Add admin API for logging in as a user (#8617)

25 files changed, 475 insertions, 87 deletions

diff --git a/changelog.d/8617.feature b/changelog.d/8617.feature
new file mode 100644
index 0000000000..4f1e788506
--- /dev/null
+++ b/changelog.d/8617.feature
@@ -0,0 +1 @@
+Add admin API for logging in as a user.
diff --git a/docs/admin_api/user_admin_api.rst b/docs/admin_api/user_admin_api.rst
index d4051d0257..95b3da27c4 100644
--- a/docs/admin_api/user_admin_api.rst
+++ b/docs/admin_api/user_admin_api.rst
@@ -424,6 +424,41 @@ The following fields are returned in the JSON response body:
 - ``next_token``: integer - Indication for pagination. See above.
 - ``total`` - integer - Total number of media.
 
+Login as a user
+===============
+
+Get an access token that can be used to authenticate as that user. Useful for
+when admins wish to do actions on behalf of a user.
+
+The API is::
+
+    POST /_synapse/admin/v1/users/<user_id>/login
+    {}
+
+An optional ``valid_until_ms`` field can be specified in the request body as an
+integer timestamp that specifies when the token should expire. By default tokens
+do not expire.
+
+A response body like the following is returned:
+
+.. code:: json
+
+    {
+        "access_token": "<opaque_access_token_string>"
+    }
+
+
+This API does *not* generate a new device for the user, and so will not appear
+their ``/devices`` list, and in general the target user should not be able to
+tell they have been logged in as.
+
+To expire the token call the standard ``/logout`` API with the token.
+
+Note: The token will expire if the *admin* user calls ``/logout/all`` from any
+of their devices, but the token will *not* expire if the target user does the
+same.
+
+
 User devices
 ============
 
diff --git a/synapse/api/auth_blocking.py b/synapse/api/auth_blocking.py
index d8fafd7cb8..9c227218e0 100644
--- a/synapse/api/auth_blocking.py
+++ b/synapse/api/auth_blocking.py
@@ -14,10 +14,12 @@
 # limitations under the License.
 
 import logging
+from typing import Optional
 
 from synapse.api.constants import LimitBlockingTypes, UserTypes
 from synapse.api.errors import Codes, ResourceLimitError
 from synapse.config.server import is_threepid_reserved
+from synapse.types import Requester
 
 logger = logging.getLogger(__name__)
 
@@ -33,24 +35,47 @@ class AuthBlocking:
         self._max_mau_value = hs.config.max_mau_value
         self._limit_usage_by_mau = hs.config.limit_usage_by_mau
         self._mau_limits_reserved_threepids = hs.config.mau_limits_reserved_threepids
+        self._server_name = hs.hostname
 
-    async def check_auth_blocking(self, user_id=None, threepid=None, user_type=None):
+    async def check_auth_blocking(
+        self,
+        user_id: Optional[str] = None,
+        threepid: Optional[dict] = None,
+        user_type: Optional[str] = None,
+        requester: Optional[Requester] = None,
+    ):
         """Checks if the user should be rejected for some external reason,
         such as monthly active user limiting or global disable flag
 
         Args:
-            user_id(str|None): If present, checks for presence against existing
+            user_id: If present, checks for presence against existing
                 MAU cohort
 
-            threepid(dict|None): If present, checks for presence against configured
+            threepid: If present, checks for presence against configured
                 reserved threepid. Used in cases where the user is trying register
                 with a MAU blocked server, normally they would be rejected but their
                 threepid is on the reserved list. user_id and
                 threepid should never be set at the same time.
 
-            user_type(str|None): If present, is used to decide whether to check against
+            user_type: If present, is used to decide whether to check against
                 certain blocking reasons like MAU.
+
+            requester: If present, and the authenticated entity is a user, checks for
+                presence against existing MAU cohort. Passing in both a `user_id` and
+                `requester` is an error.
         """
+        if requester and user_id:
+            raise Exception(
+                "Passed in both 'user_id' and 'requester' to 'check_auth_blocking'"
+            )
+
+        if requester:
+            if requester.authenticated_entity.startswith("@"):
+                user_id = requester.authenticated_entity
+            elif requester.authenticated_entity == self._server_name:
+                # We never block the server from doing actions on behalf of
+                # users.
+                return
 
         # Never fail an auth check for the server notices users or support user
         # This can be a problem where event creation is prohibited due to blocking
diff --git a/synapse/handlers/_base.py b/synapse/handlers/_base.py
index bd8e71ae56..bb81c0e81d 100644
--- a/synapse/handlers/_base.py
+++ b/synapse/handlers/_base.py
@@ -169,7 +169,9 @@ class BaseHandler:
                 # and having homeservers have their own users leave keeps more
                 # of that decision-making and control local to the guest-having
                 # homeserver.
-                requester = synapse.types.create_requester(target_user, is_guest=True)
+                requester = synapse.types.create_requester(
+                    target_user, is_guest=True, authenticated_entity=self.server_name
+                )
                 handler = self.hs.get_room_member_handler()
                 await handler.update_membership(
                     requester,
diff --git a/synapse/handlers/auth.py b/synapse/handlers/auth.py
index 213baea2e3..5163afd86c 100644
--- a/synapse/handlers/auth.py
+++ b/synapse/handlers/auth.py
@@ -698,8 +698,12 @@ class AuthHandler(BaseHandler):
         }
 
     async def get_access_token_for_user_id(
-        self, user_id: str, device_id: Optional[str], valid_until_ms: Optional[int]
-    ):
+        self,
+        user_id: str,
+        device_id: Optional[str],
+        valid_until_ms: Optional[int],
+        puppets_user_id: Optional[str] = None,
+    ) -> str:
         """
         Creates a new access token for the user with the given user ID.
 
@@ -725,13 +729,25 @@ class AuthHandler(BaseHandler):
             fmt_expiry = time.strftime(
                 " until %Y-%m-%d %H:%M:%S", time.localtime(valid_until_ms / 1000.0)
             )
-        logger.info("Logging in user %s on device %s%s", user_id, device_id, fmt_expiry)
+
+        if puppets_user_id:
+            logger.info(
+                "Logging in user %s as %s%s", user_id, puppets_user_id, fmt_expiry
+            )
+        else:
+            logger.info(
+                "Logging in user %s on device %s%s", user_id, device_id, fmt_expiry
+            )
 
         await self.auth.check_auth_blocking(user_id)
 
         access_token = self.macaroon_gen.generate_access_token(user_id)
         await self.store.add_access_token_to_user(
-            user_id, access_token, device_id, valid_until_ms
+            user_id=user_id,
+            token=access_token,
+            device_id=device_id,
+            valid_until_ms=valid_until_ms,
+            puppets_user_id=puppets_user_id,
         )
 
         # the device *should* have been registered before we got here; however,
diff --git a/synapse/handlers/deactivate_account.py b/synapse/handlers/deactivate_account.py
index 4efe6c530a..e808142365 100644
--- a/synapse/handlers/deactivate_account.py
+++ b/synapse/handlers/deactivate_account.py
@@ -39,6 +39,7 @@ class DeactivateAccountHandler(BaseHandler):
         self._room_member_handler = hs.get_room_member_handler()
         self._identity_handler = hs.get_identity_handler()
         self.user_directory_handler = hs.get_user_directory_handler()
+        self._server_name = hs.hostname
 
         # Flag that indicates whether the process to part users from rooms is running
         self._user_parter_running = False
@@ -152,7 +153,7 @@ class DeactivateAccountHandler(BaseHandler):
         for room in pending_invites:
             try:
                 await self._room_member_handler.update_membership(
-                    create_requester(user),
+                    create_requester(user, authenticated_entity=self._server_name),
                     user,
                     room.room_id,
                     "leave",
@@ -208,7 +209,7 @@ class DeactivateAccountHandler(BaseHandler):
             logger.info("User parter parting %r from %r", user_id, room_id)
             try:
                 await self._room_member_handler.update_membership(
-                    create_requester(user),
+                    create_requester(user, authenticated_entity=self._server_name),
                     user,
                     room_id,
                     "leave",
diff --git a/synapse/handlers/message.py b/synapse/handlers/message.py
index c6791fb912..96843338ae 100644
--- a/synapse/handlers/message.py
+++ b/synapse/handlers/message.py
@@ -472,7 +472,7 @@ class EventCreationHandler:
         Returns:
             Tuple of created event, Context
         """
-        await self.auth.check_auth_blocking(requester.user.to_string())
+        await self.auth.check_auth_blocking(requester=requester)
 
         if event_dict["type"] == EventTypes.Create and event_dict["state_key"] == "":
             room_version = event_dict["content"]["room_version"]
@@ -619,7 +619,13 @@ class EventCreationHandler:
         if requester.app_service is not None:
             return
 
-        user_id = requester.user.to_string()
+        user_id = requester.authenticated_entity
+        if not user_id.startswith("@"):
+            # The authenticated entity might not be a user, e.g. if it's the
+            # server puppetting the user.
+            return
+
+        user = UserID.from_string(user_id)
 
         # exempt the system notices user
         if (
@@ -639,9 +645,7 @@ class EventCreationHandler:
         if u["consent_version"] == self.config.user_consent_version:
             return
 
-        consent_uri = self._consent_uri_builder.build_user_consent_uri(
-            requester.user.localpart
-        )
+        consent_uri = self._consent_uri_builder.build_user_consent_uri(user.localpart)
         msg = self._block_events_without_consent_error % {"consent_uri": consent_uri}
         raise ConsentNotGivenError(msg=msg, consent_uri=consent_uri)
 
@@ -1252,7 +1256,7 @@ class EventCreationHandler:
         for user_id in members:
             if not self.hs.is_mine_id(user_id):
                 continue
-            requester = create_requester(user_id)
+            requester = create_requester(user_id, authenticated_entity=self.server_name)
             try:
                 event, context = await self.create_event(
                     requester,
@@ -1273,11 +1277,6 @@ class EventCreationHandler:
                     requester, event, context, ratelimit=False, ignore_shadow_ban=True,
                 )
                 return True
-            except ConsentNotGivenError:
-                logger.info(
-                    "Failed to send dummy event into room %s for user %s due to "
-                    "lack of consent. Will try another user" % (room_id, user_id)
-                )
             except AuthError:
                 logger.info(
                     "Failed to send dummy event into room %s for user %s due to "
diff --git a/synapse/handlers/profile.py b/synapse/handlers/profile.py
index 74a1ddd780..dee0ef45e7 100644
--- a/synapse/handlers/profile.py
+++ b/synapse/handlers/profile.py
@@ -206,7 +206,9 @@ class ProfileHandler(BaseHandler):
         # the join event to update the displayname in the rooms.
         # This must be done by the target user himself.
         if by_admin:
-            requester = create_requester(target_user)
+            requester = create_requester(
+                target_user, authenticated_entity=requester.authenticated_entity,
+            )
 
         await self.store.set_profile_displayname(
             target_user.localpart, displayname_to_set
@@ -286,7 +288,9 @@ class ProfileHandler(BaseHandler):
 
         # Same like set_displayname
         if by_admin:
-            requester = create_requester(target_user)
+            requester = create_requester(
+                target_user, authenticated_entity=requester.authenticated_entity
+            )
 
         await self.store.set_profile_avatar_url(target_user.localpart, new_avatar_url)
 
diff --git a/synapse/handlers/register.py b/synapse/handlers/register.py
index ed1ff62599..252f700786 100644
--- a/synapse/handlers/register.py
+++ b/synapse/handlers/register.py
@@ -52,6 +52,7 @@ class RegistrationHandler(BaseHandler):
         self.ratelimiter = hs.get_registration_ratelimiter()
         self.macaroon_gen = hs.get_macaroon_generator()
         self._server_notices_mxid = hs.config.server_notices_mxid
+        self._server_name = hs.hostname
 
         self.spam_checker = hs.get_spam_checker()
 
@@ -317,7 +318,8 @@ class RegistrationHandler(BaseHandler):
         requires_join = False
         if self.hs.config.registration.auto_join_user_id:
             fake_requester = create_requester(
-                self.hs.config.registration.auto_join_user_id
+                self.hs.config.registration.auto_join_user_id,
+                authenticated_entity=self._server_name,
             )
 
             # If the room requires an invite, add the user to the list of invites.
@@ -329,7 +331,9 @@ class RegistrationHandler(BaseHandler):
             # being necessary this will occur after the invite was sent.
             requires_join = True
         else:
-            fake_requester = create_requester(user_id)
+            fake_requester = create_requester(
+                user_id, authenticated_entity=self._server_name
+            )
 
         # Choose whether to federate the new room.
         if not self.hs.config.registration.autocreate_auto_join_rooms_federated:
@@ -362,7 +366,9 @@ class RegistrationHandler(BaseHandler):
                     # created it, then ensure the first user joins it.
                     if requires_join:
                         await room_member_handler.update_membership(
-                            requester=create_requester(user_id),
+                            requester=create_requester(
+                                user_id, authenticated_entity=self._server_name
+                            ),
                             target=UserID.from_string(user_id),
                             room_id=info["room_id"],
                             # Since it was just created, there are no remote hosts.
@@ -370,11 +376,6 @@ class RegistrationHandler(BaseHandler):
                             action="join",
                             ratelimit=False,
                         )
-
-            except ConsentNotGivenError as e:
-                # Technically not necessary to pull out this error though
-                # moving away from bare excepts is a good thing to do.
-                logger.error("Failed to join new user to %r: %r", r, e)
             except Exception as e:
                 logger.error("Failed to join new user to %r: %r", r, e)
 
@@ -426,7 +427,8 @@ class RegistrationHandler(BaseHandler):
                 if requires_invite:
                     await room_member_handler.update_membership(
                         requester=create_requester(
-                            self.hs.config.registration.auto_join_user_id
+                            self.hs.config.registration.auto_join_user_id,
+                            authenticated_entity=self._server_name,
                         ),
                         target=UserID.from_string(user_id),
                         room_id=room_id,
@@ -437,7 +439,9 @@ class RegistrationHandler(BaseHandler):
 
                 # Send the join.
                 await room_member_handler.update_membership(
-                    requester=create_requester(user_id),
+                    requester=create_requester(
+                        user_id, authenticated_entity=self._server_name
+                    ),
                     target=UserID.from_string(user_id),
                     room_id=room_id,
                     remote_room_hosts=remote_room_hosts,
diff --git a/synapse/handlers/room.py b/synapse/handlers/room.py
index e73031475f..930047e730 100644
--- a/synapse/handlers/room.py
+++ b/synapse/handlers/room.py
@@ -587,7 +587,7 @@ class RoomCreationHandler(BaseHandler):
         """
         user_id = requester.user.to_string()
 
-        await self.auth.check_auth_blocking(user_id)
+        await self.auth.check_auth_blocking(requester=requester)
 
         if (
             self._server_notices_mxid is not None
@@ -1257,7 +1257,9 @@ class RoomShutdownHandler:
                     400, "User must be our own: %s" % (new_room_user_id,)
                 )
 
-            room_creator_requester = create_requester(new_room_user_id)
+            room_creator_requester = create_requester(
+                new_room_user_id, authenticated_entity=requester_user_id
+            )
 
             info, stream_id = await self._room_creation_handler.create_room(
                 room_creator_requester,
@@ -1297,7 +1299,9 @@ class RoomShutdownHandler:
 
             try:
                 # Kick users from room
-                target_requester = create_requester(user_id)
+                target_requester = create_requester(
+                    user_id, authenticated_entity=requester_user_id
+                )
                 _, stream_id = await self.room_member_handler.update_membership(
                     requester=target_requester,
                     target=target_requester.user,
diff --git a/synapse/handlers/room_member.py b/synapse/handlers/room_member.py
index fd85e08973..70f8966267 100644
--- a/synapse/handlers/room_member.py
+++ b/synapse/handlers/room_member.py
@@ -965,6 +965,7 @@ class RoomMemberMasterHandler(RoomMemberHandler):
 
         self.distributor = hs.get_distributor()
         self.distributor.declare("user_left_room")
+        self._server_name = hs.hostname
 
     async def _is_remote_room_too_complex(
         self, room_id: str, remote_room_hosts: List[str]
@@ -1059,7 +1060,9 @@ class RoomMemberMasterHandler(RoomMemberHandler):
                 return event_id, stream_id
 
             # The room is too large. Leave.
-            requester = types.create_requester(user, None, False, False, None)
+            requester = types.create_requester(
+                user, authenticated_entity=self._server_name
+            )
             await self.update_membership(
                 requester=requester, target=user, room_id=room_id, action="leave"
             )
diff --git a/synapse/handlers/sync.py b/synapse/handlers/sync.py
index 32e53c2d25..9827c7eb8d 100644
--- a/synapse/handlers/sync.py
+++ b/synapse/handlers/sync.py
@@ -31,6 +31,7 @@ from synapse.types import (
     Collection,
     JsonDict,
     MutableStateMap,
+    Requester,
     RoomStreamToken,
     StateMap,
     StreamToken,
@@ -260,6 +261,7 @@ class SyncHandler:
 
     async def wait_for_sync_for_user(
         self,
+        requester: Requester,
         sync_config: SyncConfig,
         since_token: Optional[StreamToken] = None,
         timeout: int = 0,
@@ -273,7 +275,7 @@ class SyncHandler:
         # not been exceeded (if not part of the group by this point, almost certain
         # auth_blocking will occur)
         user_id = sync_config.user.to_string()
-        await self.auth.check_auth_blocking(user_id)
+        await self.auth.check_auth_blocking(requester=requester)
 
         res = await self.response_cache.wrap(
             sync_config.request_key,
diff --git a/synapse/module_api/__init__.py b/synapse/module_api/__init__.py
index 0142542852..72ab5750cc 100644
--- a/synapse/module_api/__init__.py
+++ b/synapse/module_api/__init__.py
@@ -49,6 +49,7 @@ class ModuleApi:
         self._store = hs.get_datastore()
         self._auth = hs.get_auth()
         self._auth_handler = auth_handler
+        self._server_name = hs.hostname
 
         # We expose these as properties below in order to attach a helpful docstring.
         self._http_client = hs.get_simple_http_client()  # type: SimpleHttpClient
@@ -336,7 +337,9 @@ class ModuleApi:
             SynapseError if the event was not allowed.
         """
         # Create a requester object
-        requester = create_requester(event_dict["sender"])
+        requester = create_requester(
+            event_dict["sender"], authenticated_entity=self._server_name
+        )
 
         # Create and send the event
         (
diff --git a/synapse/rest/admin/__init__.py b/synapse/rest/admin/__init__.py
index 2a4f7a1740..7a3a5c46ca 100644
--- a/synapse/rest/admin/__init__.py
+++ b/synapse/rest/admin/__init__.py
@@ -61,6 +61,7 @@ from synapse.rest.admin.users import (
     UserRestServletV2,
     UsersRestServlet,
     UsersRestServletV2,
+    UserTokenRestServlet,
     WhoisRestServlet,
 )
 from synapse.types import RoomStreamToken
@@ -223,6 +224,7 @@ def register_servlets(hs, http_server):
     UserAdminServlet(hs).register(http_server)
     UserMediaRestServlet(hs).register(http_server)
     UserMembershipRestServlet(hs).register(http_server)
+    UserTokenRestServlet(hs).register(http_server)
     UserRestServletV2(hs).register(http_server)
     UsersRestServletV2(hs).register(http_server)
     DeviceRestServlet(hs).register(http_server)
diff --git a/synapse/rest/admin/rooms.py b/synapse/rest/admin/rooms.py
index f5304ff43d..ee345e12ce 100644
--- a/synapse/rest/admin/rooms.py
+++ b/synapse/rest/admin/rooms.py
@@ -309,7 +309,9 @@ class JoinRoomAliasServlet(RestServlet):
                 400, "%s was not legal room ID or room alias" % (room_identifier,)
             )
 
-        fake_requester = create_requester(target_user)
+        fake_requester = create_requester(
+            target_user, authenticated_entity=requester.authenticated_entity
+        )
 
         # send invite if room has "JoinRules.INVITE"
         room_state = await self.state_handler.get_current_state(room_id)
diff --git a/synapse/rest/admin/users.py b/synapse/rest/admin/users.py
index 3638e219f2..fa8d8e6d91 100644
--- a/synapse/rest/admin/users.py
+++ b/synapse/rest/admin/users.py
@@ -16,7 +16,7 @@ import hashlib
 import hmac
 import logging
 from http import HTTPStatus
-from typing import Tuple
+from typing import TYPE_CHECKING, Tuple
 
 from synapse.api.constants import UserTypes
 from synapse.api.errors import Codes, NotFoundError, SynapseError
@@ -37,6 +37,9 @@ from synapse.rest.admin._base import (
 )
 from synapse.types import JsonDict, UserID
 
+if TYPE_CHECKING:
+    from synapse.server import HomeServer
+
 logger = logging.getLogger(__name__)
 
 _GET_PUSHERS_ALLOWED_KEYS = {
@@ -828,3 +831,52 @@ class UserMediaRestServlet(RestServlet):
             ret["next_token"] = start + len(media)
 
         return 200, ret
+
+
+class UserTokenRestServlet(RestServlet):
+    """An admin API for logging in as a user.
+
+    Example:
+
+        POST /_synapse/admin/v1/users/@test:example.com/login
+        {}
+
+        200 OK
+        {
+            "access_token": "<some_token>"
+        }
+    """
+
+    PATTERNS = admin_patterns("/users/(?P<user_id>[^/]*)/login$")
+
+    def __init__(self, hs: "HomeServer"):
+        self.hs = hs
+        self.store = hs.get_datastore()
+        self.auth = hs.get_auth()
+        self.auth_handler = hs.get_auth_handler()
+
+    async def on_POST(self, request, user_id):
+        requester = await self.auth.get_user_by_req(request)
+        await assert_user_is_admin(self.auth, requester.user)
+        auth_user = requester.user
+
+        if not self.hs.is_mine_id(user_id):
+            raise SynapseError(400, "Only local users can be logged in as")
+
+        body = parse_json_object_from_request(request, allow_empty_body=True)
+
+        valid_until_ms = body.get("valid_until_ms")
+        if valid_until_ms and not isinstance(valid_until_ms, int):
+            raise SynapseError(400, "'valid_until_ms' parameter must be an int")
+
+        if auth_user.to_string() == user_id:
+            raise SynapseError(400, "Cannot use admin API to login as self")
+
+        token = await self.auth_handler.get_access_token_for_user_id(
+            user_id=auth_user.to_string(),
+            device_id=None,
+            valid_until_ms=valid_until_ms,
+            puppets_user_id=user_id,
+        )
+
+        return 200, {"access_token": token}
diff --git a/synapse/rest/client/v2_alpha/sync.py b/synapse/rest/client/v2_alpha/sync.py
index 2b84eb89c0..8e52e4cca4 100644
--- a/synapse/rest/client/v2_alpha/sync.py
+++ b/synapse/rest/client/v2_alpha/sync.py
@@ -171,6 +171,7 @@ class SyncRestServlet(RestServlet):
         )
         with context:
             sync_result = await self.sync_handler.wait_for_sync_for_user(
+                requester,
                 sync_config,
                 since_token=since_token,
                 timeout=timeout,
diff --git a/synapse/server_notices/server_notices_manager.py b/synapse/server_notices/server_notices_manager.py
index d464c75c03..100dbd5e2c 100644
--- a/synapse/server_notices/server_notices_manager.py
+++ b/synapse/server_notices/server_notices_manager.py
@@ -39,6 +39,7 @@ class ServerNoticesManager:
         self._room_member_handler = hs.get_room_member_handler()
         self._event_creation_handler = hs.get_event_creation_handler()
         self._is_mine_id = hs.is_mine_id
+        self._server_name = hs.hostname
 
         self._notifier = hs.get_notifier()
         self.server_notices_mxid = self._config.server_notices_mxid
@@ -72,7 +73,9 @@ class ServerNoticesManager:
         await self.maybe_invite_user_to_room(user_id, room_id)
 
         system_mxid = self._config.server_notices_mxid
-        requester = create_requester(system_mxid)
+        requester = create_requester(
+            system_mxid, authenticated_entity=self._server_name
+        )
 
         logger.info("Sending server notice to %s", user_id)
 
@@ -145,7 +148,9 @@ class ServerNoticesManager:
                 "avatar_url": self._config.server_notices_mxid_avatar_url,
             }
 
-        requester = create_requester(self.server_notices_mxid)
+        requester = create_requester(
+            self.server_notices_mxid, authenticated_entity=self._server_name
+        )
         info, _ = await self._room_creation_handler.create_room(
             requester,
             config={
@@ -174,7 +179,9 @@ class ServerNoticesManager:
             user_id: The ID of the user to invite.
             room_id: The ID of the room to invite the user to.
         """
-        requester = create_requester(self.server_notices_mxid)
+        requester = create_requester(
+            self.server_notices_mxid, authenticated_entity=self._server_name
+        )
 
         # Check whether the user has already joined or been invited to this room. If
         # that's the case, there is no need to re-invite them.
diff --git a/synapse/storage/databases/main/registration.py b/synapse/storage/databases/main/registration.py
index e5d07ce72a..fedb8a6c26 100644
--- a/synapse/storage/databases/main/registration.py
+++ b/synapse/storage/databases/main/registration.py
@@ -1110,6 +1110,7 @@ class RegistrationStore(StatsStore, RegistrationBackgroundUpdateStore):
         token: str,
         device_id: Optional[str],
         valid_until_ms: Optional[int],
+        puppets_user_id: Optional[str] = None,
     ) -> int:
         """Adds an access token for the given user.
 
@@ -1133,6 +1134,7 @@ class RegistrationStore(StatsStore, RegistrationBackgroundUpdateStore):
                 "token": token,
                 "device_id": device_id,
                 "valid_until_ms": valid_until_ms,
+                "puppets_user_id": puppets_user_id,
             },
             desc="add_access_token_to_user",
         )
diff --git a/tests/api/test_auth.py b/tests/api/test_auth.py
index 0fd55f428a..ee5217b074 100644
--- a/tests/api/test_auth.py
+++ b/tests/api/test_auth.py
@@ -282,7 +282,11 @@ class AuthTestCase(unittest.TestCase):
             )
         )
         self.store.add_access_token_to_user.assert_called_with(
-            USER_ID, token, "DEVICE", None
+            user_id=USER_ID,
+            token=token,
+            device_id="DEVICE",
+            valid_until_ms=None,
+            puppets_user_id=None,
         )
 
         def get_user(tok):
diff --git a/tests/handlers/test_sync.py b/tests/handlers/test_sync.py
index e178d7765b..e62586142e 100644
--- a/tests/handlers/test_sync.py
+++ b/tests/handlers/test_sync.py
@@ -16,7 +16,7 @@
 from synapse.api.errors import Codes, ResourceLimitError
 from synapse.api.filtering import DEFAULT_FILTER_COLLECTION
 from synapse.handlers.sync import SyncConfig
-from synapse.types import UserID
+from synapse.types import UserID, create_requester
 
 import tests.unittest
 import tests.utils
@@ -38,6 +38,7 @@ class SyncTestCase(tests.unittest.HomeserverTestCase):
         user_id1 = "@user1:test"
         user_id2 = "@user2:test"
         sync_config = self._generate_sync_config(user_id1)
+        requester = create_requester(user_id1)
 
         self.reactor.advance(100)  # So we get not 0 time
         self.auth_blocking._limit_usage_by_mau = True
@@ -45,21 +46,26 @@ class SyncTestCase(tests.unittest.HomeserverTestCase):
 
         # Check that the happy case does not throw errors
         self.get_success(self.store.upsert_monthly_active_user(user_id1))
-        self.get_success(self.sync_handler.wait_for_sync_for_user(sync_config))
+        self.get_success(
+            self.sync_handler.wait_for_sync_for_user(requester, sync_config)
+        )
 
         # Test that global lock works
         self.auth_blocking._hs_disabled = True
         e = self.get_failure(
-            self.sync_handler.wait_for_sync_for_user(sync_config), ResourceLimitError
+            self.sync_handler.wait_for_sync_for_user(requester, sync_config),
+            ResourceLimitError,
         )
         self.assertEquals(e.value.errcode, Codes.RESOURCE_LIMIT_EXCEEDED)
 
         self.auth_blocking._hs_disabled = False
 
         sync_config = self._generate_sync_config(user_id2)
+        requester = create_requester(user_id2)
 
         e = self.get_failure(
-            self.sync_handler.wait_for_sync_for_user(sync_config), ResourceLimitError
+            self.sync_handler.wait_for_sync_for_user(requester, sync_config),
+            ResourceLimitError,
         )
         self.assertEquals(e.value.errcode, Codes.RESOURCE_LIMIT_EXCEEDED)
 
diff --git a/tests/module_api/test_api.py b/tests/module_api/test_api.py
index 9b573ac24d..27206ca3db 100644
--- a/tests/module_api/test_api.py
+++ b/tests/module_api/test_api.py
@@ -94,12 +94,13 @@ class ModuleApiTestCase(HomeserverTestCase):
         self.assertFalse(hasattr(event, "state_key"))
         self.assertDictEqual(event.content, content)
 
+        expected_requester = create_requester(
+            user_id, authenticated_entity=self.hs.hostname
+        )
+
         # Check that the event was sent
         self.event_creation_handler.create_and_send_nonmember_event.assert_called_with(
-            create_requester(user_id),
-            event_dict,
-            ratelimit=False,
-            ignore_shadow_ban=True,
+            expected_requester, event_dict, ratelimit=False, ignore_shadow_ban=True,
         )
 
         # Create and send a state event
@@ -128,7 +129,7 @@ class ModuleApiTestCase(HomeserverTestCase):
 
         # Check that the event was sent
         self.event_creation_handler.create_and_send_nonmember_event.assert_called_with(
-            create_requester(user_id),
+            expected_requester,
             {
                 "type": "m.room.power_levels",
                 "content": content,
diff --git a/tests/rest/admin/test_user.py b/tests/rest/admin/test_user.py
index d74efede06..94cd45066c 100644
--- a/tests/rest/admin/test_user.py
+++ b/tests/rest/admin/test_user.py
@@ -24,8 +24,8 @@ from mock import Mock
 import synapse.rest.admin
 from synapse.api.constants import UserTypes
 from synapse.api.errors import Codes, HttpResponseException, ResourceLimitError
-from synapse.rest.client.v1 import login, profile, room
-from synapse.rest.client.v2_alpha import sync
+from synapse.rest.client.v1 import login, logout, profile, room
+from synapse.rest.client.v2_alpha import devices, sync
 
 from tests import unittest
 from tests.test_utils import make_awaitable
@@ -1638,3 +1638,244 @@ class UserMediaRestTestCase(unittest.HomeserverTestCase):
             self.assertIn("last_access_ts", m)
             self.assertIn("quarantined_by", m)
             self.assertIn("safe_from_quarantine", m)
+
+
+class UserTokenRestTestCase(unittest.HomeserverTestCase):
+    """Test for /_synapse/admin/v1/users/<user>/login
+    """
+
+    servlets = [
+        synapse.rest.admin.register_servlets,
+        login.register_servlets,
+        sync.register_servlets,
+        room.register_servlets,
+        devices.register_servlets,
+        logout.register_servlets,
+    ]
+
+    def prepare(self, reactor, clock, hs):
+        self.store = hs.get_datastore()
+
+        self.admin_user = self.register_user("admin", "pass", admin=True)
+        self.admin_user_tok = self.login("admin", "pass")
+
+        self.other_user = self.register_user("user", "pass")
+        self.other_user_tok = self.login("user", "pass")
+        self.url = "/_synapse/admin/v1/users/%s/login" % urllib.parse.quote(
+            self.other_user
+        )
+
+    def _get_token(self) -> str:
+        request, channel = self.make_request(
+            "POST", self.url, b"{}", access_token=self.admin_user_tok
+        )
+        self.render(request)
+        self.assertEqual(200, int(channel.result["code"]), msg=channel.result["body"])
+        return channel.json_body["access_token"]
+
+    def test_no_auth(self):
+        """Try to login as a user without authentication.
+        """
+        request, channel = self.make_request("POST", self.url, b"{}")
+        self.render(request)
+
+        self.assertEqual(401, int(channel.result["code"]), msg=channel.result["body"])
+        self.assertEqual(Codes.MISSING_TOKEN, channel.json_body["errcode"])
+
+    def test_not_admin(self):
+        """Try to login as a user as a non-admin user.
+        """
+        request, channel = self.make_request(
+            "POST", self.url, b"{}", access_token=self.other_user_tok
+        )
+        self.render(request)
+
+        self.assertEqual(403, int(channel.result["code"]), msg=channel.result["body"])
+
+    def test_send_event(self):
+        """Test that sending event as a user works.
+        """
+        # Create a room.
+        room_id = self.helper.create_room_as(self.other_user, tok=self.other_user_tok)
+
+        # Login in as the user
+        puppet_token = self._get_token()
+
+        # Test that sending works, and generates the event as the right user.
+        resp = self.helper.send_event(room_id, "com.example.test", tok=puppet_token)
+        event_id = resp["event_id"]
+        event = self.get_success(self.store.get_event(event_id))
+        self.assertEqual(event.sender, self.other_user)
+
+    def test_devices(self):
+        """Tests that logging in as a user doesn't create a new device for them.
+        """
+        # Login in as the user
+        self._get_token()
+
+        # Check that we don't see a new device in our devices list
+        request, channel = self.make_request(
+            "GET", "devices", b"{}", access_token=self.other_user_tok
+        )
+        self.render(request)
+        self.assertEqual(200, int(channel.result["code"]), msg=channel.result["body"])
+
+        # We should only see the one device (from the login in `prepare`)
+        self.assertEqual(len(channel.json_body["devices"]), 1)
+
+    def test_logout(self):
+        """Test that calling `/logout` with the token works.
+        """
+        # Login in as the user
+        puppet_token = self._get_token()
+
+        # Test that we can successfully make a request
+        request, channel = self.make_request(
+            "GET", "devices", b"{}", access_token=puppet_token
+        )
+        self.render(request)
+        self.assertEqual(200, int(channel.result["code"]), msg=channel.result["body"])
+
+        # Logout with the puppet token
+        request, channel = self.make_request(
+            "POST", "logout", b"{}", access_token=puppet_token
+        )
+        self.render(request)
+        self.assertEqual(200, int(channel.result["code"]), msg=channel.result["body"])
+
+        # The puppet token should no longer work
+        request, channel = self.make_request(
+            "GET", "devices", b"{}", access_token=puppet_token
+        )
+        self.render(request)
+        self.assertEqual(401, int(channel.result["code"]), msg=channel.result["body"])
+
+        # .. but the real user's tokens should still work
+        request, channel = self.make_request(
+            "GET", "devices", b"{}", access_token=self.other_user_tok
+        )
+        self.render(request)
+        self.assertEqual(200, int(channel.result["code"]), msg=channel.result["body"])
+
+    def test_user_logout_all(self):
+        """Tests that the target user calling `/logout/all` does *not* expire
+        the token.
+        """
+        # Login in as the user
+        puppet_token = self._get_token()
+
+        # Test that we can successfully make a request
+        request, channel = self.make_request(
+            "GET", "devices", b"{}", access_token=puppet_token
+        )
+        self.render(request)
+        self.assertEqual(200, int(channel.result["code"]), msg=channel.result["body"])
+
+        # Logout all with the real user token
+        request, channel = self.make_request(
+            "POST", "logout/all", b"{}", access_token=self.other_user_tok
+        )
+        self.render(request)
+        self.assertEqual(200, int(channel.result["code"]), msg=channel.result["body"])
+
+        # The puppet token should still work
+        request, channel = self.make_request(
+            "GET", "devices", b"{}", access_token=puppet_token
+        )
+        self.render(request)
+        self.assertEqual(200, int(channel.result["code"]), msg=channel.result["body"])
+
+        # .. but the real user's tokens shouldn't
+        request, channel = self.make_request(
+            "GET", "devices", b"{}", access_token=self.other_user_tok
+        )
+        self.render(request)
+        self.assertEqual(401, int(channel.result["code"]), msg=channel.result["body"])
+
+    def test_admin_logout_all(self):
+        """Tests that the admin user calling `/logout/all` does expire the
+        token.
+        """
+        # Login in as the user
+        puppet_token = self._get_token()
+
+        # Test that we can successfully make a request
+        request, channel = self.make_request(
+            "GET", "devices", b"{}", access_token=puppet_token
+        )
+        self.render(request)
+        self.assertEqual(200, int(channel.result["code"]), msg=channel.result["body"])
+
+        # Logout all with the admin user token
+        request, channel = self.make_request(
+            "POST", "logout/all", b"{}", access_token=self.admin_user_tok
+        )
+        self.render(request)
+        self.assertEqual(200, int(channel.result["code"]), msg=channel.result["body"])
+
+        # The puppet token should no longer work
+        request, channel = self.make_request(
+            "GET", "devices", b"{}", access_token=puppet_token
+        )
+        self.render(request)
+        self.assertEqual(401, int(channel.result["code"]), msg=channel.result["body"])
+
+        # .. but the real user's tokens should still work
+        request, channel = self.make_request(
+            "GET", "devices", b"{}", access_token=self.other_user_tok
+        )
+        self.render(request)
+        self.assertEqual(200, int(channel.result["code"]), msg=channel.result["body"])
+
+    @unittest.override_config(
+        {
+            "public_baseurl": "https://example.org/",
+            "user_consent": {
+                "version": "1.0",
+                "policy_name": "My Cool Privacy Policy",
+                "template_dir": "/",
+                "require_at_registration": True,
+                "block_events_error": "You should accept the policy",
+            },
+            "form_secret": "123secret",
+        }
+    )
+    def test_consent(self):
+        """Test that sending a message is not subject to the privacy policies.
+        """
+        # Have the admin user accept the terms.
+        self.get_success(self.store.user_set_consent_version(self.admin_user, "1.0"))
+
+        # First, cheekily accept the terms and create a room
+        self.get_success(self.store.user_set_consent_version(self.other_user, "1.0"))
+        room_id = self.helper.create_room_as(self.other_user, tok=self.other_user_tok)
+        self.helper.send_event(room_id, "com.example.test", tok=self.other_user_tok)
+
+        # Now unaccept it and check that we can't send an event
+        self.get_success(self.store.user_set_consent_version(self.other_user, "0.0"))
+        self.helper.send_event(
+            room_id, "com.example.test", tok=self.other_user_tok, expect_code=403
+        )
+
+        # Login in as the user
+        puppet_token = self._get_token()
+
+        # Sending an event on their behalf should work fine
+        self.helper.send_event(room_id, "com.example.test", tok=puppet_token)
+
+    @override_config(
+        {"limit_usage_by_mau": True, "max_mau_value": 1, "mau_trial_days": 0}
+    )
+    def test_mau_limit(self):
+        # Create a room as the admin user. This will bump the monthly active users to 1.
+        room_id = self.helper.create_room_as(self.admin_user, tok=self.admin_user_tok)
+
+        # Trying to join as the other user should fail due to reaching MAU limit.
+        self.helper.join(
+            room_id, user=self.other_user, tok=self.other_user_tok, expect_code=403
+        )
+
+        # Logging in as the other user and joining a room should work, even
+        # though the MAU limit would stop the user doing so.
+        puppet_token = self._get_token()
+        self.helper.join(room_id, user=self.other_user, tok=puppet_token)
diff --git a/tests/storage/test_cleanup_extrems.py b/tests/storage/test_cleanup_extrems.py
index 5a1e5c4e66..c13a57dad1 100644
--- a/tests/storage/test_cleanup_extrems.py
+++ b/tests/storage/test_cleanup_extrems.py
@@ -309,36 +309,6 @@ class CleanupExtremDummyEventsTestCase(HomeserverTestCase):
         )
         self.assertTrue(len(latest_event_ids) < 10, len(latest_event_ids))
 
-    @patch("synapse.handlers.message._DUMMY_EVENT_ROOM_EXCLUSION_EXPIRY", new=0)
-    def test_send_dummy_event_without_consent(self):
-        self._create_extremity_rich_graph()
-        self._enable_consent_checking()
-
-        # Pump the reactor repeatedly so that the background updates have a
-        # chance to run. Attempt to add dummy event with user that has not consented
-        # Check that dummy event send fails.
-        self.pump(10 * 60)
-        latest_event_ids = self.get_success(
-            self.store.get_latest_event_ids_in_room(self.room_id)
-        )
-        self.assertTrue(len(latest_event_ids) == self.EXTREMITIES_COUNT)
-
-        # Create new user, and add consent
-        user2 = self.register_user("user2", "password")
-        token2 = self.login("user2", "password")
-        self.get_success(
-            self.store.user_set_consent_version(user2, self.CONSENT_VERSION)
-        )
-        self.helper.join(self.room_id, user2, tok=token2)
-
-        # Background updates should now cause a dummy event to be added to the graph
-        self.pump(10 * 60)
-
-        latest_event_ids = self.get_success(
-            self.store.get_latest_event_ids_in_room(self.room_id)
-        )
-        self.assertTrue(len(latest_event_ids) < 10, len(latest_event_ids))
-
     @patch("synapse.handlers.message._DUMMY_EVENT_ROOM_EXCLUSION_EXPIRY", new=250)
     def test_expiry_logic(self):
         """Simple test to ensure that _expire_rooms_to_exclude_from_dummy_event_insertion()
diff --git a/tests/test_state.py b/tests/test_state.py
index 80b0ccbc40..6227a3ba95 100644
--- a/tests/test_state.py
+++ b/tests/test_state.py
@@ -169,6 +169,7 @@ class StateTestCase(unittest.TestCase):
                 "get_state_handler",
                 "get_clock",
                 "get_state_resolution_handler",
+                "hostname",
             ]
         )
         hs.config = default_config("tesths", True)