diff --git a/synapse/api/auth/base.py b/synapse/api/auth/base.py
new file mode 100644
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--- /dev/null
+++ b/synapse/api/auth/base.py
@@ -0,0 +1,273 @@
+# Copyright 2023 The Matrix.org Foundation.
+#
+# 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 logging
+from typing import TYPE_CHECKING, Optional, Tuple
+
+from twisted.web.server import Request
+
+from synapse import event_auth
+from synapse.api.constants import EventTypes, HistoryVisibility, Membership
+from synapse.api.errors import (
+ AuthError,
+ Codes,
+ MissingClientTokenError,
+ UnstableSpecAuthError,
+)
+from synapse.appservice import ApplicationService
+from synapse.logging.opentracing import trace
+from synapse.types import Requester
+
+if TYPE_CHECKING:
+ from synapse.server import HomeServer
+
+logger = logging.getLogger(__name__)
+
+
+class BaseAuth:
+ """Common base class for all auth implementations."""
+
+ def __init__(self, hs: "HomeServer"):
+ self.hs = hs
+ self.store = hs.get_datastores().main
+ self._storage_controllers = hs.get_storage_controllers()
+
+ async def check_user_in_room(
+ self,
+ room_id: str,
+ requester: Requester,
+ allow_departed_users: bool = False,
+ ) -> Tuple[str, Optional[str]]:
+ """Check if the user is in the room, or was at some point.
+ Args:
+ room_id: The room to check.
+
+ requester: The user making the request, according to the access token.
+
+ current_state: Optional map of the current state of the room.
+ If provided then that map is used to check whether they are a
+ member of the room. Otherwise the current membership is
+ loaded from the database.
+
+ allow_departed_users: if True, accept users that were previously
+ members but have now departed.
+
+ Raises:
+ AuthError if the user is/was not in the room.
+ Returns:
+ The current membership of the user in the room and the
+ membership event ID of the user.
+ """
+
+ user_id = requester.user.to_string()
+ (
+ membership,
+ member_event_id,
+ ) = await self.store.get_local_current_membership_for_user_in_room(
+ user_id=user_id,
+ room_id=room_id,
+ )
+
+ if membership:
+ if membership == Membership.JOIN:
+ return membership, member_event_id
+
+ # XXX this looks totally bogus. Why do we not allow users who have been banned,
+ # or those who were members previously and have been re-invited?
+ if allow_departed_users and membership == Membership.LEAVE:
+ forgot = await self.store.did_forget(user_id, room_id)
+ if not forgot:
+ return membership, member_event_id
+ raise UnstableSpecAuthError(
+ 403,
+ "User %s not in room %s" % (user_id, room_id),
+ errcode=Codes.NOT_JOINED,
+ )
+
+ @trace
+ async def check_user_in_room_or_world_readable(
+ self, room_id: str, requester: Requester, allow_departed_users: bool = False
+ ) -> Tuple[str, Optional[str]]:
+ """Checks that the user is or was in the room or the room is world
+ readable. If it isn't then an exception is raised.
+
+ Args:
+ room_id: room to check
+ user_id: user to check
+ allow_departed_users: if True, accept users that were previously
+ members but have now departed
+
+ Returns:
+ Resolves to the current membership of the user in the room and the
+ membership event ID of the user. If the user is not in the room and
+ never has been, then `(Membership.JOIN, None)` is returned.
+ """
+
+ try:
+ # check_user_in_room will return the most recent membership
+ # event for the user if:
+ # * The user is a non-guest user, and was ever in the room
+ # * The user is a guest user, and has joined the room
+ # else it will throw.
+ return await self.check_user_in_room(
+ room_id, requester, allow_departed_users=allow_departed_users
+ )
+ except AuthError:
+ visibility = await self._storage_controllers.state.get_current_state_event(
+ room_id, EventTypes.RoomHistoryVisibility, ""
+ )
+ if (
+ visibility
+ and visibility.content.get("history_visibility")
+ == HistoryVisibility.WORLD_READABLE
+ ):
+ return Membership.JOIN, None
+ raise AuthError(
+ 403,
+ "User %r not in room %s, and room previews are disabled"
+ % (requester.user, room_id),
+ )
+
+ async def validate_appservice_can_control_user_id(
+ self, app_service: ApplicationService, user_id: str
+ ) -> None:
+ """Validates that the app service is allowed to control
+ the given user.
+
+ Args:
+ app_service: The app service that controls the user
+ user_id: The author MXID that the app service is controlling
+
+ Raises:
+ AuthError: If the application service is not allowed to control the user
+ (user namespace regex does not match, wrong homeserver, etc)
+ or if the user has not been registered yet.
+ """
+
+ # It's ok if the app service is trying to use the sender from their registration
+ if app_service.sender == user_id:
+ pass
+ # Check to make sure the app service is allowed to control the user
+ elif not app_service.is_interested_in_user(user_id):
+ raise AuthError(
+ 403,
+ "Application service cannot masquerade as this user (%s)." % user_id,
+ )
+ # Check to make sure the user is already registered on the homeserver
+ elif not (await self.store.get_user_by_id(user_id)):
+ raise AuthError(
+ 403, "Application service has not registered this user (%s)" % user_id
+ )
+
+ async def is_server_admin(self, requester: Requester) -> bool:
+ """Check if the given user is a local server admin.
+
+ Args:
+ requester: user to check
+
+ Returns:
+ True if the user is an admin
+ """
+ raise NotImplementedError()
+
+ async def check_can_change_room_list(
+ self, room_id: str, requester: Requester
+ ) -> bool:
+ """Determine whether the user is allowed to edit the room's entry in the
+ published room list.
+
+ Args:
+ room_id
+ user
+ """
+
+ is_admin = await self.is_server_admin(requester)
+ if is_admin:
+ return True
+
+ await self.check_user_in_room(room_id, requester)
+
+ # We currently require the user is a "moderator" in the room. We do this
+ # by checking if they would (theoretically) be able to change the
+ # m.room.canonical_alias events
+
+ power_level_event = (
+ await self._storage_controllers.state.get_current_state_event(
+ room_id, EventTypes.PowerLevels, ""
+ )
+ )
+
+ auth_events = {}
+ if power_level_event:
+ auth_events[(EventTypes.PowerLevels, "")] = power_level_event
+
+ send_level = event_auth.get_send_level(
+ EventTypes.CanonicalAlias, "", power_level_event
+ )
+ user_level = event_auth.get_user_power_level(
+ requester.user.to_string(), auth_events
+ )
+
+ return user_level >= send_level
+
+ @staticmethod
+ def has_access_token(request: Request) -> bool:
+ """Checks if the request has an access_token.
+
+ Returns:
+ False if no access_token was given, True otherwise.
+ """
+ # This will always be set by the time Twisted calls us.
+ assert request.args is not None
+
+ query_params = request.args.get(b"access_token")
+ auth_headers = request.requestHeaders.getRawHeaders(b"Authorization")
+ return bool(query_params) or bool(auth_headers)
+
+ @staticmethod
+ def get_access_token_from_request(request: Request) -> str:
+ """Extracts the access_token from the request.
+
+ Args:
+ request: The http request.
+ Returns:
+ The access_token
+ Raises:
+ MissingClientTokenError: If there isn't a single access_token in the
+ request
+ """
+ # This will always be set by the time Twisted calls us.
+ assert request.args is not None
+
+ auth_headers = request.requestHeaders.getRawHeaders(b"Authorization")
+ query_params = request.args.get(b"access_token")
+ if auth_headers:
+ # Try the get the access_token from a "Authorization: Bearer"
+ # header
+ if query_params is not None:
+ raise MissingClientTokenError(
+ "Mixing Authorization headers and access_token query parameters."
+ )
+ if len(auth_headers) > 1:
+ raise MissingClientTokenError("Too many Authorization headers.")
+ parts = auth_headers[0].split(b" ")
+ if parts[0] == b"Bearer" and len(parts) == 2:
+ return parts[1].decode("ascii")
+ else:
+ raise MissingClientTokenError("Invalid Authorization header.")
+ else:
+ # Try to get the access_token from the query params.
+ if not query_params:
+ raise MissingClientTokenError()
+
+ return query_params[0].decode("ascii")
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