Make the api.auth.Auth a Protocol

3 files changed, 817 insertions, 0 deletions

diff --git a/synapse/api/auth/__init__.py b/synapse/api/auth/__init__.py
new file mode 100644
index 0000000000..90cfe39d76
--- /dev/null
+++ b/synapse/api/auth/__init__.py
@@ -0,0 +1,175 @@
+# 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.
+from typing import Optional, Tuple
+
+from typing_extensions import Protocol
+
+from twisted.web.server import Request
+
+from synapse.appservice import ApplicationService
+from synapse.http.site import SynapseRequest
+from synapse.types import Requester
+
+# guests always get this device id.
+GUEST_DEVICE_ID = "guest_device"
+
+
+class Auth(Protocol):
+    """The interface that an auth provider must implement."""
+
+    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.
+
+            user_id: The user to check.
+
+            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.
+        """
+
+    async def get_user_by_req(
+        self,
+        request: SynapseRequest,
+        allow_guest: bool = False,
+        allow_expired: bool = False,
+    ) -> Requester:
+        """Get a registered user's ID.
+
+        Args:
+            request: An HTTP request with an access_token query parameter.
+            allow_guest: If False, will raise an AuthError if the user making the
+                request is a guest.
+            allow_expired: If True, allow the request through even if the account
+                is expired, or session token lifetime has ended. Note that
+                /login will deliver access tokens regardless of expiration.
+
+        Returns:
+            Resolves to the requester
+        Raises:
+            InvalidClientCredentialsError if no user by that token exists or the token
+                is invalid.
+            AuthError if access is denied for the user in the access token
+        """
+
+    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.
+        """
+
+    async def get_user_by_access_token(
+        self,
+        token: str,
+        allow_expired: bool = False,
+    ) -> Requester:
+        """Validate access token and get user_id from it
+
+        Args:
+            token: The access token to get the user by
+            allow_expired: If False, raises an InvalidClientTokenError
+                if the token is expired
+
+        Raises:
+            InvalidClientTokenError if a user by that token exists, but the token is
+                expired
+            InvalidClientCredentialsError if no user by that token exists or the token
+                is invalid
+        """
+
+    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
+        """
+
+    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
+        """
+
+    @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.
+        """
+
+    @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
+        """
+
+    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.
+        """
diff --git a/synapse/api/auth/base.py b/synapse/api/auth/base.py
new file mode 100644
index 0000000000..240f2b90de
--- /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")
diff --git a/synapse/api/auth/internal.py b/synapse/api/auth/internal.py
new file mode 100644
index 0000000000..813d537e53
--- /dev/null
+++ b/synapse/api/auth/internal.py
@@ -0,0 +1,369 @@
+# 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
+
+import pymacaroons
+from netaddr import IPAddress
+
+from twisted.web.server import Request
+
+from synapse.api.errors import (
+    AuthError,
+    Codes,
+    InvalidClientTokenError,
+    MissingClientTokenError,
+)
+from synapse.http import get_request_user_agent
+from synapse.http.site import SynapseRequest
+from synapse.logging.opentracing import active_span, force_tracing, start_active_span
+from synapse.types import Requester, create_requester
+from synapse.util.cancellation import cancellable
+
+from . import GUEST_DEVICE_ID
+from .base import BaseAuth
+
+if TYPE_CHECKING:
+    from synapse.server import HomeServer
+
+logger = logging.getLogger(__name__)
+
+
+class InternalAuth(BaseAuth):
+    """
+    This class contains functions for authenticating users of our client-server API.
+    """
+
+    def __init__(self, hs: "HomeServer"):
+        super().__init__(hs)
+        self.clock = hs.get_clock()
+        self._account_validity_handler = hs.get_account_validity_handler()
+        self._macaroon_generator = hs.get_macaroon_generator()
+
+        self._track_appservice_user_ips = hs.config.appservice.track_appservice_user_ips
+        self._track_puppeted_user_ips = hs.config.api.track_puppeted_user_ips
+        self._force_tracing_for_users = hs.config.tracing.force_tracing_for_users
+
+    @cancellable
+    async def get_user_by_req(
+        self,
+        request: SynapseRequest,
+        allow_guest: bool = False,
+        allow_expired: bool = False,
+    ) -> Requester:
+        """Get a registered user's ID.
+
+        Args:
+            request: An HTTP request with an access_token query parameter.
+            allow_guest: If False, will raise an AuthError if the user making the
+                request is a guest.
+            allow_expired: If True, allow the request through even if the account
+                is expired, or session token lifetime has ended. Note that
+                /login will deliver access tokens regardless of expiration.
+
+        Returns:
+            Resolves to the requester
+        Raises:
+            InvalidClientCredentialsError if no user by that token exists or the token
+                is invalid.
+            AuthError if access is denied for the user in the access token
+        """
+        parent_span = active_span()
+        with start_active_span("get_user_by_req"):
+            requester = await self._wrapped_get_user_by_req(
+                request, allow_guest, allow_expired
+            )
+
+            if parent_span:
+                if requester.authenticated_entity in self._force_tracing_for_users:
+                    # request tracing is enabled for this user, so we need to force it
+                    # tracing on for the parent span (which will be the servlet span).
+                    #
+                    # It's too late for the get_user_by_req span to inherit the setting,
+                    # so we also force it on for that.
+                    force_tracing()
+                    force_tracing(parent_span)
+                parent_span.set_tag(
+                    "authenticated_entity", requester.authenticated_entity
+                )
+                parent_span.set_tag("user_id", requester.user.to_string())
+                if requester.device_id is not None:
+                    parent_span.set_tag("device_id", requester.device_id)
+                if requester.app_service is not None:
+                    parent_span.set_tag("appservice_id", requester.app_service.id)
+            return requester
+
+    @cancellable
+    async def _wrapped_get_user_by_req(
+        self,
+        request: SynapseRequest,
+        allow_guest: bool,
+        allow_expired: bool,
+    ) -> Requester:
+        """Helper for get_user_by_req
+
+        Once get_user_by_req has set up the opentracing span, this does the actual work.
+        """
+        try:
+            ip_addr = request.getClientAddress().host
+            user_agent = get_request_user_agent(request)
+
+            access_token = self.get_access_token_from_request(request)
+
+            # First check if it could be a request from an appservice
+            requester = await self._get_appservice_user(request)
+            if not requester:
+                # If not, it should be from a regular user
+                requester = await self.get_user_by_access_token(
+                    access_token, allow_expired=allow_expired
+                )
+
+                # Deny the request if the user account has expired.
+                # This check is only done for regular users, not appservice ones.
+                if not allow_expired:
+                    if await self._account_validity_handler.is_user_expired(
+                        requester.user.to_string()
+                    ):
+                        # Raise the error if either an account validity module has determined
+                        # the account has expired, or the legacy account validity
+                        # implementation is enabled and determined the account has expired
+                        raise AuthError(
+                            403,
+                            "User account has expired",
+                            errcode=Codes.EXPIRED_ACCOUNT,
+                        )
+
+            if ip_addr and (
+                not requester.app_service or self._track_appservice_user_ips
+            ):
+                # XXX(quenting): I'm 95% confident that we could skip setting the
+                # device_id to "dummy-device" for appservices, and that the only impact
+                # would be some rows which whould not deduplicate in the 'user_ips'
+                # table during the transition
+                recorded_device_id = (
+                    "dummy-device"
+                    if requester.device_id is None and requester.app_service is not None
+                    else requester.device_id
+                )
+                await self.store.insert_client_ip(
+                    user_id=requester.authenticated_entity,
+                    access_token=access_token,
+                    ip=ip_addr,
+                    user_agent=user_agent,
+                    device_id=recorded_device_id,
+                )
+
+                # Track also the puppeted user client IP if enabled and the user is puppeting
+                if (
+                    requester.user.to_string() != requester.authenticated_entity
+                    and self._track_puppeted_user_ips
+                ):
+                    await self.store.insert_client_ip(
+                        user_id=requester.user.to_string(),
+                        access_token=access_token,
+                        ip=ip_addr,
+                        user_agent=user_agent,
+                        device_id=requester.device_id,
+                    )
+
+            if requester.is_guest and not allow_guest:
+                raise AuthError(
+                    403,
+                    "Guest access not allowed",
+                    errcode=Codes.GUEST_ACCESS_FORBIDDEN,
+                )
+
+            request.requester = requester
+            return requester
+        except KeyError:
+            raise MissingClientTokenError()
+
+    @cancellable
+    async def _get_appservice_user(self, request: Request) -> Optional[Requester]:
+        """
+        Given a request, reads the request parameters to determine:
+        - whether it's an application service that's making this request
+        - what user the application service should be treated as controlling
+          (the user_id URI parameter allows an application service to masquerade
+          any applicable user in its namespace)
+        - what device the application service should be treated as controlling
+          (the device_id[^1] URI parameter allows an application service to masquerade
+          as any device that exists for the relevant user)
+
+        [^1] Unstable and provided by MSC3202.
+             Must use `org.matrix.msc3202.device_id` in place of `device_id` for now.
+
+        Returns:
+            the application service `Requester` of that request
+
+        Postconditions:
+        - The `app_service` field in the returned `Requester` is set
+        - The `user_id` field in the returned `Requester` is either the application
+          service sender or the controlled user set by the `user_id` URI parameter
+        - The returned application service is permitted to control the returned user ID.
+        - The returned device ID, if present, has been checked to be a valid device ID
+          for the returned user ID.
+        """
+        DEVICE_ID_ARG_NAME = b"org.matrix.msc3202.device_id"
+
+        app_service = self.store.get_app_service_by_token(
+            self.get_access_token_from_request(request)
+        )
+        if app_service is None:
+            return None
+
+        if app_service.ip_range_whitelist:
+            ip_address = IPAddress(request.getClientAddress().host)
+            if ip_address not in app_service.ip_range_whitelist:
+                return None
+
+        # This will always be set by the time Twisted calls us.
+        assert request.args is not None
+
+        if b"user_id" in request.args:
+            effective_user_id = request.args[b"user_id"][0].decode("utf8")
+            await self.validate_appservice_can_control_user_id(
+                app_service, effective_user_id
+            )
+        else:
+            effective_user_id = app_service.sender
+
+        effective_device_id: Optional[str] = None
+
+        if (
+            self.hs.config.experimental.msc3202_device_masquerading_enabled
+            and DEVICE_ID_ARG_NAME in request.args
+        ):
+            effective_device_id = request.args[DEVICE_ID_ARG_NAME][0].decode("utf8")
+            # We only just set this so it can't be None!
+            assert effective_device_id is not None
+            device_opt = await self.store.get_device(
+                effective_user_id, effective_device_id
+            )
+            if device_opt is None:
+                # For now, use 400 M_EXCLUSIVE if the device doesn't exist.
+                # This is an open thread of discussion on MSC3202 as of 2021-12-09.
+                raise AuthError(
+                    400,
+                    f"Application service trying to use a device that doesn't exist ('{effective_device_id}' for {effective_user_id})",
+                    Codes.EXCLUSIVE,
+                )
+
+        return create_requester(
+            effective_user_id, app_service=app_service, device_id=effective_device_id
+        )
+
+    async def get_user_by_access_token(
+        self,
+        token: str,
+        allow_expired: bool = False,
+    ) -> Requester:
+        """Validate access token and get user_id from it
+
+        Args:
+            token: The access token to get the user by
+            allow_expired: If False, raises an InvalidClientTokenError
+                if the token is expired
+
+        Raises:
+            InvalidClientTokenError if a user by that token exists, but the token is
+                expired
+            InvalidClientCredentialsError if no user by that token exists or the token
+                is invalid
+        """
+
+        # First look in the database to see if the access token is present
+        # as an opaque token.
+        user_info = await self.store.get_user_by_access_token(token)
+        if user_info:
+            valid_until_ms = user_info.valid_until_ms
+            if (
+                not allow_expired
+                and valid_until_ms is not None
+                and valid_until_ms < self.clock.time_msec()
+            ):
+                # there was a valid access token, but it has expired.
+                # soft-logout the user.
+                raise InvalidClientTokenError(
+                    msg="Access token has expired", soft_logout=True
+                )
+
+            # Mark the token as used. This is used to invalidate old refresh
+            # tokens after some time.
+            await self.store.mark_access_token_as_used(user_info.token_id)
+
+            requester = create_requester(
+                user_id=user_info.user_id,
+                access_token_id=user_info.token_id,
+                is_guest=user_info.is_guest,
+                shadow_banned=user_info.shadow_banned,
+                device_id=user_info.device_id,
+                authenticated_entity=user_info.token_owner,
+            )
+
+            return requester
+
+        # If the token isn't found in the database, then it could still be a
+        # macaroon for a guest, so we check that here.
+        try:
+            user_id = self._macaroon_generator.verify_guest_token(token)
+
+            # Guest access tokens are not stored in the database (there can
+            # only be one access token per guest, anyway).
+            #
+            # In order to prevent guest access tokens being used as regular
+            # user access tokens (and hence getting around the invalidation
+            # process), we look up the user id and check that it is indeed
+            # a guest user.
+            #
+            # It would of course be much easier to store guest access
+            # tokens in the database as well, but that would break existing
+            # guest tokens.
+            stored_user = await self.store.get_user_by_id(user_id)
+            if not stored_user:
+                raise InvalidClientTokenError("Unknown user_id %s" % user_id)
+            if not stored_user["is_guest"]:
+                raise InvalidClientTokenError(
+                    "Guest access token used for regular user"
+                )
+
+            return create_requester(
+                user_id=user_id,
+                is_guest=True,
+                # all guests get the same device id
+                device_id=GUEST_DEVICE_ID,
+                authenticated_entity=user_id,
+            )
+        except (
+            pymacaroons.exceptions.MacaroonException,
+            TypeError,
+            ValueError,
+        ) as e:
+            logger.warning(
+                "Invalid access token in auth: %s %s.",
+                type(e),
+                e,
+            )
+            raise InvalidClientTokenError("Invalid access token passed.")
+
+    async def is_server_admin(self, requester: Requester) -> bool:
+        """Check if the given user is a local server admin.
+
+        Args:
+            requester: The user making the request, according to the access token.
+
+        Returns:
+            True if the user is an admin
+        """
+        return await self.store.is_server_admin(requester.user)