diff --git a/synapse/third_party_rules/access_rules.py b/synapse/third_party_rules/access_rules.py
new file mode 100644
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+++ b/synapse/third_party_rules/access_rules.py
@@ -0,0 +1,372 @@
+# -*- coding: utf-8 -*-
+# Copyright 2019 New Vector Ltd
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+# http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+import email.utils
+
+from twisted.internet import defer
+
+from synapse.api.constants import EventTypes
+from synapse.api.errors import SynapseError
+from synapse.config._base import ConfigError
+from synapse.types import get_domain_from_id
+
+ACCESS_RULES_TYPE = "im.vector.room.access_rules"
+ACCESS_RULE_RESTRICTED = "restricted"
+ACCESS_RULE_UNRESTRICTED = "unrestricted"
+ACCESS_RULE_DIRECT = "direct"
+
+VALID_ACCESS_RULES = (
+ ACCESS_RULE_DIRECT,
+ ACCESS_RULE_RESTRICTED,
+ ACCESS_RULE_UNRESTRICTED,
+)
+
+
+class RoomAccessRules(object):
+ """Implementation of the ThirdPartyEventRules module API that allows federation admins
+ to define custom rules for specific events and actions.
+ Implements the custom behaviour for the "im.vector.room.access_rules" state event.
+
+ Takes a config in the format:
+
+ third_party_event_rules:
+ module: third_party_rules.RoomAccessRules
+ config:
+ # List of domains (server names) that can't be invited to rooms if the
+ # "restricted" rule is set. Defaults to an empty list.
+ domains_forbidden_when_restricted: []
+
+ # Identity server to use when checking the HS an email address belongs to
+ # using the /info endpoint. Required.
+ id_server: "vector.im"
+
+ Don't forget to consider if you can invite users from your own domain.
+ """
+
+ def __init__(self, config, http_client):
+ self.http_client = http_client
+
+ self.id_server = config["id_server"]
+
+ self.domains_forbidden_when_restricted = config.get(
+ "domains_forbidden_when_restricted", [],
+ )
+
+ @staticmethod
+ def parse_config(config):
+ if "id_server" in config:
+ return config
+ else:
+ raise ConfigError("No IS for event rules TchapEventRules")
+
+ def on_create_room(self, requester, config, is_requester_admin):
+ """Implements synapse.events.ThirdPartyEventRules.on_create_room
+
+ Checks if a im.vector.room.access_rules event is being set during room creation.
+ If yes, make sure the event is correct. Otherwise, append an event with the
+ default rule to the initial state.
+ """
+ is_direct = config.get("is_direct")
+ rules_in_initial_state = False
+
+ # If there's a rules event in the initial state, check if it complies with the
+ # spec for im.vector.room.access_rules and deny the request if not.
+ for event in config.get("initial_state", []):
+ if event["type"] == ACCESS_RULES_TYPE:
+ rules_in_initial_state = True
+
+ rule = event["content"].get("rule")
+
+ # Make sure the event has a valid content.
+ if rule is None:
+ raise SynapseError(400, "Invalid access rule",)
+
+ # Make sure the rule name is valid.
+ if rule not in VALID_ACCESS_RULES:
+ raise SynapseError(400, "Invalid access rule", )
+
+ # Make sure the rule is "direct" if the room is a direct chat.
+ if (
+ (is_direct and rule != ACCESS_RULE_DIRECT)
+ or (rule == ACCESS_RULE_DIRECT and not is_direct)
+ ):
+ raise SynapseError(400, "Invalid access rule",)
+
+ # If there's no rules event in the initial state, create one with the default
+ # setting.
+ if not rules_in_initial_state:
+ if is_direct:
+ default_rule = ACCESS_RULE_DIRECT
+ else:
+ default_rule = ACCESS_RULE_RESTRICTED
+
+ if not config.get("initial_state"):
+ config["initial_state"] = []
+
+ config["initial_state"].append({
+ "type": ACCESS_RULES_TYPE,
+ "state_key": "",
+ "content": {
+ "rule": default_rule,
+ }
+ })
+
+ @defer.inlineCallbacks
+ def check_threepid_can_be_invited(self, medium, address, state_events):
+ """Implements synapse.events.ThirdPartyEventRules.check_threepid_can_be_invited
+
+ Check if a threepid can be invited to the room via a 3PID invite given the current
+ rules and the threepid's address, by retrieving the HS it's mapped to from the
+ configured identity server, and checking if we can invite users from it.
+ """
+ rule = self._get_rule_from_state(state_events)
+
+ if medium != "email":
+ defer.returnValue(False)
+
+ if rule != ACCESS_RULE_RESTRICTED:
+ # Only "restricted" requires filtering 3PID invites. We don't need to do
+ # anything for "direct" here, because only "restricted" requires filtering
+ # based on the HS the address is mapped to.
+ defer.returnValue(True)
+
+ parsed_address = email.utils.parseaddr(address)[1]
+ if parsed_address != address:
+ # Avoid reproducing the security issue described here:
+ # https://matrix.org/blog/2019/04/18/security-update-sydent-1-0-2
+ # It's probably not worth it but let's just be overly safe here.
+ defer.returnValue(False)
+
+ # Get the HS this address belongs to from the identity server.
+ res = yield self.http_client.get_json(
+ "https://%s/_matrix/identity/api/v1/info" % (self.id_server,),
+ {
+ "medium": medium,
+ "address": address,
+ }
+ )
+
+ # Look for a domain that's not forbidden from being invited.
+ if not res.get("hs"):
+ defer.returnValue(False)
+ if res.get("hs") in self.domains_forbidden_when_restricted:
+ defer.returnValue(False)
+
+ defer.returnValue(True)
+
+ def check_event_allowed(self, event, state_events):
+ """Implements synapse.events.ThirdPartyEventRules.check_event_allowed
+
+ Checks the event's type and the current rule and calls the right function to
+ determine whether the event can be allowed.
+ """
+ # Special-case the access rules event.
+ if event.type == ACCESS_RULES_TYPE:
+ return self._on_rules_change(event, state_events)
+
+ rule = self._get_rule_from_state(state_events)
+
+ if rule == ACCESS_RULE_RESTRICTED:
+ ret = self._apply_restricted(event)
+ elif rule == ACCESS_RULE_UNRESTRICTED:
+ ret = self._apply_unrestricted()
+ elif rule == ACCESS_RULE_DIRECT:
+ ret = self._apply_direct(event, state_events)
+ else:
+ # We currently apply the default (restricted) if we don't know the rule, we
+ # might want to change that in the future.
+ ret = self._apply_restricted(event)
+
+ return ret
+
+ def _on_rules_change(self, event, state_events):
+ """Implement the checks and behaviour specified on allowing or forbidding a new
+ im.vector.room.access_rules event.
+
+ Args:
+ event (synapse.events.EventBase): The event to check.
+ state_events (dict[tuple[event type, state key], EventBase]): The state of the
+ room before the event was sent.
+ Returns:
+ bool, True if the event can be allowed, False otherwise.
+ """
+ new_rule = event.content.get("rule")
+
+ # Check for invalid values.
+ if new_rule not in VALID_ACCESS_RULES:
+ return False
+
+ # Make sure we don't apply "direct" if the room has more than two members.
+ if new_rule == ACCESS_RULE_DIRECT:
+ existing_members, threepid_tokens = self._get_members_and_tokens_from_state(
+ state_events,
+ )
+
+ if len(existing_members) > 2 or len(threepid_tokens) > 1:
+ return False
+
+ prev_rules_event = state_events.get((ACCESS_RULES_TYPE, ""))
+
+ # Now that we know the new rule doesn't break the "direct" case, we can allow any
+ # new rule in rooms that had none before.
+ if prev_rules_event is None:
+ return True
+
+ prev_rule = prev_rules_event.content.get("rule")
+
+ # Currently, we can only go from "restricted" to "unrestricted".
+ if prev_rule == ACCESS_RULE_RESTRICTED and new_rule == ACCESS_RULE_UNRESTRICTED:
+ return True
+
+ return False
+
+ def _apply_restricted(self, event):
+ """Implements the checks and behaviour specified for the "restricted" rule.
+
+ Args:
+ event (synapse.events.EventBase): The event to check.
+ Returns:
+ bool, True if the event can be allowed, False otherwise.
+ """
+ # "restricted" currently means that users can only invite users if their server is
+ # included in a limited list of domains.
+ # We're not filtering on m.room.third_party_member events here because the
+ # filtering on threepids is done in check_threepid_can_be_invited.
+ if event.type != EventTypes.Member:
+ return True
+ invitee_domain = get_domain_from_id(event.state_key)
+ return invitee_domain not in self.domains_forbidden_when_restricted
+
+ def _apply_unrestricted(self):
+ """Implements the checks and behaviour specified for the "unrestricted" rule.
+
+ Returns:
+ bool, True if the event can be allowed, False otherwise.
+ """
+ # "unrestricted" currently means that every event is allowed.
+ return True
+
+ def _apply_direct(self, event, state_events):
+ """Implements the checks and behaviour specified for the "direct" rule.
+
+ Args:
+ event (synapse.events.EventBase): The event to check.
+ state_events (dict[tuple[event type, state key], EventBase]): The state of the
+ room before the event was sent.
+ Returns:
+ bool, True if the event can be allowed, False otherwise.
+ """
+ # "direct" currently means that no member is allowed apart from the two initial
+ # members the room was created for (i.e. the room's creator and their first
+ # invitee).
+ if event.type != EventTypes.Member and event.type != EventTypes.ThirdPartyInvite:
+ return True
+
+ # Get the room memberships and 3PID invite tokens from the room's state.
+ existing_members, threepid_tokens = self._get_members_and_tokens_from_state(
+ state_events,
+ )
+
+ # There should never be more than one 3PID invite in the room state: if the second
+ # original user came and left, and we're inviting them using their email address,
+ # given we know they have a Matrix account binded to the address (so they could
+ # join the first time), Synapse will successfully look it up before attempting to
+ # store an invite on the IS.
+ if len(threepid_tokens) == 1 and event.type == EventTypes.ThirdPartyInvite:
+ # If we already have a 3PID invite in flight, don't accept another one.
+ return False
+
+ if len(existing_members) == 2:
+ # If the user was within the two initial user of the room, Synapse would have
+ # looked it up successfully and thus sent a m.room.member here instead of
+ # m.room.third_party_invite.
+ if event.type == EventTypes.ThirdPartyInvite:
+ return False
+
+ # We can only have m.room.member events here. The rule in this case is to only
+ # allow the event if its target is one of the initial two members in the room,
+ # i.e. the state key of one of the two m.room.member states in the room.
+ return event.state_key in existing_members
+
+ # We're alone in the room (and always have been) and there's one 3PID invite in
+ # flight.
+ if len(existing_members) == 1 and len(threepid_tokens) == 1:
+ # We can only have m.room.member events here. In this case, we can only allow
+ # the event if it's either a m.room.member from the joined user (we can assume
+ # that the only m.room.member event is a join otherwise we wouldn't be able to
+ # send an event to the room) or an an invite event which target is the invited
+ # user.
+ target = event.state_key
+ is_from_threepid_invite = self._is_invite_from_threepid(
+ event, threepid_tokens[0],
+ )
+ if is_from_threepid_invite or target == existing_members[0]:
+ return True
+
+ return False
+
+ return True
+
+ @staticmethod
+ def _get_rule_from_state(state_events):
+ """Extract the rule to be applied from the given set of state events.
+
+ Args:
+ state_events (dict[tuple[event type, state key], EventBase]): The set of state
+ events.
+ Returns:
+ str, the name of the rule (either "direct", "restricted" or "unrestricted")
+ """
+ access_rules = state_events.get((ACCESS_RULES_TYPE, ""))
+ if access_rules is None:
+ rule = ACCESS_RULE_RESTRICTED
+ else:
+ rule = access_rules.content.get("rule")
+ return rule
+
+ @staticmethod
+ def _get_members_and_tokens_from_state(state_events):
+ """Retrieves from a list of state events the list of users that have a
+ m.room.member event in the room, and the tokens of 3PID invites in the room.
+
+ Args:
+ state_events (dict[tuple[event type, state key], EventBase]): The set of state
+ events.
+ Returns:
+ existing_members (list[str]): List of targets of the m.room.member events in
+ the state.
+ threepid_invite_tokens (list[str]): List of tokens of the 3PID invites in the
+ state.
+ """
+ existing_members = []
+ threepid_invite_tokens = []
+ for key, event in state_events.items():
+ if key[0] == EventTypes.Member:
+ existing_members.append(event.state_key)
+ if key[0] == EventTypes.ThirdPartyInvite:
+ threepid_invite_tokens.append(event.state_key)
+
+ return existing_members, threepid_invite_tokens
+
+ @staticmethod
+ def _is_invite_from_threepid(invite, threepid_invite_token):
+ """Checks whether the given invite follows the given 3PID invite.
+
+ Args:
+ invite (EventBase): The m.room.member event with "invite" membership.
+ threepid_invite_token (str): The state key from the 3PID invite.
+ """
+ token = invite.content.get("third_party_signed", {}).get("token", "")
+ return token == threepid_invite_token
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