matrix-spec/content/rooms/fragments/v8-auth-rules.md

Events must be signed by the server denoted by the sender property.

The types of state events that affect authorization are:

• m.room.create
• m.room.member
• m.room.join_rules
• m.room.power_levels
• m.room.third_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 %}}

{{% boxes/note %}} m.room.redaction events are subject to auth rules in the same way as any other event. In practice, that means they will normally be allowed by the auth rules, unless the m.room.power_levels event sets a power level requirement for m.room.redaction events via the events or events_default properties. In particular, the redact level is not considered by the auth rules.

The ability to send a redaction event does not mean that the redaction itself should be performed. Receiving servers must perform additional checks, as described in the Handling Redactions section. {{% /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 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, 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.member:
   1. If there is no state_key property, or no membership property in content, reject.
   2. {{% added-in v=8 %}} If content has a join_authorised_via_users_server property:
      1. If the event is not validly signed by the homeserver of the user ID denoted by the key, reject.
   3. 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 or knock then allow if membership state is invite or join.
      5. {{% added-in v=8 %}} If the join_rule is restricted:
         1. If membership state is join or invite, allow.
         2. If the join_authorised_via_users_server key in content is not a user with sufficient permission to invite other users, reject.
         3. Otherwise, allow.
      6. If the join_rule is public, allow.
      7. Otherwise, reject.
   4. 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.
   5. If membership is leave:
      1. If the sender matches state_key, allow if and only if that user's current membership state is invite, join, or knock.
      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.
   6. 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.
   7. If membership is knock:
      1. If the join_rule is anything other than knock, reject.
      2. If sender does not match state_key, reject.
      3. If the sender's current membership is not ban, invite, or join, allow.
      4. Otherwise, reject.
   8. Otherwise, the membership is unknown. Reject.
5. If the sender's current membership state is not join, reject.
6. 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.
7. If the event type's required power level is greater than the sender's power level, reject.
8. If the event has a state_key that starts with an @ and does not match the sender, reject.
9. If type is m.room.power_levels:
   1. If the users property in content is not an object 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 properties 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 changed in, or removed from, the events or notifications properties:
      1. If the current value is greater than the sender's current power level, reject.
   5. For each entry being added to, or changed in the events or notifications properties:
      1. If the new value is greater than the sender's current power level, reject.
   6. For each entry being changed in, or removed from, the users property, other than the sender's own entry:
      1. If the current value is greater than or equal to the sender's current power level, reject.
   7. For each entry being added to, or changed in, the users property:
      1. If the new value is greater than the sender's current power level, reject.
   8. Otherwise, allow.
10. 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 %}}