Allow admins to require a manual approval process before new accounts can be used (using MSC3866) (#13556)

changelog.d/13556.feature

@@ -0,0 +1 @@
+Allow server admins to require a manual approval process before new accounts can be used (using [MSC3866](https://github.com/matrix-org/matrix-spec-proposals/pull/3866)).

synapse/_scripts/synapse_port_db.py

@@ -107,7 +107,7 @@ BOOLEAN_COLUMNS = {
     "redactions": ["have_censored"],
     "room_stats_state": ["is_federatable"],
     "local_media_repository": ["safe_from_quarantine"],
-    "users": ["shadow_banned"],
+    "users": ["shadow_banned", "approved"],
     "e2e_fallback_keys_json": ["used"],
     "access_tokens": ["used"],
     "device_lists_changes_in_room": ["converted_to_destinations"],

synapse/api/constants.py

@@ -269,3 +269,14 @@ class PublicRoomsFilterFields:
 
     GENERIC_SEARCH_TERM: Final = "generic_search_term"
     ROOM_TYPES: Final = "room_types"
+
+
+class ApprovalNoticeMedium:
+    """Identifier for the medium this server will use to serve notice of approval for a
+    specific user's registration.
+
+    As defined in https://github.com/matrix-org/matrix-spec-proposals/blob/babolivier/m_not_approved/proposals/3866-user-not-approved-error.md
+    """
+
+    NONE = "org.matrix.msc3866.none"
+    EMAIL = "org.matrix.msc3866.email"

synapse/api/errors.py

@@ -106,6 +106,8 @@ class Codes(str, Enum):
     # Part of MSC3895.
     UNABLE_DUE_TO_PARTIAL_STATE = "ORG.MATRIX.MSC3895_UNABLE_DUE_TO_PARTIAL_STATE"
 
+    USER_AWAITING_APPROVAL = "ORG.MATRIX.MSC3866_USER_AWAITING_APPROVAL"
+
 
 class CodeMessageException(RuntimeError):
     """An exception with integer code and message string attributes.
@@ -566,6 +568,20 @@ class UnredactedContentDeletedError(SynapseError):
         return cs_error(self.msg, self.errcode, **extra)
 
 
+class NotApprovedError(SynapseError):
+    def __init__(
+        self,
+        msg: str,
+        approval_notice_medium: str,
+    ):
+        super().__init__(
+            code=403,
+            msg=msg,
+            errcode=Codes.USER_AWAITING_APPROVAL,
+            additional_fields={"approval_notice_medium": approval_notice_medium},
+        )
+
+
 def cs_error(msg: str, code: str = Codes.UNKNOWN, **kwargs: Any) -> "JsonDict":
     """Utility method for constructing an error response for client-server
     interactions.

synapse/config/experimental.py

@@ -14,10 +14,25 @@
 
 from typing import Any
 
+import attr
+
 from synapse.config._base import Config
 from synapse.types import JsonDict
 
 
+@attr.s(auto_attribs=True, frozen=True, slots=True)
+class MSC3866Config:
+    """Configuration for MSC3866 (mandating approval for new users)"""
+
+    # Whether the base support for the approval process is enabled. This includes the
+    # ability for administrators to check and update the approval of users, even if no
+    # approval is currently required.
+    enabled: bool = False
+    # Whether to require that new users are approved by an admin before their account
+    # can be used. Note that this setting is ignored if 'enabled' is false.
+    require_approval_for_new_accounts: bool = False
+
+
 class ExperimentalConfig(Config):
     """Config section for enabling experimental features"""
 
@@ -97,6 +112,10 @@ class ExperimentalConfig(Config):
         # MSC3852: Expose last seen user agent field on /_matrix/client/v3/devices.
         self.msc3852_enabled: bool = experimental.get("msc3852_enabled", False)
 
+        # MSC3866: M_USER_AWAITING_APPROVAL error code
+        raw_msc3866_config = experimental.get("msc3866", {})
+        self.msc3866 = MSC3866Config(**raw_msc3866_config)
+
         # MSC3881: Remotely toggle push notifications for another client
         self.msc3881_enabled: bool = experimental.get("msc3881_enabled", False)
 

synapse/handlers/admin.py

@@ -32,6 +32,7 @@ class AdminHandler:
         self.store = hs.get_datastores().main
         self._storage_controllers = hs.get_storage_controllers()
         self._state_storage_controller = self._storage_controllers.state
+        self._msc3866_enabled = hs.config.experimental.msc3866.enabled
 
     async def get_whois(self, user: UserID) -> JsonDict:
         connections = []
@@ -75,6 +76,10 @@ class AdminHandler:
             "is_guest",
         }
 
+        if self._msc3866_enabled:
+            # Only include the approved flag if support for MSC3866 is enabled.
+            user_info_to_return.add("approved")
+
         # Restrict returned keys to a known set.
         user_info_dict = {
             key: value

synapse/handlers/auth.py

@@ -1009,6 +1009,17 @@ class AuthHandler:
             return res[0]
         return None
 
+    async def is_user_approved(self, user_id: str) -> bool:
+        """Checks if a user is approved and therefore can be allowed to log in.
+
+        Args:
+            user_id: the user to check the approval status of.
+
+        Returns:
+            A boolean that is True if the user is approved, False otherwise.
+        """
+        return await self.store.is_user_approved(user_id)
+
     async def _find_user_id_and_pwd_hash(
         self, user_id: str
     ) -> Optional[Tuple[str, str]]:

synapse/handlers/register.py

@@ -220,6 +220,7 @@ class RegistrationHandler:
         by_admin: bool = False,
         user_agent_ips: Optional[List[Tuple[str, str]]] = None,
         auth_provider_id: Optional[str] = None,
+        approved: bool = False,
     ) -> str:
         """Registers a new client on the server.
 
@@ -246,6 +247,8 @@ class RegistrationHandler:
             user_agent_ips: Tuples of user-agents and IP addresses used
                 during the registration process.
             auth_provider_id: The SSO IdP the user used, if any.
+            approved: True if the new user should be considered already
+                approved by an administrator.
         Returns:
             The registered user_id.
         Raises:
@@ -307,6 +310,7 @@ class RegistrationHandler:
                 user_type=user_type,
                 address=address,
                 shadow_banned=shadow_banned,
+                approved=approved,
             )
 
             profile = await self.store.get_profileinfo(localpart)
@@ -695,6 +699,7 @@ class RegistrationHandler:
         user_type: Optional[str] = None,
         address: Optional[str] = None,
         shadow_banned: bool = False,
+        approved: bool = False,
     ) -> None:
         """Register user in the datastore.
 
@@ -713,6 +718,7 @@ class RegistrationHandler:
                 api.constants.UserTypes, or None for a normal user.
             address: the IP address used to perform the registration.
             shadow_banned: Whether to shadow-ban the user
+            approved: Whether to mark the user as approved by an administrator
         """
         if self.hs.config.worker.worker_app:
             await self._register_client(
@@ -726,6 +732,7 @@ class RegistrationHandler:
                 user_type=user_type,
                 address=address,
                 shadow_banned=shadow_banned,
+                approved=approved,
             )
         else:
             await self.store.register_user(
@@ -738,6 +745,7 @@ class RegistrationHandler:
                 admin=admin,
                 user_type=user_type,
                 shadow_banned=shadow_banned,
+                approved=approved,
             )
 
             # Only call the account validity module(s) on the main process, to avoid

synapse/replication/http/register.py

@@ -51,6 +51,7 @@ class ReplicationRegisterServlet(ReplicationEndpoint):
         user_type: Optional[str],
         address: Optional[str],
         shadow_banned: bool,
+        approved: bool,
     ) -> JsonDict:
         """
         Args:
@@ -68,6 +69,8 @@ class ReplicationRegisterServlet(ReplicationEndpoint):
                 or None for a normal user.
             address: the IP address used to perform the regitration.
             shadow_banned: Whether to shadow-ban the user
+            approved: Whether the user should be considered already approved by an
+                administrator.
         """
         return {
             "password_hash": password_hash,
@@ -79,6 +82,7 @@ class ReplicationRegisterServlet(ReplicationEndpoint):
             "user_type": user_type,
             "address": address,
             "shadow_banned": shadow_banned,
+            "approved": approved,
         }
 
     async def _handle_request(  # type: ignore[override]
@@ -99,6 +103,7 @@ class ReplicationRegisterServlet(ReplicationEndpoint):
             user_type=content["user_type"],
             address=content["address"],
             shadow_banned=content["shadow_banned"],
+            approved=content["approved"],
         )
 
         return 200, {}

synapse/rest/admin/users.py

@@ -69,6 +69,7 @@ class UsersRestServletV2(RestServlet):
         self.store = hs.get_datastores().main
         self.auth = hs.get_auth()
         self.admin_handler = hs.get_admin_handler()
+        self._msc3866_enabled = hs.config.experimental.msc3866.enabled
 
     async def on_GET(self, request: SynapseRequest) -> Tuple[int, JsonDict]:
         await assert_requester_is_admin(self.auth, request)
@@ -95,6 +96,13 @@ class UsersRestServletV2(RestServlet):
         guests = parse_boolean(request, "guests", default=True)
         deactivated = parse_boolean(request, "deactivated", default=False)
 
+        # If support for MSC3866 is not enabled, apply no filtering based on the
+        # `approved` column.
+        if self._msc3866_enabled:
+            approved = parse_boolean(request, "approved", default=True)
+        else:
+            approved = True
+
         order_by = parse_string(
             request,
             "order_by",
@@ -115,8 +123,22 @@ class UsersRestServletV2(RestServlet):
         direction = parse_string(request, "dir", default="f", allowed_values=("f", "b"))
 
         users, total = await self.store.get_users_paginate(
-            start, limit, user_id, name, guests, deactivated, order_by, direction
+            start,
+            limit,
+            user_id,
+            name,
+            guests,
+            deactivated,
+            order_by,
+            direction,
+            approved,
         )
+
+        # If support for MSC3866 is not enabled, don't show the approval flag.
+        if not self._msc3866_enabled:
+            for user in users:
+                del user["approved"]
+
         ret = {"users": users, "total": total}
         if (start + limit) < total:
             ret["next_token"] = str(start + len(users))
@@ -163,6 +185,7 @@ class UserRestServletV2(RestServlet):
         self.deactivate_account_handler = hs.get_deactivate_account_handler()
         self.registration_handler = hs.get_registration_handler()
         self.pusher_pool = hs.get_pusherpool()
+        self._msc3866_enabled = hs.config.experimental.msc3866.enabled
 
     async def on_GET(
         self, request: SynapseRequest, user_id: str
@@ -239,6 +262,15 @@ class UserRestServletV2(RestServlet):
                 HTTPStatus.BAD_REQUEST, "'deactivated' parameter is not of type boolean"
             )
 
+        approved: Optional[bool] = None
+        if "approved" in body and self._msc3866_enabled:
+            approved = body["approved"]
+            if not isinstance(approved, bool):
+                raise SynapseError(
+                    HTTPStatus.BAD_REQUEST,
+                    "'approved' parameter is not of type boolean",
+                )
+
         # convert List[Dict[str, str]] into List[Tuple[str, str]]
         if external_ids is not None:
             new_external_ids = [
@@ -343,6 +375,9 @@ class UserRestServletV2(RestServlet):
             if "user_type" in body:
                 await self.store.set_user_type(target_user, user_type)
 
+            if approved is not None:
+                await self.store.update_user_approval_status(target_user, approved)
+
             user = await self.admin_handler.get_user(target_user)
             assert user is not None
 
@@ -355,6 +390,10 @@ class UserRestServletV2(RestServlet):
             if password is not None:
                 password_hash = await self.auth_handler.hash(password)
 
+            new_user_approved = True
+            if self._msc3866_enabled and approved is not None:
+                new_user_approved = approved
+
             user_id = await self.registration_handler.register_user(
                 localpart=target_user.localpart,
                 password_hash=password_hash,
@@ -362,6 +401,7 @@ class UserRestServletV2(RestServlet):
                 default_display_name=displayname,
                 user_type=user_type,
                 by_admin=True,
+                approved=new_user_approved,
             )
 
             if threepids is not None:
@@ -550,6 +590,7 @@ class UserRegisterServlet(RestServlet):
             user_type=user_type,
             default_display_name=displayname,
             by_admin=True,
+            approved=True,
         )
 
         result = await register._create_registration_details(user_id, body)

synapse/rest/client/login.py

@@ -28,7 +28,14 @@ from typing import (
 
 from typing_extensions import TypedDict
 
-from synapse.api.errors import Codes, InvalidClientTokenError, LoginError, SynapseError
+from synapse.api.constants import ApprovalNoticeMedium
+from synapse.api.errors import (
+    Codes,
+    InvalidClientTokenError,
+    LoginError,
+    NotApprovedError,
+    SynapseError,
+)
 from synapse.api.ratelimiting import Ratelimiter
 from synapse.api.urls import CLIENT_API_PREFIX
 from synapse.appservice import ApplicationService
@@ -55,11 +62,11 @@ logger = logging.getLogger(__name__)
 
 class LoginResponse(TypedDict, total=False):
     user_id: str
-    access_token: str
+    access_token: Optional[str]
     home_server: str
     expires_in_ms: Optional[int]
     refresh_token: Optional[str]
-    device_id: str
+    device_id: Optional[str]
     well_known: Optional[Dict[str, Any]]
 
 
@@ -92,6 +99,12 @@ class LoginRestServlet(RestServlet):
             hs.config.registration.refreshable_access_token_lifetime is not None
         )
 
+        # Whether we need to check if the user has been approved or not.
+        self._require_approval = (
+            hs.config.experimental.msc3866.enabled
+            and hs.config.experimental.msc3866.require_approval_for_new_accounts
+        )
+
         self.auth = hs.get_auth()
 
         self.clock = hs.get_clock()
@@ -220,6 +233,14 @@ class LoginRestServlet(RestServlet):
         except KeyError:
             raise SynapseError(400, "Missing JSON keys.")
 
+        if self._require_approval:
+            approved = await self.auth_handler.is_user_approved(result["user_id"])
+            if not approved:
+                raise NotApprovedError(
+                    msg="This account is pending approval by a server administrator.",
+                    approval_notice_medium=ApprovalNoticeMedium.NONE,
+                )
+
         well_known_data = self._well_known_builder.get_well_known()
         if well_known_data:
             result["well_known"] = well_known_data
@@ -356,6 +377,16 @@ class LoginRestServlet(RestServlet):
                 errcode=Codes.INVALID_PARAM,
             )
 
+        if self._require_approval:
+            approved = await self.auth_handler.is_user_approved(user_id)
+            if not approved:
+                # If the user isn't approved (and needs to be) we won't allow them to
+                # actually log in, so we don't want to create a device/access token.
+                return LoginResponse(
+                    user_id=user_id,
+                    home_server=self.hs.hostname,
+                )
+
         initial_display_name = login_submission.get("initial_device_display_name")
         (
             device_id,

synapse/rest/client/register.py

@@ -21,10 +21,15 @@ from twisted.web.server import Request
 import synapse
 import synapse.api.auth
 import synapse.types
-from synapse.api.constants import APP_SERVICE_REGISTRATION_TYPE, LoginType
+from synapse.api.constants import (
+    APP_SERVICE_REGISTRATION_TYPE,
+    ApprovalNoticeMedium,
+    LoginType,
+)
 from synapse.api.errors import (
     Codes,
     InteractiveAuthIncompleteError,
+    NotApprovedError,
     SynapseError,
     ThreepidValidationError,
     UnrecognizedRequestError,
@@ -414,6 +419,11 @@ class RegisterRestServlet(RestServlet):
             hs.config.registration.inhibit_user_in_use_error
         )
 
+        self._require_approval = (
+            hs.config.experimental.msc3866.enabled
+            and hs.config.experimental.msc3866.require_approval_for_new_accounts
+        )
+
         self._registration_flows = _calculate_registration_flows(
             hs.config, self.auth_handler
         )
@@ -734,6 +744,12 @@ class RegisterRestServlet(RestServlet):
                 access_token=return_dict.get("access_token"),
             )
 
+            if self._require_approval:
+                raise NotApprovedError(
+                    msg="This account needs to be approved by an administrator before it can be used.",
+                    approval_notice_medium=ApprovalNoticeMedium.NONE,
+                )
+
         return 200, return_dict
 
     async def _do_appservice_registration(
@@ -778,7 +794,9 @@ class RegisterRestServlet(RestServlet):
             "user_id": user_id,
             "home_server": self.hs.hostname,
         }
-        if not params.get("inhibit_login", False):
+        # We don't want to log the user in if we're going to deny them access because
+        # they need to be approved first.
+        if not params.get("inhibit_login", False) and not self._require_approval:
             device_id = params.get("device_id")
             initial_display_name = params.get("initial_device_display_name")
             (

synapse/storage/databases/main/__init__.py

@@ -203,6 +203,7 @@ class DataStore(
         deactivated: bool = False,
         order_by: str = UserSortOrder.USER_ID.value,
         direction: str = "f",
+        approved: bool = True,
     ) -> Tuple[List[JsonDict], int]:
         """Function to retrieve a paginated list of users from
         users list. This will return a json list of users and the
@@ -217,6 +218,7 @@ class DataStore(
             deactivated: whether to include deactivated users
             order_by: the sort order of the returned list
             direction: sort ascending or descending
+            approved: whether to include approved users
         Returns:
             A tuple of a list of mappings from user to information and a count of total users.
         """
@@ -249,6 +251,11 @@ class DataStore(
             if not deactivated:
                 filters.append("deactivated = 0")
 
+            if not approved:
+                # We ignore NULL values for the approved flag because these should only
+                # be already existing users that we consider as already approved.
+                filters.append("approved IS FALSE")
+
             where_clause = "WHERE " + " AND ".join(filters) if len(filters) > 0 else ""
 
             sql_base = f"""
@@ -262,7 +269,7 @@ class DataStore(
 
             sql = f"""
                 SELECT name, user_type, is_guest, admin, deactivated, shadow_banned,
-                displayname, avatar_url, creation_ts * 1000 as creation_ts
+                displayname, avatar_url, creation_ts * 1000 as creation_ts, approved
                 {sql_base}
                 ORDER BY {order_by_column} {order}, u.name ASC
                 LIMIT ? OFFSET ?

synapse/storage/databases/main/registration.py

@@ -166,27 +166,49 @@ class RegistrationWorkerStore(CacheInvalidationWorkerStore):
     @cached()
     async def get_user_by_id(self, user_id: str) -> Optional[Dict[str, Any]]:
         """Deprecated: use get_userinfo_by_id instead"""
-        return await self.db_pool.simple_select_one(
-            table="users",
-            keyvalues={"name": user_id},
-            retcols=[
-                "name",
-                "password_hash",
-                "is_guest",
-                "admin",
-                "consent_version",
-                "consent_ts",
-                "consent_server_notice_sent",
-                "appservice_id",
-                "creation_ts",
-                "user_type",
-                "deactivated",
-                "shadow_banned",
-            ],
-            allow_none=True,
-            desc="get_user_by_id",
+
+        def get_user_by_id_txn(txn: LoggingTransaction) -> Optional[Dict[str, Any]]:
+            # We could technically use simple_select_one here, but it would not perform
+            # the COALESCEs (unless hacked into the column names), which could yield
+            # confusing results.
+            txn.execute(
+                """
+                SELECT
+                    name, password_hash, is_guest, admin, consent_version, consent_ts,
+                    consent_server_notice_sent, appservice_id, creation_ts, user_type,
+                    deactivated, COALESCE(shadow_banned, FALSE) AS shadow_banned,
+                    COALESCE(approved, TRUE) AS approved
+                FROM users
+                WHERE name = ?
+                """,
+                (user_id,),
             )
 
+            rows = self.db_pool.cursor_to_dict(txn)
+
+            if len(rows) == 0:
+                return None
+
+            return rows[0]
+
+        row = await self.db_pool.runInteraction(
+            desc="get_user_by_id",
+            func=get_user_by_id_txn,
+        )
+
+        if row is not None:
+            # If we're using SQLite our boolean values will be integers. Because we
+            # present some of this data as is to e.g. server admins via REST APIs, we
+            # want to make sure we're returning the right type of data.
+            # Note: when adding a column name to this list, be wary of NULLable columns,
+            # since NULL values will be turned into False.
+            boolean_columns = ["admin", "deactivated", "shadow_banned", "approved"]
+            for column in boolean_columns:
+                if not isinstance(row[column], bool):
+                    row[column] = bool(row[column])
+
+        return row
+
     async def get_userinfo_by_id(self, user_id: str) -> Optional[UserInfo]:
         """Get a UserInfo object for a user by user ID.
 
@@ -1779,6 +1801,40 @@ class RegistrationWorkerStore(CacheInvalidationWorkerStore):
 
         return res if res else False
 
+    @cached()
+    async def is_user_approved(self, user_id: str) -> bool:
+        """Checks if a user is approved and therefore can be allowed to log in.
+
+        If the user's 'approved' column is NULL, we consider it as true given it means
+        the user was registered when support for an approval flow was either disabled
+        or nonexistent.
+
+        Args:
+            user_id: the user to check the approval status of.
+
+        Returns:
+            A boolean that is True if the user is approved, False otherwise.
+        """
+
+        def is_user_approved_txn(txn: LoggingTransaction) -> bool:
+            txn.execute(
+                """
+                SELECT COALESCE(approved, TRUE) AS approved FROM users WHERE name = ?
+                """,
+                (user_id,),
+            )
+
+            rows = self.db_pool.cursor_to_dict(txn)
+
+            # We cast to bool because the value returned by the database engine might
+            # be an integer if we're using SQLite.
+            return bool(rows[0]["approved"])
+
+        return await self.db_pool.runInteraction(
+            desc="is_user_pending_approval",
+            func=is_user_approved_txn,
+        )
+
 
 class RegistrationBackgroundUpdateStore(RegistrationWorkerStore):
     def __init__(
@@ -1916,6 +1972,29 @@ class RegistrationBackgroundUpdateStore(RegistrationWorkerStore):
         self._invalidate_cache_and_stream(txn, self.get_user_by_id, (user_id,))
         txn.call_after(self.is_guest.invalidate, (user_id,))
 
+    def update_user_approval_status_txn(
+        self, txn: LoggingTransaction, user_id: str, approved: bool
+    ) -> None:
+        """Set the user's 'approved' flag to the given value.
+
+        The boolean is turned into an int because the column is a smallint.
+
+        Args:
+            txn: the current database transaction.
+            user_id: the user to update the flag for.
+            approved: the value to set the flag to.
+        """
+        self.db_pool.simple_update_one_txn(
+            txn=txn,
+            table="users",
+            keyvalues={"name": user_id},
+            updatevalues={"approved": approved},
+        )
+
+        # Invalidate the caches of methods that read the value of the 'approved' flag.
+        self._invalidate_cache_and_stream(txn, self.get_user_by_id, (user_id,))
+        self._invalidate_cache_and_stream(txn, self.is_user_approved, (user_id,))
+
 
 class RegistrationStore(StatsStore, RegistrationBackgroundUpdateStore):
     def __init__(
@@ -1933,6 +2012,13 @@ class RegistrationStore(StatsStore, RegistrationBackgroundUpdateStore):
         self._access_tokens_id_gen = IdGenerator(db_conn, "access_tokens", "id")
         self._refresh_tokens_id_gen = IdGenerator(db_conn, "refresh_tokens", "id")
 
+        # If support for MSC3866 is enabled and configured to require approval for new
+        # account, we will create new users with an 'approved' flag set to false.
+        self._require_approval = (
+            hs.config.experimental.msc3866.enabled
+            and hs.config.experimental.msc3866.require_approval_for_new_accounts
+        )
+
     async def add_access_token_to_user(
         self,
         user_id: str,
@@ -2065,6 +2151,7 @@ class RegistrationStore(StatsStore, RegistrationBackgroundUpdateStore):
         admin: bool = False,
         user_type: Optional[str] = None,
         shadow_banned: bool = False,
+        approved: bool = False,
     ) -> None:
         """Attempts to register an account.
 
@@ -2083,6 +2170,8 @@ class RegistrationStore(StatsStore, RegistrationBackgroundUpdateStore):
                 or None for a normal user.
             shadow_banned: Whether the user is shadow-banned, i.e. they may be
                 told their requests succeeded but we ignore them.
+            approved: Whether to consider the user has already been approved by an
+                administrator.
 
         Raises:
             StoreError if the user_id could not be registered.
@@ -2099,6 +2188,7 @@ class RegistrationStore(StatsStore, RegistrationBackgroundUpdateStore):
             admin,
             user_type,
             shadow_banned,
+            approved,
         )
 
     def _register_user(
@@ -2113,11 +2203,14 @@ class RegistrationStore(StatsStore, RegistrationBackgroundUpdateStore):
         admin: bool,
         user_type: Optional[str],
         shadow_banned: bool,
+        approved: bool,
     ) -> None:
         user_id_obj = UserID.from_string(user_id)
 
         now = int(self._clock.time())
 
+        user_approved = approved or not self._require_approval
+
         try:
             if was_guest:
                 # Ensure that the guest user actually exists
@@ -2143,6 +2236,7 @@ class RegistrationStore(StatsStore, RegistrationBackgroundUpdateStore):
                         "admin": 1 if admin else 0,
                         "user_type": user_type,
                         "shadow_banned": shadow_banned,
+                        "approved": user_approved,
                     },
                 )
             else:
@@ -2158,6 +2252,7 @@ class RegistrationStore(StatsStore, RegistrationBackgroundUpdateStore):
                         "admin": 1 if admin else 0,
                         "user_type": user_type,
                         "shadow_banned": shadow_banned,
+                        "approved": user_approved,
                     },
                 )
 
@@ -2503,6 +2598,25 @@ class RegistrationStore(StatsStore, RegistrationBackgroundUpdateStore):
             start_or_continue_validation_session_txn,
         )
 
+    async def update_user_approval_status(
+        self, user_id: UserID, approved: bool
+    ) -> None:
+        """Set the user's 'approved' flag to the given value.
+
+        The boolean will be turned into an int (in update_user_approval_status_txn)
+        because the column is a smallint.
+
+        Args:
+            user_id: the user to update the flag for.
+            approved: the value to set the flag to.
+        """
+        await self.db_pool.runInteraction(
+            "update_user_approval_status",
+            self.update_user_approval_status_txn,
+            user_id.to_string(),
+            approved,
+        )
+
 
 def find_max_generated_user_id_localpart(cur: Cursor) -> int:
     """

synapse/storage/schema/main/delta/73/03users_approved_column.sql

@@ -0,0 +1,20 @@
+/* Copyright 2022 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.
+ */
+
+-- Add a column to the users table to track whether the user needs to be approved by an
+-- administrator.
+-- A NULL column means the user was created before this feature was supported by Synapse,
+-- and should be considered as TRUE.
+ALTER TABLE users ADD COLUMN approved BOOLEAN;

tests/rest/admin/test_user.py

@@ -25,10 +25,10 @@ from parameterized import parameterized, parameterized_class
 from twisted.test.proto_helpers import MemoryReactor
 
 import synapse.rest.admin
-from synapse.api.constants import UserTypes
+from synapse.api.constants import ApprovalNoticeMedium, LoginType, UserTypes
 from synapse.api.errors import Codes, HttpResponseException, ResourceLimitError
 from synapse.api.room_versions import RoomVersions
-from synapse.rest.client import devices, login, logout, profile, room, sync
+from synapse.rest.client import devices, login, logout, profile, register, room, sync
 from synapse.rest.media.v1.filepath import MediaFilePaths
 from synapse.server import HomeServer
 from synapse.types import JsonDict, UserID
@@ -578,6 +578,16 @@ class UsersListTestCase(unittest.HomeserverTestCase):
         _search_test(None, "foo", "user_id")
         _search_test(None, "bar", "user_id")
 
+    @override_config(
+        {
+            "experimental_features": {
+                "msc3866": {
+                    "enabled": True,
+                    "require_approval_for_new_accounts": True,
+                }
+            }
+        }
+    )
     def test_invalid_parameter(self) -> None:
         """
         If parameters are invalid, an error is returned.
@@ -623,6 +633,16 @@ class UsersListTestCase(unittest.HomeserverTestCase):
         self.assertEqual(400, channel.code, msg=channel.json_body)
         self.assertEqual(Codes.INVALID_PARAM, channel.json_body["errcode"])
 
+        # invalid approved
+        channel = self.make_request(
+            "GET",
+            self.url + "?approved=not_bool",
+            access_token=self.admin_user_tok,
+        )
+
+        self.assertEqual(400, channel.code, msg=channel.json_body)
+        self.assertEqual(Codes.INVALID_PARAM, channel.json_body["errcode"])
+
         # unkown order_by
         channel = self.make_request(
             "GET",
@@ -841,6 +861,69 @@ class UsersListTestCase(unittest.HomeserverTestCase):
         self._order_test([self.admin_user, user1, user2], "creation_ts", "f")
         self._order_test([user2, user1, self.admin_user], "creation_ts", "b")
 
+    @override_config(
+        {
+            "experimental_features": {
+                "msc3866": {
+                    "enabled": True,
+                    "require_approval_for_new_accounts": True,
+                }
+            }
+        }
+    )
+    def test_filter_out_approved(self) -> None:
+        """Tests that the endpoint can filter out approved users."""
+        # Create our users.
+        self._create_users(2)
+
+        # Get the list of users.
+        channel = self.make_request(
+            "GET",
+            self.url,
+            access_token=self.admin_user_tok,
+        )
+        self.assertEqual(200, channel.code, channel.result)
+
+        # Exclude the admin, because we don't want to accidentally un-approve the admin.
+        non_admin_user_ids = [
+            user["name"]
+            for user in channel.json_body["users"]
+            if user["name"] != self.admin_user
+        ]
+
+        self.assertEqual(2, len(non_admin_user_ids), non_admin_user_ids)
+
+        # Select a user and un-approve them. We do this rather than the other way around
+        # because, since these users are created by an admin, we consider them already
+        # approved.
+        not_approved_user = non_admin_user_ids[0]
+
+        channel = self.make_request(
+            "PUT",
+            f"/_synapse/admin/v2/users/{not_approved_user}",
+            {"approved": False},
+            access_token=self.admin_user_tok,
+        )
+        self.assertEqual(200, channel.code, channel.result)
+
+        # Now get the list of users again, this time filtering out approved users.
+        channel = self.make_request(
+            "GET",
+            self.url + "?approved=false",
+            access_token=self.admin_user_tok,
+        )
+        self.assertEqual(200, channel.code, channel.result)
+
+        non_admin_user_ids = [
+            user["name"]
+            for user in channel.json_body["users"]
+            if user["name"] != self.admin_user
+        ]
+
+        # We should only have our unapproved user now.
+        self.assertEqual(1, len(non_admin_user_ids), non_admin_user_ids)
+        self.assertEqual(not_approved_user, non_admin_user_ids[0])
+
     def _order_test(
         self,
         expected_user_list: List[str],
@@ -1272,6 +1355,7 @@ class UserRestTestCase(unittest.HomeserverTestCase):
         synapse.rest.admin.register_servlets,
         login.register_servlets,
         sync.register_servlets,
+        register.register_servlets,
     ]
 
     def prepare(self, reactor: MemoryReactor, clock: Clock, hs: HomeServer) -> None:
@@ -2536,6 +2620,104 @@ class UserRestTestCase(unittest.HomeserverTestCase):
         # Ensure they're still alive
         self.assertEqual(0, channel.json_body["deactivated"])
 
+    @override_config(
+        {
+            "experimental_features": {
+                "msc3866": {
+                    "enabled": True,
+                    "require_approval_for_new_accounts": True,
+                }
+            }
+        }
+    )
+    def test_approve_account(self) -> None:
+        """Tests that approving an account correctly sets the approved flag for the user."""
+        url = self.url_prefix % "@bob:test"
+
+        # Create the user using the client-server API since otherwise the user will be
+        # marked as approved automatically.
+        channel = self.make_request(
+            "POST",
+            "register",
+            {
+                "username": "bob",
+                "password": "test",
+                "auth": {"type": LoginType.DUMMY},
+            },
+        )
+        self.assertEqual(403, channel.code, channel.result)
+        self.assertEqual(Codes.USER_AWAITING_APPROVAL, channel.json_body["errcode"])
+        self.assertEqual(
+            ApprovalNoticeMedium.NONE, channel.json_body["approval_notice_medium"]
+        )
+
+        # Get user
+        channel = self.make_request(
+            "GET",
+            url,
+            access_token=self.admin_user_tok,
+        )
+
+        self.assertEqual(200, channel.code, msg=channel.json_body)
+        self.assertIs(False, channel.json_body["approved"])
+
+        # Approve user
+        channel = self.make_request(
+            "PUT",
+            url,
+            access_token=self.admin_user_tok,
+            content={"approved": True},
+        )
+
+        self.assertEqual(200, channel.code, msg=channel.json_body)
+        self.assertIs(True, channel.json_body["approved"])
+
+        # Check that the user is now approved
+        channel = self.make_request(
+            "GET",
+            url,
+            access_token=self.admin_user_tok,
+        )
+
+        self.assertEqual(200, channel.code, msg=channel.json_body)
+        self.assertIs(True, channel.json_body["approved"])
+
+    @override_config(
+        {
+            "experimental_features": {
+                "msc3866": {
+                    "enabled": True,
+                    "require_approval_for_new_accounts": True,
+                }
+            }
+        }
+    )
+    def test_register_approved(self) -> None:
+        url = self.url_prefix % "@bob:test"
+
+        # Create user
+        channel = self.make_request(
+            "PUT",
+            url,
+            access_token=self.admin_user_tok,
+            content={"password": "abc123", "approved": True},
+        )
+
+        self.assertEqual(201, channel.code, msg=channel.json_body)
+        self.assertEqual("@bob:test", channel.json_body["name"])
+        self.assertEqual(1, channel.json_body["approved"])
+
+        # Get user
+        channel = self.make_request(
+            "GET",
+            url,
+            access_token=self.admin_user_tok,
+        )
+
+        self.assertEqual(200, channel.code, msg=channel.json_body)
+        self.assertEqual("@bob:test", channel.json_body["name"])
+        self.assertEqual(1, channel.json_body["approved"])
+
     def _is_erased(self, user_id: str, expect: bool) -> None:
         """Assert that the user is erased or not"""
         d = self.store.is_user_erased(user_id)

tests/rest/client/test_auth.py

@@ -20,7 +20,8 @@ from twisted.test.proto_helpers import MemoryReactor
 from twisted.web.resource import Resource
 
 import synapse.rest.admin
-from synapse.api.constants import LoginType
+from synapse.api.constants import ApprovalNoticeMedium, LoginType
+from synapse.api.errors import Codes
 from synapse.handlers.ui_auth.checkers import UserInteractiveAuthChecker
 from synapse.rest.client import account, auth, devices, login, logout, register
 from synapse.rest.synapse.client import build_synapse_client_resource_tree
@@ -567,6 +568,36 @@ class UIAuthTests(unittest.HomeserverTestCase):
             body={"auth": {"session": session_id}},
         )
 
+    @skip_unless(HAS_OIDC, "requires OIDC")
+    @override_config(
+        {
+            "oidc_config": TEST_OIDC_CONFIG,
+            "experimental_features": {
+                "msc3866": {
+                    "enabled": True,
+                    "require_approval_for_new_accounts": True,
+                }
+            },
+        }
+    )
+    def test_sso_not_approved(self) -> None:
+        """Tests that if we register a user via SSO while requiring approval for new
+        accounts, we still raise the correct error before logging the user in.
+        """
+        login_resp = self.helper.login_via_oidc("username", expected_status=403)
+
+        self.assertEqual(login_resp["errcode"], Codes.USER_AWAITING_APPROVAL)
+        self.assertEqual(
+            ApprovalNoticeMedium.NONE, login_resp["approval_notice_medium"]
+        )
+
+        # Check that we didn't register a device for the user during the login attempt.
+        devices = self.get_success(
+            self.hs.get_datastores().main.get_devices_by_user("@username:test")
+        )
+
+        self.assertEqual(len(devices), 0)
+
 
 class RefreshAuthTests(unittest.HomeserverTestCase):
     servlets = [

tests/rest/client/test_login.py

@@ -23,6 +23,8 @@ from twisted.test.proto_helpers import MemoryReactor
 from twisted.web.resource import Resource
 
 import synapse.rest.admin
+from synapse.api.constants import ApprovalNoticeMedium, LoginType
+from synapse.api.errors import Codes
 from synapse.appservice import ApplicationService
 from synapse.rest.client import devices, login, logout, register
 from synapse.rest.client.account import WhoamiRestServlet
@@ -94,6 +96,7 @@ class LoginRestServletTestCase(unittest.HomeserverTestCase):
         logout.register_servlets,
         devices.register_servlets,
         lambda hs, http_server: WhoamiRestServlet(hs).register(http_server),
+        register.register_servlets,
     ]
 
     def make_homeserver(self, reactor: MemoryReactor, clock: Clock) -> HomeServer:
@@ -406,6 +409,44 @@ class LoginRestServletTestCase(unittest.HomeserverTestCase):
         self.assertEqual(channel.code, 400)
         self.assertEqual(channel.json_body["errcode"], "M_INVALID_PARAM")
 
+    @override_config(
+        {
+            "experimental_features": {
+                "msc3866": {
+                    "enabled": True,
+                    "require_approval_for_new_accounts": True,
+                }
+            }
+        }
+    )
+    def test_require_approval(self) -> None:
+        channel = self.make_request(
+            "POST",
+            "register",
+            {
+                "username": "kermit",
+                "password": "monkey",
+                "auth": {"type": LoginType.DUMMY},
+            },
+        )
+        self.assertEqual(403, channel.code, channel.result)
+        self.assertEqual(Codes.USER_AWAITING_APPROVAL, channel.json_body["errcode"])
+        self.assertEqual(
+            ApprovalNoticeMedium.NONE, channel.json_body["approval_notice_medium"]
+        )
+
+        params = {
+            "type": LoginType.PASSWORD,
+            "identifier": {"type": "m.id.user", "user": "kermit"},
+            "password": "monkey",
+        }
+        channel = self.make_request("POST", LOGIN_URL, params)
+        self.assertEqual(403, channel.code, channel.result)
+        self.assertEqual(Codes.USER_AWAITING_APPROVAL, channel.json_body["errcode"])
+        self.assertEqual(
+            ApprovalNoticeMedium.NONE, channel.json_body["approval_notice_medium"]
+        )
+
 
 @skip_unless(has_saml2 and HAS_OIDC, "Requires SAML2 and OIDC")
 class MultiSSOTestCase(unittest.HomeserverTestCase):

tests/rest/client/test_register.py

@@ -22,7 +22,11 @@ import pkg_resources
 from twisted.test.proto_helpers import MemoryReactor
 
 import synapse.rest.admin
-from synapse.api.constants import APP_SERVICE_REGISTRATION_TYPE, LoginType
+from synapse.api.constants import (
+    APP_SERVICE_REGISTRATION_TYPE,
+    ApprovalNoticeMedium,
+    LoginType,
+)
 from synapse.api.errors import Codes
 from synapse.appservice import ApplicationService
 from synapse.rest.client import account, account_validity, login, logout, register, sync
@@ -765,6 +769,32 @@ class RegisterRestServletTestCase(unittest.HomeserverTestCase):
         self.assertEqual(channel.code, 400, channel.json_body)
         self.assertEqual(channel.json_body["errcode"], Codes.USER_IN_USE)
 
+    @override_config(
+        {
+            "experimental_features": {
+                "msc3866": {
+                    "enabled": True,
+                    "require_approval_for_new_accounts": True,
+                }
+            }
+        }
+    )
+    def test_require_approval(self) -> None:
+        channel = self.make_request(
+            "POST",
+            "register",
+            {
+                "username": "kermit",
+                "password": "monkey",
+                "auth": {"type": LoginType.DUMMY},
+            },
+        )
+        self.assertEqual(403, channel.code, channel.result)
+        self.assertEqual(Codes.USER_AWAITING_APPROVAL, channel.json_body["errcode"])
+        self.assertEqual(
+            ApprovalNoticeMedium.NONE, channel.json_body["approval_notice_medium"]
+        )
+
 
 class AccountValidityTestCase(unittest.HomeserverTestCase):
 

tests/rest/client/utils.py

@@ -543,8 +543,12 @@ class RestHelper:
 
         return channel.json_body
 
-    def login_via_oidc(self, remote_user_id: str) -> JsonDict:
-        """Log in (as a new user) via OIDC
+    def login_via_oidc(
+        self,
+        remote_user_id: str,
+        expected_status: int = 200,
+    ) -> JsonDict:
+        """Log in via OIDC
 
         Returns the result of the final token login.
 
@@ -578,7 +582,9 @@ class RestHelper:
             "/login",
             content={"type": "m.login.token", "token": login_token},
         )
-        assert channel.code == HTTPStatus.OK
+        assert (
+            channel.code == expected_status
+        ), f"unexpected status in response: {channel.code}"
         return channel.json_body
 
     def auth_via_oidc(

tests/storage/test_registration.py

@@ -16,9 +16,10 @@ from twisted.test.proto_helpers import MemoryReactor
 from synapse.api.constants import UserTypes
 from synapse.api.errors import ThreepidValidationError
 from synapse.server import HomeServer
+from synapse.types import JsonDict, UserID
 from synapse.util import Clock
 
-from tests.unittest import HomeserverTestCase
+from tests.unittest import HomeserverTestCase, override_config
 
 
 class RegistrationStoreTestCase(HomeserverTestCase):
@@ -48,6 +49,7 @@ class RegistrationStoreTestCase(HomeserverTestCase):
                 "user_type": None,
                 "deactivated": 0,
                 "shadow_banned": 0,
+                "approved": 1,
             },
             (self.get_success(self.store.get_user_by_id(self.user_id))),
         )
@@ -166,3 +168,101 @@ class RegistrationStoreTestCase(HomeserverTestCase):
             ThreepidValidationError,
         )
         self.assertEqual(e.value.msg, "Validation token not found or has expired", e)
+
+
+class ApprovalRequiredRegistrationTestCase(HomeserverTestCase):
+    def default_config(self) -> JsonDict:
+        config = super().default_config()
+
+        # If there's already some config for this feature in the default config, it
+        # means we're overriding it with @override_config. In this case we don't want
+        # to do anything more with it.
+        msc3866_config = config.get("experimental_features", {}).get("msc3866")
+        if msc3866_config is not None:
+            return config
+
+        # Require approval for all new accounts.
+        config["experimental_features"] = {
+            "msc3866": {
+                "enabled": True,
+                "require_approval_for_new_accounts": True,
+            }
+        }
+        return config
+
+    def prepare(self, reactor: MemoryReactor, clock: Clock, hs: HomeServer) -> None:
+        self.store = hs.get_datastores().main
+        self.user_id = "@my-user:test"
+        self.pwhash = "{xx1}123456789"
+
+    @override_config(
+        {
+            "experimental_features": {
+                "msc3866": {
+                    "enabled": True,
+                    "require_approval_for_new_accounts": False,
+                }
+            }
+        }
+    )
+    def test_approval_not_required(self) -> None:
+        """Tests that if we don't require approval for new accounts, newly created
+        accounts are automatically marked as approved.
+        """
+        self.get_success(self.store.register_user(self.user_id, self.pwhash))
+
+        user = self.get_success(self.store.get_user_by_id(self.user_id))
+        assert user is not None
+        self.assertTrue(user["approved"])
+
+        approved = self.get_success(self.store.is_user_approved(self.user_id))
+        self.assertTrue(approved)
+
+    def test_approval_required(self) -> None:
+        """Tests that if we require approval for new accounts, newly created accounts
+        are not automatically marked as approved.
+        """
+        self.get_success(self.store.register_user(self.user_id, self.pwhash))
+
+        user = self.get_success(self.store.get_user_by_id(self.user_id))
+        assert user is not None
+        self.assertFalse(user["approved"])
+
+        approved = self.get_success(self.store.is_user_approved(self.user_id))
+        self.assertFalse(approved)
+
+    def test_override(self) -> None:
+        """Tests that if we require approval for new accounts, but we explicitly say the
+        new user should be considered approved, they're marked as approved.
+        """
+        self.get_success(
+            self.store.register_user(
+                self.user_id,
+                self.pwhash,
+                approved=True,
+            )
+        )
+
+        user = self.get_success(self.store.get_user_by_id(self.user_id))
+        self.assertIsNotNone(user)
+        assert user is not None
+        self.assertEqual(user["approved"], 1)
+
+        approved = self.get_success(self.store.is_user_approved(self.user_id))
+        self.assertTrue(approved)
+
+    def test_approve_user(self) -> None:
+        """Tests that approving the user updates their approval status."""
+        self.get_success(self.store.register_user(self.user_id, self.pwhash))
+
+        approved = self.get_success(self.store.is_user_approved(self.user_id))
+        self.assertFalse(approved)
+
+        self.get_success(
+            self.store.update_user_approval_status(
+                UserID.from_string(self.user_id), True
+            )
+        )
+
+        approved = self.get_success(self.store.is_user_approved(self.user_id))
+        self.assertTrue(approved)