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-rw-r--r--changelog.d/8617.feature1
-rw-r--r--changelog.d/8765.misc1
-rw-r--r--changelog.d/8770.misc1
-rw-r--r--changelog.d/8771.doc1
-rw-r--r--docs/admin_api/user_admin_api.rst37
-rw-r--r--synapse/api/auth_blocking.py33
-rw-r--r--synapse/events/spamcheck.py5
-rw-r--r--synapse/handlers/_base.py4
-rw-r--r--synapse/handlers/auth.py24
-rw-r--r--synapse/handlers/deactivate_account.py5
-rw-r--r--synapse/handlers/message.py21
-rw-r--r--synapse/handlers/oidc_handler.py92
-rw-r--r--synapse/handlers/presence.py5
-rw-r--r--synapse/handlers/profile.py8
-rw-r--r--synapse/handlers/register.py24
-rw-r--r--synapse/handlers/room.py10
-rw-r--r--synapse/handlers/room_member.py5
-rw-r--r--synapse/handlers/saml_handler.py77
-rw-r--r--synapse/handlers/sso.py90
-rw-r--r--synapse/handlers/sync.py4
-rw-r--r--synapse/module_api/__init__.py5
-rw-r--r--synapse/rest/admin/__init__.py2
-rw-r--r--synapse/rest/admin/rooms.py4
-rw-r--r--synapse/rest/admin/users.py54
-rw-r--r--synapse/rest/client/v1/room.py5
-rw-r--r--synapse/rest/client/v2_alpha/sync.py1
-rw-r--r--synapse/server.py5
-rw-r--r--synapse/server_notices/server_notices_manager.py13
-rw-r--r--synapse/storage/databases/main/registration.py2
-rw-r--r--tests/api/test_auth.py6
-rw-r--r--tests/handlers/test_oidc.py14
-rw-r--r--tests/handlers/test_sync.py14
-rw-r--r--tests/module_api/test_api.py11
-rw-r--r--tests/rest/admin/test_user.py245
-rw-r--r--tests/storage/test_cleanup_extrems.py30
-rw-r--r--tests/test_state.py1
36 files changed, 643 insertions, 217 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/changelog.d/8765.misc b/changelog.d/8765.misc
new file mode 100644
index 0000000000..053f9acc9c
--- /dev/null
+++ b/changelog.d/8765.misc
@@ -0,0 +1 @@
+Consolidate logic between the OpenID Connect and SAML code.
diff --git a/changelog.d/8770.misc b/changelog.d/8770.misc
new file mode 100644
index 0000000000..b5876a82f9
--- /dev/null
+++ b/changelog.d/8770.misc
@@ -0,0 +1 @@
+Use `TYPE_CHECKING` instead of magic `MYPY` variable.
diff --git a/changelog.d/8771.doc b/changelog.d/8771.doc
new file mode 100644
index 0000000000..297cf61e98
--- /dev/null
+++ b/changelog.d/8771.doc
@@ -0,0 +1 @@
+Remove extraneous comma from JSON example in User Admin API docs.
\ No newline at end of file
diff --git a/docs/admin_api/user_admin_api.rst b/docs/admin_api/user_admin_api.rst
index d4051d0257..84863296e3 100644
--- a/docs/admin_api/user_admin_api.rst
+++ b/docs/admin_api/user_admin_api.rst
@@ -254,7 +254,7 @@ with a body of:
 
    {
        "new_password": "<secret>",
-       "logout_devices": true,
+       "logout_devices": true
    }
 
 To use it, you will need to authenticate by providing an ``access_token`` for a
@@ -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/events/spamcheck.py b/synapse/events/spamcheck.py
index bad18f7fdf..936896656a 100644
--- a/synapse/events/spamcheck.py
+++ b/synapse/events/spamcheck.py
@@ -15,13 +15,12 @@
 # limitations under the License.
 
 import inspect
-from typing import Any, Dict, List, Optional, Tuple
+from typing import TYPE_CHECKING, Any, Dict, List, Optional, Tuple
 
 from synapse.spam_checker_api import RegistrationBehaviour
 from synapse.types import Collection
 
-MYPY = False
-if MYPY:
+if TYPE_CHECKING:
     import synapse.events
     import synapse.server
 
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/oidc_handler.py b/synapse/handlers/oidc_handler.py
index 331d4e7e96..be8562d47b 100644
--- a/synapse/handlers/oidc_handler.py
+++ b/synapse/handlers/oidc_handler.py
@@ -34,7 +34,8 @@ from typing_extensions import TypedDict
 from twisted.web.client import readBody
 
 from synapse.config import ConfigError
-from synapse.http.server import respond_with_html
+from synapse.handlers._base import BaseHandler
+from synapse.handlers.sso import MappingException
 from synapse.http.site import SynapseRequest
 from synapse.logging.context import make_deferred_yieldable
 from synapse.types import JsonDict, UserID, map_username_to_mxid_localpart
@@ -83,17 +84,12 @@ class OidcError(Exception):
         return self.error
 
 
-class MappingException(Exception):
-    """Used to catch errors when mapping the UserInfo object
-    """
-
-
-class OidcHandler:
+class OidcHandler(BaseHandler):
     """Handles requests related to the OpenID Connect login flow.
     """
 
     def __init__(self, hs: "HomeServer"):
-        self.hs = hs
+        super().__init__(hs)
         self._callback_url = hs.config.oidc_callback_url  # type: str
         self._scopes = hs.config.oidc_scopes  # type: List[str]
         self._user_profile_method = hs.config.oidc_user_profile_method  # type: str
@@ -120,36 +116,13 @@ class OidcHandler:
         self._http_client = hs.get_proxied_http_client()
         self._auth_handler = hs.get_auth_handler()
         self._registration_handler = hs.get_registration_handler()
-        self._datastore = hs.get_datastore()
-        self._clock = hs.get_clock()
-        self._hostname = hs.hostname  # type: str
         self._server_name = hs.config.server_name  # type: str
         self._macaroon_secret_key = hs.config.macaroon_secret_key
-        self._error_template = hs.config.sso_error_template
 
         # identifier for the external_ids table
         self._auth_provider_id = "oidc"
 
-    def _render_error(
-        self, request, error: str, error_description: Optional[str] = None
-    ) -> None:
-        """Render the error template and respond to the request with it.
-
-        This is used to show errors to the user. The template of this page can
-        be found under `synapse/res/templates/sso_error.html`.
-
-        Args:
-            request: The incoming request from the browser.
-                We'll respond with an HTML page describing the error.
-            error: A technical identifier for this error. Those include
-                well-known OAuth2/OIDC error types like invalid_request or
-                access_denied.
-            error_description: A human-readable description of the error.
-        """
-        html = self._error_template.render(
-            error=error, error_description=error_description
-        )
-        respond_with_html(request, 400, html)
+        self._sso_handler = hs.get_sso_handler()
 
     def _validate_metadata(self):
         """Verifies the provider metadata.
@@ -571,7 +544,7 @@ class OidcHandler:
 
         Since we might want to display OIDC-related errors in a user-friendly
         way, we don't raise SynapseError from here. Instead, we call
-        ``self._render_error`` which displays an HTML page for the error.
+        ``self._sso_handler.render_error`` which displays an HTML page for the error.
 
         Most of the OpenID Connect logic happens here:
 
@@ -609,7 +582,7 @@ class OidcHandler:
             if error != "access_denied":
                 logger.error("Error from the OIDC provider: %s %s", error, description)
 
-            self._render_error(request, error, description)
+            self._sso_handler.render_error(request, error, description)
             return
 
         # otherwise, it is presumably a successful response. see:
@@ -619,7 +592,9 @@ class OidcHandler:
         session = request.getCookie(SESSION_COOKIE_NAME)  # type: Optional[bytes]
         if session is None:
             logger.info("No session cookie found")
-            self._render_error(request, "missing_session", "No session cookie found")
+            self._sso_handler.render_error(
+                request, "missing_session", "No session cookie found"
+            )
             return
 
         # Remove the cookie. There is a good chance that if the callback failed
@@ -637,7 +612,9 @@ class OidcHandler:
         # Check for the state query parameter
         if b"state" not in request.args:
             logger.info("State parameter is missing")
-            self._render_error(request, "invalid_request", "State parameter is missing")
+            self._sso_handler.render_error(
+                request, "invalid_request", "State parameter is missing"
+            )
             return
 
         state = request.args[b"state"][0].decode()
@@ -651,17 +628,19 @@ class OidcHandler:
             ) = self._verify_oidc_session_token(session, state)
         except MacaroonDeserializationException as e:
             logger.exception("Invalid session")
-            self._render_error(request, "invalid_session", str(e))
+            self._sso_handler.render_error(request, "invalid_session", str(e))
             return
         except MacaroonInvalidSignatureException as e:
             logger.exception("Could not verify session")
-            self._render_error(request, "mismatching_session", str(e))
+            self._sso_handler.render_error(request, "mismatching_session", str(e))
             return
 
         # Exchange the code with the provider
         if b"code" not in request.args:
             logger.info("Code parameter is missing")
-            self._render_error(request, "invalid_request", "Code parameter is missing")
+            self._sso_handler.render_error(
+                request, "invalid_request", "Code parameter is missing"
+            )
             return
 
         logger.debug("Exchanging code")
@@ -670,7 +649,7 @@ class OidcHandler:
             token = await self._exchange_code(code)
         except OidcError as e:
             logger.exception("Could not exchange code")
-            self._render_error(request, e.error, e.error_description)
+            self._sso_handler.render_error(request, e.error, e.error_description)
             return
 
         logger.debug("Successfully obtained OAuth2 access token")
@@ -683,7 +662,7 @@ class OidcHandler:
                 userinfo = await self._fetch_userinfo(token)
             except Exception as e:
                 logger.exception("Could not fetch userinfo")
-                self._render_error(request, "fetch_error", str(e))
+                self._sso_handler.render_error(request, "fetch_error", str(e))
                 return
         else:
             logger.debug("Extracting userinfo from id_token")
@@ -691,7 +670,7 @@ class OidcHandler:
                 userinfo = await self._parse_id_token(token, nonce=nonce)
             except Exception as e:
                 logger.exception("Invalid id_token")
-                self._render_error(request, "invalid_token", str(e))
+                self._sso_handler.render_error(request, "invalid_token", str(e))
                 return
 
         # Pull out the user-agent and IP from the request.
@@ -705,7 +684,7 @@ class OidcHandler:
             )
         except MappingException as e:
             logger.exception("Could not map user")
-            self._render_error(request, "mapping_error", str(e))
+            self._sso_handler.render_error(request, "mapping_error", str(e))
             return
 
         # Mapping providers might not have get_extra_attributes: only call this
@@ -770,7 +749,7 @@ class OidcHandler:
             macaroon.add_first_party_caveat(
                 "ui_auth_session_id = %s" % (ui_auth_session_id,)
             )
-        now = self._clock.time_msec()
+        now = self.clock.time_msec()
         expiry = now + duration_in_ms
         macaroon.add_first_party_caveat("time < %d" % (expiry,))
 
@@ -845,7 +824,7 @@ class OidcHandler:
         if not caveat.startswith(prefix):
             return False
         expiry = int(caveat[len(prefix) :])
-        now = self._clock.time_msec()
+        now = self.clock.time_msec()
         return now < expiry
 
     async def _map_userinfo_to_user(
@@ -885,20 +864,14 @@ class OidcHandler:
         # to be strings.
         remote_user_id = str(remote_user_id)
 
-        logger.info(
-            "Looking for existing mapping for user %s:%s",
-            self._auth_provider_id,
-            remote_user_id,
-        )
-
-        registered_user_id = await self._datastore.get_user_by_external_id(
+        # first of all, check if we already have a mapping for this user
+        previously_registered_user_id = await self._sso_handler.get_sso_user_by_remote_user_id(
             self._auth_provider_id, remote_user_id,
         )
+        if previously_registered_user_id:
+            return previously_registered_user_id
 
-        if registered_user_id is not None:
-            logger.info("Found existing mapping %s", registered_user_id)
-            return registered_user_id
-
+        # Otherwise, generate a new user.
         try:
             attributes = await self._user_mapping_provider.map_user_attributes(
                 userinfo, token
@@ -917,8 +890,8 @@ class OidcHandler:
 
         localpart = map_username_to_mxid_localpart(attributes["localpart"])
 
-        user_id = UserID(localpart, self._hostname).to_string()
-        users = await self._datastore.get_users_by_id_case_insensitive(user_id)
+        user_id = UserID(localpart, self.server_name).to_string()
+        users = await self.store.get_users_by_id_case_insensitive(user_id)
         if users:
             if self._allow_existing_users:
                 if len(users) == 1:
@@ -942,7 +915,8 @@ class OidcHandler:
                 default_display_name=attributes["display_name"],
                 user_agent_ips=(user_agent, ip_address),
             )
-        await self._datastore.record_user_external_id(
+
+        await self.store.record_user_external_id(
             self._auth_provider_id, remote_user_id, registered_user_id,
         )
         return registered_user_id
diff --git a/synapse/handlers/presence.py b/synapse/handlers/presence.py
index 8e014c9bb5..22d1e9d35c 100644
--- a/synapse/handlers/presence.py
+++ b/synapse/handlers/presence.py
@@ -25,7 +25,7 @@ The methods that define policy are:
 import abc
 import logging
 from contextlib import contextmanager
-from typing import Dict, Iterable, List, Set, Tuple
+from typing import TYPE_CHECKING, Dict, Iterable, List, Set, Tuple
 
 from prometheus_client import Counter
 from typing_extensions import ContextManager
@@ -46,8 +46,7 @@ from synapse.util.caches.descriptors import cached
 from synapse.util.metrics import Measure
 from synapse.util.wheel_timer import WheelTimer
 
-MYPY = False
-if MYPY:
+if TYPE_CHECKING:
     from synapse.server import HomeServer
 
 logger = logging.getLogger(__name__)
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/saml_handler.py b/synapse/handlers/saml_handler.py
index fd6c5e9ea8..aee772239a 100644
--- a/synapse/handlers/saml_handler.py
+++ b/synapse/handlers/saml_handler.py
@@ -24,7 +24,8 @@ from saml2.client import Saml2Client
 from synapse.api.errors import SynapseError
 from synapse.config import ConfigError
 from synapse.config.saml2_config import SamlAttributeRequirement
-from synapse.http.server import respond_with_html
+from synapse.handlers._base import BaseHandler
+from synapse.handlers.sso import MappingException
 from synapse.http.servlet import parse_string
 from synapse.http.site import SynapseRequest
 from synapse.module_api import ModuleApi
@@ -42,10 +43,6 @@ if TYPE_CHECKING:
 logger = logging.getLogger(__name__)
 
 
-class MappingException(Exception):
-    """Used to catch errors when mapping the SAML2 response to a user."""
-
-
 @attr.s(slots=True)
 class Saml2SessionData:
     """Data we track about SAML2 sessions"""
@@ -57,17 +54,13 @@ class Saml2SessionData:
     ui_auth_session_id = attr.ib(type=Optional[str], default=None)
 
 
-class SamlHandler:
+class SamlHandler(BaseHandler):
     def __init__(self, hs: "synapse.server.HomeServer"):
-        self.hs = hs
+        super().__init__(hs)
         self._saml_client = Saml2Client(hs.config.saml2_sp_config)
-        self._auth = hs.get_auth()
         self._auth_handler = hs.get_auth_handler()
         self._registration_handler = hs.get_registration_handler()
 
-        self._clock = hs.get_clock()
-        self._datastore = hs.get_datastore()
-        self._hostname = hs.hostname
         self._saml2_session_lifetime = hs.config.saml2_session_lifetime
         self._grandfathered_mxid_source_attribute = (
             hs.config.saml2_grandfathered_mxid_source_attribute
@@ -88,26 +81,9 @@ class SamlHandler:
         self._outstanding_requests_dict = {}  # type: Dict[str, Saml2SessionData]
 
         # a lock on the mappings
-        self._mapping_lock = Linearizer(name="saml_mapping", clock=self._clock)
-
-    def _render_error(
-        self, request, error: str, error_description: Optional[str] = None
-    ) -> None:
-        """Render the error template and respond to the request with it.
+        self._mapping_lock = Linearizer(name="saml_mapping", clock=self.clock)
 
-        This is used to show errors to the user. The template of this page can
-        be found under `synapse/res/templates/sso_error.html`.
-
-        Args:
-            request: The incoming request from the browser.
-                We'll respond with an HTML page describing the error.
-            error: A technical identifier for this error.
-            error_description: A human-readable description of the error.
-        """
-        html = self._error_template.render(
-            error=error, error_description=error_description
-        )
-        respond_with_html(request, 400, html)
+        self._sso_handler = hs.get_sso_handler()
 
     def handle_redirect_request(
         self, client_redirect_url: bytes, ui_auth_session_id: Optional[str] = None
@@ -130,7 +106,7 @@ class SamlHandler:
         # Since SAML sessions timeout it is useful to log when they were created.
         logger.info("Initiating a new SAML session: %s" % (reqid,))
 
-        now = self._clock.time_msec()
+        now = self.clock.time_msec()
         self._outstanding_requests_dict[reqid] = Saml2SessionData(
             creation_time=now, ui_auth_session_id=ui_auth_session_id,
         )
@@ -171,12 +147,12 @@ class SamlHandler:
             # in the (user-visible) exception message, so let's log the exception here
             # so we can track down the session IDs later.
             logger.warning(str(e))
-            self._render_error(
+            self._sso_handler.render_error(
                 request, "unsolicited_response", "Unexpected SAML2 login."
             )
             return
         except Exception as e:
-            self._render_error(
+            self._sso_handler.render_error(
                 request,
                 "invalid_response",
                 "Unable to parse SAML2 response: %s." % (e,),
@@ -184,7 +160,7 @@ class SamlHandler:
             return
 
         if saml2_auth.not_signed:
-            self._render_error(
+            self._sso_handler.render_error(
                 request, "unsigned_respond", "SAML2 response was not signed."
             )
             return
@@ -210,7 +186,7 @@ class SamlHandler:
         # attributes.
         for requirement in self._saml2_attribute_requirements:
             if not _check_attribute_requirement(saml2_auth.ava, requirement):
-                self._render_error(
+                self._sso_handler.render_error(
                     request, "unauthorised", "You are not authorised to log in here."
                 )
                 return
@@ -226,7 +202,7 @@ class SamlHandler:
             )
         except MappingException as e:
             logger.exception("Could not map user")
-            self._render_error(request, "mapping_error", str(e))
+            self._sso_handler.render_error(request, "mapping_error", str(e))
             return
 
         # Complete the interactive auth session or the login.
@@ -274,17 +250,11 @@ class SamlHandler:
 
         with (await self._mapping_lock.queue(self._auth_provider_id)):
             # first of all, check if we already have a mapping for this user
-            logger.info(
-                "Looking for existing mapping for user %s:%s",
-                self._auth_provider_id,
-                remote_user_id,
+            previously_registered_user_id = await self._sso_handler.get_sso_user_by_remote_user_id(
+                self._auth_provider_id, remote_user_id,
             )
-            registered_user_id = await self._datastore.get_user_by_external_id(
-                self._auth_provider_id, remote_user_id
-            )
-            if registered_user_id is not None:
-                logger.info("Found existing mapping %s", registered_user_id)
-                return registered_user_id
+            if previously_registered_user_id:
+                return previously_registered_user_id
 
             # backwards-compatibility hack: see if there is an existing user with a
             # suitable mapping from the uid
@@ -294,7 +264,7 @@ class SamlHandler:
             ):
                 attrval = saml2_auth.ava[self._grandfathered_mxid_source_attribute][0]
                 user_id = UserID(
-                    map_username_to_mxid_localpart(attrval), self._hostname
+                    map_username_to_mxid_localpart(attrval), self.server_name
                 ).to_string()
                 logger.info(
                     "Looking for existing account based on mapped %s %s",
@@ -302,11 +272,11 @@ class SamlHandler:
                     user_id,
                 )
 
-                users = await self._datastore.get_users_by_id_case_insensitive(user_id)
+                users = await self.store.get_users_by_id_case_insensitive(user_id)
                 if users:
                     registered_user_id = list(users.keys())[0]
                     logger.info("Grandfathering mapping to %s", registered_user_id)
-                    await self._datastore.record_user_external_id(
+                    await self.store.record_user_external_id(
                         self._auth_provider_id, remote_user_id, registered_user_id
                     )
                     return registered_user_id
@@ -335,8 +305,8 @@ class SamlHandler:
                 emails = attribute_dict.get("emails", [])
 
                 # Check if this mxid already exists
-                if not await self._datastore.get_users_by_id_case_insensitive(
-                    UserID(localpart, self._hostname).to_string()
+                if not await self.store.get_users_by_id_case_insensitive(
+                    UserID(localpart, self.server_name).to_string()
                 ):
                     # This mxid is free
                     break
@@ -348,7 +318,6 @@ class SamlHandler:
                 )
 
             logger.info("Mapped SAML user to local part %s", localpart)
-
             registered_user_id = await self._registration_handler.register_user(
                 localpart=localpart,
                 default_display_name=displayname,
@@ -356,13 +325,13 @@ class SamlHandler:
                 user_agent_ips=(user_agent, ip_address),
             )
 
-            await self._datastore.record_user_external_id(
+            await self.store.record_user_external_id(
                 self._auth_provider_id, remote_user_id, registered_user_id
             )
             return registered_user_id
 
     def expire_sessions(self):
-        expire_before = self._clock.time_msec() - self._saml2_session_lifetime
+        expire_before = self.clock.time_msec() - self._saml2_session_lifetime
         to_expire = set()
         for reqid, data in self._outstanding_requests_dict.items():
             if data.creation_time < expire_before:
diff --git a/synapse/handlers/sso.py b/synapse/handlers/sso.py
new file mode 100644
index 0000000000..9cb1866a71
--- /dev/null
+++ b/synapse/handlers/sso.py
@@ -0,0 +1,90 @@
+# -*- coding: utf-8 -*-
+# Copyright 2020 The Matrix.org Foundation C.I.C.
+#
+# 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
+
+from synapse.handlers._base import BaseHandler
+from synapse.http.server import respond_with_html
+
+if TYPE_CHECKING:
+    from synapse.server import HomeServer
+
+logger = logging.getLogger(__name__)
+
+
+class MappingException(Exception):
+    """Used to catch errors when mapping the UserInfo object
+    """
+
+
+class SsoHandler(BaseHandler):
+    def __init__(self, hs: "HomeServer"):
+        super().__init__(hs)
+        self._error_template = hs.config.sso_error_template
+
+    def render_error(
+        self, request, error: str, error_description: Optional[str] = None
+    ) -> None:
+        """Renders the error template and responds with it.
+
+        This is used to show errors to the user. The template of this page can
+        be found under `synapse/res/templates/sso_error.html`.
+
+        Args:
+            request: The incoming request from the browser.
+                We'll respond with an HTML page describing the error.
+            error: A technical identifier for this error.
+            error_description: A human-readable description of the error.
+        """
+        html = self._error_template.render(
+            error=error, error_description=error_description
+        )
+        respond_with_html(request, 400, html)
+
+    async def get_sso_user_by_remote_user_id(
+        self, auth_provider_id: str, remote_user_id: str
+    ) -> Optional[str]:
+        """
+        Maps the user ID of a remote IdP to a mxid for a previously seen user.
+
+        If the user has not been seen yet, this will return None.
+
+        Args:
+            auth_provider_id: A unique identifier for this SSO provider, e.g.
+                "oidc" or "saml".
+            remote_user_id: The user ID according to the remote IdP. This might
+                be an e-mail address, a GUID, or some other form. It must be
+                unique and immutable.
+
+        Returns:
+            The mxid of a previously seen user.
+        """
+        # Check if we already have a mapping for this user.
+        logger.info(
+            "Looking for existing mapping for user %s:%s",
+            auth_provider_id,
+            remote_user_id,
+        )
+        previously_registered_user_id = await self.store.get_user_by_external_id(
+            auth_provider_id, remote_user_id,
+        )
+
+        # A match was found, return the user ID.
+        if previously_registered_user_id is not None:
+            logger.info("Found existing mapping %s", previously_registered_user_id)
+            return previously_registered_user_id
+
+        # No match.
+        return None
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/v1/room.py b/synapse/rest/client/v1/room.py
index 25d3cc6148..93c06afe27 100644
--- a/synapse/rest/client/v1/room.py
+++ b/synapse/rest/client/v1/room.py
@@ -18,7 +18,7 @@
 
 import logging
 import re
-from typing import List, Optional
+from typing import TYPE_CHECKING, List, Optional
 from urllib import parse as urlparse
 
 from synapse.api.constants import EventTypes, Membership
@@ -48,8 +48,7 @@ from synapse.types import RoomAlias, RoomID, StreamToken, ThirdPartyInstanceID,
 from synapse.util import json_decoder
 from synapse.util.stringutils import random_string
 
-MYPY = False
-if MYPY:
+if TYPE_CHECKING:
     import synapse.server
 
 logger = logging.getLogger(__name__)
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.py b/synapse/server.py
index 21a232bbd9..12a783de17 100644
--- a/synapse/server.py
+++ b/synapse/server.py
@@ -89,6 +89,7 @@ from synapse.handlers.room_member import RoomMemberMasterHandler
 from synapse.handlers.room_member_worker import RoomMemberWorkerHandler
 from synapse.handlers.search import SearchHandler
 from synapse.handlers.set_password import SetPasswordHandler
+from synapse.handlers.sso import SsoHandler
 from synapse.handlers.stats import StatsHandler
 from synapse.handlers.sync import SyncHandler
 from synapse.handlers.typing import FollowerTypingHandler, TypingWriterHandler
@@ -391,6 +392,10 @@ class HomeServer(metaclass=abc.ABCMeta):
             return FollowerTypingHandler(self)
 
     @cache_in_self
+    def get_sso_handler(self) -> SsoHandler:
+        return SsoHandler(self)
+
+    @cache_in_self
     def get_sync_handler(self) -> SyncHandler:
         return SyncHandler(self)
 
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_oidc.py b/tests/handlers/test_oidc.py
index 0d51705849..630e6da808 100644
--- a/tests/handlers/test_oidc.py
+++ b/tests/handlers/test_oidc.py
@@ -154,6 +154,9 @@ class OidcHandlerTestCase(HomeserverTestCase):
         )
 
         self.handler = OidcHandler(hs)
+        # Mock the render error method.
+        self.render_error = Mock(return_value=None)
+        self.handler._sso_handler.render_error = self.render_error
 
         return hs
 
@@ -161,12 +164,12 @@ class OidcHandlerTestCase(HomeserverTestCase):
         return patch.dict(self.handler._provider_metadata, values)
 
     def assertRenderedError(self, error, error_description=None):
-        args = self.handler._render_error.call_args[0]
+        args = self.render_error.call_args[0]
         self.assertEqual(args[1], error)
         if error_description is not None:
             self.assertEqual(args[2], error_description)
         # Reset the render_error mock
-        self.handler._render_error.reset_mock()
+        self.render_error.reset_mock()
 
     def test_config(self):
         """Basic config correctly sets up the callback URL and client auth correctly."""
@@ -356,7 +359,6 @@ class OidcHandlerTestCase(HomeserverTestCase):
 
     def test_callback_error(self):
         """Errors from the provider returned in the callback are displayed."""
-        self.handler._render_error = Mock()
         request = Mock(args={})
         request.args[b"error"] = [b"invalid_client"]
         self.get_success(self.handler.handle_oidc_callback(request))
@@ -387,7 +389,6 @@ class OidcHandlerTestCase(HomeserverTestCase):
             "preferred_username": "bar",
         }
         user_id = "@foo:domain.org"
-        self.handler._render_error = Mock(return_value=None)
         self.handler._exchange_code = simple_async_mock(return_value=token)
         self.handler._parse_id_token = simple_async_mock(return_value=userinfo)
         self.handler._fetch_userinfo = simple_async_mock(return_value=userinfo)
@@ -435,7 +436,7 @@ class OidcHandlerTestCase(HomeserverTestCase):
             userinfo, token, user_agent, ip_address
         )
         self.handler._fetch_userinfo.assert_not_called()
-        self.handler._render_error.assert_not_called()
+        self.render_error.assert_not_called()
 
         # Handle mapping errors
         self.handler._map_userinfo_to_user = simple_async_mock(
@@ -469,7 +470,7 @@ class OidcHandlerTestCase(HomeserverTestCase):
             userinfo, token, user_agent, ip_address
         )
         self.handler._fetch_userinfo.assert_called_once_with(token)
-        self.handler._render_error.assert_not_called()
+        self.render_error.assert_not_called()
 
         # Handle userinfo fetching error
         self.handler._fetch_userinfo = simple_async_mock(raises=Exception())
@@ -485,7 +486,6 @@ class OidcHandlerTestCase(HomeserverTestCase):
 
     def test_callback_session(self):
         """The callback verifies the session presence and validity"""
-        self.handler._render_error = Mock(return_value=None)
         request = Mock(spec=["args", "getCookie", "addCookie"])
 
         # Missing cookie
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 5699c41eb4..cc8a70be04 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
@@ -1543,3 +1543,244 @@ 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)