ts": 1436829458432,
+        "m.asset": {
+            "type": "m.self"
+        }
+    }
+}
+```
+
+Since `:` is not permitted in the localpart and `_` is not permitted in the domain part of an mxid (see [Historical User IDs](https://spec.matrix.org/v1.2/appendices/#historical-user-ids)), it is not possible to craft an mxid that matches the beginning of a state key constructed for another user's mxid, so state keys restricted to one owner can never be overwritten by another user.
+
+## Potential issues
+
+None yet.
+
+## Alternatives
+
+As originally proposed in [MSC3489](https://github.com/matrix-org/matrix-spec-proposals/pull/3489) and [MSC3672](https://github.com/matrix-org/matrix-spec-proposals/pull/3672), we can require
+the use of a state key equal to the sender's mxid, but this means we can only
+have one such event of each type, so those MSCs proposed using different types
+for each unique event.
+
+An earlier draft of this MSC proposed putting a flag on the contents of the 
+event (outside of the E2EE payload) called `m.peer_unwritable: true` to indicate
+if other users were prohibited from overwriting the event or not.  However, this
+unravelled when it became clear that there wasn't a good value for the state_key,
+which needs to be unique and not subject to races from other malicious users.
+By scoping who can set the state_key to be the mxid of the sender, this problem
+goes away.
+
+[MSC3760](https://github.com/matrix-org/matrix-spec-proposals/pull/3760)
+proposes to include a dedicated `state_subkey` as the third component of what
+makes a state event unique.
+
+## Security considerations
+
+This change requires a new room version, so will not affect old events.
+
+As this changes auth rules, we should think carefully about whether could
+introduce an attack on state resolution. For instance: if a user had higher
+PL at some point in the past, will they be able to abuse somehow this to
+overwrite the state event, despite not being its owner?
+
+When using a state_key prefix to restrict who can write the event, we have
+deliberately chosen an underscore to terminate the mxid prefix, as underscores
+are not allowed in domain names.  A pure prefix match will **not** be sufficient,
+as `@matthew:matrix.org` will match a state_key of form `@matthew:matrix.org.evil.com:id1`.
+
+This changes auth rules in a backwards incompatible way, which will break any
+use cases which assume that higher PL users cannot overwrite state events whose 
+state_key is a different mxid.  This is considered a feature rather than a bug,
+fixing an abuse vector where unprivileged users could send arbitrary state events
+which could never be overwritten.
+
+## Unstable prefix
+
+While this MSC is not considered stable, implementations should apply the behaviours of this MSC on top of room version 9 as `org.matrix.msc3757`.
+
+## Dependencies
+
+None