matrix-spec/content/rooms/fragments/v3-auth-rules.md

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{{% added-in this=true %}} In room versions 1 and 2, events need a
signature from the domain of the `event_id` in order to be considered
valid. This room version does not include an `event_id` over federation
in the same respect, so does not need a signature from that server.
The event must still be signed by the server denoted by the `sender`,
however.

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 complete list of rules, as of room version 3, is as follows:

1.  If type is `m.room.create`:
    1.  If it has any previous 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` field, reject.
    5.  Otherwise, allow.
2.  Reject if event has `auth_events` that:
    1.  have duplicate entries for a given `type` and `state_key` pair
    2.  have 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.
3.  If event does not have a `m.room.create` in its `auth_events`,
    reject.
4.  If the create event content has the field `m.federate` set to `false`
    and the sender domain of the event does not match the sender domain of
    the create event, reject.
5.  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.
6.  If type is `m.room.member`:
    1.  If no `state_key` key or `membership` key 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 `third_party_invite` key:
            1.  If *target user* is banned, reject.
            2.  If `content.third_party_invite` does not have a `signed`
                key, reject.
            3.  If `signed` does not have `mxid` and `token` keys,
                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` field.
                2.  A list of public keys in the `public_keys` field.
            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.
7.  If the `sender`'s current membership state is not `join`, reject.
8.  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*.
9.  If the event type's *required power level* is greater than the
    `sender`'s power level, reject.
10.  If the event has a `state_key` that starts with an `@` and does not
    match the `sender`, reject.
11. 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.
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 %}}