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matrix-spec/content/rooms/fragments/v1-auth-rules.md

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The types of state events that affect authorization are:
- [`m.room.create`](/client-server-api#mroomcreate)
- [`m.room.member`](/client-server-api#mroommember)
- [`m.room.join_rules`](/client-server-api#mroom)
- [`m.room.power_levels`](/client-server-api#mroompower_levels)
- [`m.room.third_party_invite`](/client-server-api#mroomthird_party_invite)
{{% boxes/note %}}
Power levels are inferred from defaults when not explicitly supplied.
For example, mentions of the `sender`'s power level can also refer to
the default power level for users in the room.
{{% /boxes/note %}}
The rules are as follows:
1. If type is `m.room.create`:
1. If it has any `prev_events`, reject.
2. If the domain of the `room_id` does not match the domain of the
`sender`, reject.
3. If `content.room_version` is present and is not a recognised
version, reject.
4. If `content` has no `creator` property, reject.
5. Otherwise, allow.
2. Considering the event's `auth_events`:
1. If there are duplicate entries for a given `type` and `state_key` pair,
reject.
2. If there are entries whose `type` and `state_key` don't match those
specified by the [auth events
selection](/server-server-api#auth-events-selection)
algorithm described in the server specification, reject.
3. If there are entries which were themselves rejected under the [checks
performed on receipt of a
PDU](server-server-api/#checks-performed-on-receipt-of-a-pdu), reject.
4. If there is no `m.room.create` event among the entries, reject.
3. If the `content` of the `m.room.create` event in the room state has the
property `m.federate` set to `false`, and the `sender` domain of the event
does not match the `sender` domain of the create event, reject.
4. If type is `m.room.aliases`:
1. If event has no `state_key`, reject.
2. If sender's domain doesn't matches `state_key`, reject.
3. Otherwise, allow.
5. If type is `m.room.member`:
1. If there is no `state_key` property, or no `membership` property in
`content`, reject.
2. If `membership` is `join`:
1. If the only previous event is an `m.room.create` and the
`state_key` is the creator, allow.
2. If the `sender` does not match `state_key`, reject.
3. If the `sender` is banned, reject.
4. If the `join_rule` is `invite` then allow if membership
state is `invite` or `join`.
5. If the `join_rule` is `public`, allow.
6. Otherwise, reject.
3. If `membership` is `invite`:
1. If `content` has a `third_party_invite` property:
1. If *target user* is banned, reject.
2. If `content.third_party_invite` does not have a `signed`
property, reject.
3. If `signed` does not have `mxid` and `token` properties,
reject.
4. If `mxid` does not match `state_key`, reject.
5. If there is no `m.room.third_party_invite` event in the
current room state with `state_key` matching `token`,
reject.
6. If `sender` does not match `sender` of the
`m.room.third_party_invite`, reject.
7. If any signature in `signed` matches any public key in
the `m.room.third_party_invite` event, allow. The public
keys are in `content` of `m.room.third_party_invite` as:
1. A single public key in the `public_key` property.
2. A list of public keys in the `public_keys` property.
8. Otherwise, reject.
2. If the `sender`'s current membership state is not `join`,
reject.
3. If *target user*'s current membership state is `join` or
`ban`, reject.
4. If the `sender`'s power level is greater than or equal to
the *invite level*, allow.
5. Otherwise, reject.
4. If `membership` is `leave`:
1. If the `sender` matches `state_key`, allow if and only if
that user's current membership state is `invite` or `join`.
2. If the `sender`'s current membership state is not `join`,
reject.
3. If the *target user*'s current membership state is `ban`,
and the `sender`'s power level is less than the *ban level*,
reject.
4. If the `sender`'s power level is greater than or equal to
the *kick level*, and the *target user*'s power level is
less than the `sender`'s power level, allow.
5. Otherwise, reject.
5. If `membership` is `ban`:
1. If the `sender`'s current membership state is not `join`,
reject.
2. If the `sender`'s power level is greater than or equal to
the *ban level*, and the *target user*'s power level is less
than the `sender`'s power level, allow.
3. Otherwise, reject.
6. Otherwise, the membership is unknown. Reject.
6. If the `sender`'s current membership state is not `join`, reject.
7. If type is `m.room.third_party_invite`:
1. Allow if and only if `sender`'s current power level is greater
than or equal to the *invite level*.
8. If the event type's *required power level* is greater than the
`sender`'s power level, reject.
9. If the event has a `state_key` that starts with an `@` and does not
match the `sender`, reject.
10. If type is `m.room.power_levels`:
1. If `users` key in `content` is not a dictionary with keys that
are valid user IDs with values that are integers (or a string
that is an integer), reject.
2. If there is no previous `m.room.power_levels` event in the room,
allow.
3. For the keys `users_default`, `events_default`, `state_default`,
`ban`, `redact`, `kick`, `invite` check if they were added,
changed or removed. For each found alteration:
1. If the current value is higher than the `sender`'s current
power level, reject.
2. If the new value is higher than the `sender`'s current power
level, reject.
4. For each entry being added, changed or removed in both the
`events` and `users` keys:
1. If the current value is higher than the `sender`'s current
power level, reject.
2. If the new value is higher than the `sender`'s current power
level, reject.
5. For each entry being changed under the `users` key, other than
the `sender`'s own entry:
1. If the current value is equal to the `sender`'s current
power level, reject.
6. Otherwise, allow.
11. If type is `m.room.redaction`:
1. If the `sender`'s power level is greater than or equal to the
*redact level*, allow.
2. If the domain of the `event_id` of the event being redacted is
the same as the domain of the `event_id` of the
`m.room.redaction`, allow.
3. Otherwise, reject.
12. Otherwise, allow.
{{% boxes/note %}}
Some consequences of these rules:
- Unless you are a member of the room, the only permitted operations
(apart from the initial create/join) are: joining a public room;
accepting or rejecting an invitation to a room.
- To unban somebody, you must have power level greater than or equal
to both the kick *and* ban levels, *and* greater than the target
user's power level.
{{% /boxes/note %}}