synapse-product/synapse/handlers/sso.py

# 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 abc
import logging
from typing import (
    TYPE_CHECKING,
    Any,
    Awaitable,
    Callable,
    Collection,
    Dict,
    Iterable,
    List,
    Mapping,
    Optional,
    Set,
)
from urllib.parse import urlencode

import attr
from typing_extensions import NoReturn, Protocol

from twisted.web.iweb import IRequest
from twisted.web.server import Request

from synapse.api.constants import LoginType
from synapse.api.errors import Codes, NotFoundError, RedirectException, SynapseError
from synapse.config.sso import SsoAttributeRequirement
from synapse.handlers.ui_auth import UIAuthSessionDataConstants
from synapse.http import get_request_user_agent
from synapse.http.server import respond_with_html, respond_with_redirect
from synapse.http.site import SynapseRequest
from synapse.types import (
    JsonDict,
    UserID,
    contains_invalid_mxid_characters,
    create_requester,
)
from synapse.util.async_helpers import Linearizer
from synapse.util.stringutils import random_string

if TYPE_CHECKING:
    from synapse.server import HomeServer

logger = logging.getLogger(__name__)


class MappingException(Exception):
    """Used to catch errors when mapping an SSO response to user attributes.

    Note that the msg that is raised is shown to end-users.
    """


class SsoIdentityProvider(Protocol):
    """Abstract base class to be implemented by SSO Identity Providers

    An Identity Provider, or IdP, is an external HTTP service which authenticates a user
    to say whether they should be allowed to log in, or perform a given action.

    Synapse supports various implementations of IdPs, including OpenID Connect, SAML,
    and CAS.

    The main entry point is `handle_redirect_request`, which should return a URI to
    redirect the user's browser to the IdP's authentication page.

    Each IdP should be registered with the SsoHandler via
    `hs.get_sso_handler().register_identity_provider()`, so that requests to
    `/_matrix/client/r0/login/sso/redirect` can be correctly dispatched.
    """

    @property
    @abc.abstractmethod
    def idp_id(self) -> str:
        """A unique identifier for this SSO provider

        Eg, "saml", "cas", "github"
        """

    @property
    @abc.abstractmethod
    def idp_name(self) -> str:
        """User-facing name for this provider"""

    @property
    def idp_icon(self) -> Optional[str]:
        """Optional MXC URI for user-facing icon"""
        return None

    @property
    def idp_brand(self) -> Optional[str]:
        """Optional branding identifier"""
        return None

    @property
    def unstable_idp_brand(self) -> Optional[str]:
        """Optional brand identifier for the unstable API (see MSC2858)."""
        return None

    @abc.abstractmethod
    async def handle_redirect_request(
        self,
        request: SynapseRequest,
        client_redirect_url: Optional[bytes],
        ui_auth_session_id: Optional[str] = None,
    ) -> str:
        """Handle an incoming request to /login/sso/redirect

        Args:
            request: the incoming HTTP request
            client_redirect_url: the URL that we should redirect the
                client to after login (or None for UI Auth).
            ui_auth_session_id: The session ID of the ongoing UI Auth (or
                None if this is a login).

        Returns:
            URL to redirect to
        """
        raise NotImplementedError()


@attr.s
class UserAttributes:
    # the localpart of the mxid that the mapper has assigned to the user.
    # if `None`, the mapper has not picked a userid, and the user should be prompted to
    # enter one.
    localpart = attr.ib(type=Optional[str])
    display_name = attr.ib(type=Optional[str], default=None)
    emails = attr.ib(type=Collection[str], default=attr.Factory(list))


@attr.s(slots=True)
class UsernameMappingSession:
    """Data we track about SSO sessions"""

    # A unique identifier for this SSO provider, e.g.  "oidc" or "saml".
    auth_provider_id = attr.ib(type=str)

    # user ID on the IdP server
    remote_user_id = attr.ib(type=str)

    # attributes returned by the ID mapper
    display_name = attr.ib(type=Optional[str])
    emails = attr.ib(type=Collection[str])

    # An optional dictionary of extra attributes to be provided to the client in the
    # login response.
    extra_login_attributes = attr.ib(type=Optional[JsonDict])

    # where to redirect the client back to
    client_redirect_url = attr.ib(type=str)

    # expiry time for the session, in milliseconds
    expiry_time_ms = attr.ib(type=int)

    # choices made by the user
    chosen_localpart = attr.ib(type=Optional[str], default=None)
    use_display_name = attr.ib(type=bool, default=True)
    emails_to_use = attr.ib(type=Collection[str], default=())
    terms_accepted_version = attr.ib(type=Optional[str], default=None)


# the HTTP cookie used to track the mapping session id
USERNAME_MAPPING_SESSION_COOKIE_NAME = b"username_mapping_session"


class SsoHandler:
    # The number of attempts to ask the mapping provider for when generating an MXID.
    _MAP_USERNAME_RETRIES = 1000

    # the time a UsernameMappingSession remains valid for
    _MAPPING_SESSION_VALIDITY_PERIOD_MS = 15 * 60 * 1000

    def __init__(self, hs: "HomeServer"):
        self._clock = hs.get_clock()
        self._store = hs.get_datastore()
        self._server_name = hs.hostname
        self._registration_handler = hs.get_registration_handler()
        self._auth_handler = hs.get_auth_handler()
        self._error_template = hs.config.sso_error_template
        self._bad_user_template = hs.config.sso_auth_bad_user_template
        self._profile_handler = hs.get_profile_handler()

        # The following template is shown after a successful user interactive
        # authentication session. It tells the user they can close the window.
        self._sso_auth_success_template = hs.config.sso_auth_success_template

        self._sso_update_profile_information = hs.config.sso_update_profile_information

        # a lock on the mappings
        self._mapping_lock = Linearizer(name="sso_user_mapping", clock=hs.get_clock())

        # a map from session id to session data
        self._username_mapping_sessions: Dict[str, UsernameMappingSession] = {}

        # map from idp_id to SsoIdentityProvider
        self._identity_providers: Dict[str, SsoIdentityProvider] = {}

        self._consent_at_registration = hs.config.consent.user_consent_at_registration

    def register_identity_provider(self, p: SsoIdentityProvider):
        p_id = p.idp_id
        assert p_id not in self._identity_providers
        self._identity_providers[p_id] = p

    def get_identity_providers(self) -> Mapping[str, SsoIdentityProvider]:
        """Get the configured identity providers"""
        return self._identity_providers

    async def get_identity_providers_for_user(
        self, user_id: str
    ) -> Mapping[str, SsoIdentityProvider]:
        """Get the SsoIdentityProviders which a user has used

        Given a user id, get the identity providers that that user has used to log in
        with in the past (and thus could use to re-identify themselves for UI Auth).

        Args:
            user_id: MXID of user to look up

        Raises:
            a map of idp_id to SsoIdentityProvider
        """
        external_ids = await self._store.get_external_ids_by_user(user_id)

        valid_idps = {}
        for idp_id, _ in external_ids:
            idp = self._identity_providers.get(idp_id)
            if not idp:
                logger.warning(
                    "User %r has an SSO mapping for IdP %r, but this is no longer "
                    "configured.",
                    user_id,
                    idp_id,
                )
            else:
                valid_idps[idp_id] = idp

        return valid_idps

    def render_error(
        self,
        request: Request,
        error: str,
        error_description: Optional[str] = None,
        code: int = 400,
    ) -> 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.
            code: The integer error code (an HTTP response code)
        """
        html = self._error_template.render(
            error=error, error_description=error_description
        )
        respond_with_html(request, code, html)

    async def handle_redirect_request(
        self,
        request: SynapseRequest,
        client_redirect_url: bytes,
        idp_id: Optional[str],
    ) -> str:
        """Handle a request to /login/sso/redirect

        Args:
            request: incoming HTTP request
            client_redirect_url: the URL that we should redirect the
                client to after login.
            idp_id: optional identity provider chosen by the client

        Returns:
             the URI to redirect to
        """
        if not self._identity_providers:
            raise SynapseError(
                400, "Homeserver not configured for SSO.", errcode=Codes.UNRECOGNIZED
            )

        # if the client chose an IdP, use that
        idp: Optional[SsoIdentityProvider] = None
        if idp_id:
            idp = self._identity_providers.get(idp_id)
            if not idp:
                raise NotFoundError("Unknown identity provider")

        # if we only have one auth provider, redirect to it directly
        elif len(self._identity_providers) == 1:
            idp = next(iter(self._identity_providers.values()))

        if idp:
            return await idp.handle_redirect_request(request, client_redirect_url)

        # otherwise, redirect to the IDP picker
        return "/_synapse/client/pick_idp?" + urlencode(
            (("redirectUrl", client_redirect_url),)
        )

    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.
        """
        logger.debug(
            "Looking for existing mapping for user %s:%s",
            auth_provider_id,
            remote_user_id,
        )

        # Check if we already have a mapping for this user.
        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 for IdP '%s' and remote_user_id '%s': %s",
                auth_provider_id,
                remote_user_id,
                previously_registered_user_id,
            )
            return previously_registered_user_id

        # No match.
        return None

    async def complete_sso_login_request(
        self,
        auth_provider_id: str,
        remote_user_id: str,
        request: SynapseRequest,
        client_redirect_url: str,
        sso_to_matrix_id_mapper: Callable[[int], Awaitable[UserAttributes]],
        grandfather_existing_users: Callable[[], Awaitable[Optional[str]]],
        extra_login_attributes: Optional[JsonDict] = None,
    ) -> None:
        """
        Given an SSO ID, retrieve the user ID for it and possibly register the user.

        This first checks if the SSO ID has previously been linked to a matrix ID,
        if it has that matrix ID is returned regardless of the current mapping
        logic.

        If a callable is provided for grandfathering users, it is called and can
        potentially return a matrix ID to use. If it does, the SSO ID is linked to
        this matrix ID for subsequent calls.

        The mapping function is called (potentially multiple times) to generate
        a localpart for the user.

        If an unused localpart is generated, the user is registered from the
        given user-agent and IP address and the SSO ID is linked to this matrix
        ID for subsequent calls.

        Finally, we generate a redirect to the supplied redirect uri, with a login token

        Args:
            auth_provider_id: A unique identifier for this SSO provider, e.g.
                "oidc" or "saml".

            remote_user_id: The unique identifier from the SSO provider.

            request: The request to respond to

            client_redirect_url: The redirect URL passed in by the client.

            sso_to_matrix_id_mapper: A callable to generate the user attributes.
                The only parameter is an integer which represents the amount of
                times the returned mxid localpart mapping has failed.

                It is expected that the mapper can raise two exceptions, which
                will get passed through to the caller:

                    MappingException if there was a problem mapping the response
                        to the user.
                    RedirectException to redirect to an additional page (e.g.
                        to prompt the user for more information).

            grandfather_existing_users: A callable which can return an previously
                existing matrix ID. The SSO ID is then linked to the returned
                matrix ID.

            extra_login_attributes: An optional dictionary of extra
                attributes to be provided to the client in the login response.

        Raises:
            MappingException if there was a problem mapping the response to a user.
            RedirectException: if the mapping provider needs to redirect the user
                to an additional page. (e.g. to prompt for more information)

        """
        new_user = False

        # grab a lock while we try to find a mapping for this user. This seems...
        # optimistic, especially for implementations that end up redirecting to
        # interstitial pages.
        with await self._mapping_lock.queue(auth_provider_id):
            # first of all, check if we already have a mapping for this user
            user_id = await self.get_sso_user_by_remote_user_id(
                auth_provider_id,
                remote_user_id,
            )

            # Check for grandfathering of users.
            if not user_id:
                user_id = await grandfather_existing_users()
                if user_id:
                    # Future logins should also match this user ID.
                    await self._store.record_user_external_id(
                        auth_provider_id, remote_user_id, user_id
                    )

            # Otherwise, generate a new user.
            if not user_id:
                attributes = await self._call_attribute_mapper(sso_to_matrix_id_mapper)

                if attributes.localpart is None:
                    # the mapper doesn't return a username. bail out with a redirect to
                    # the username picker.
                    await self._redirect_to_username_picker(
                        auth_provider_id,
                        remote_user_id,
                        attributes,
                        client_redirect_url,
                        extra_login_attributes,
                    )

                user_id = await self._register_mapped_user(
                    attributes,
                    auth_provider_id,
                    remote_user_id,
                    get_request_user_agent(request),
                    request.getClientIP(),
                )
                new_user = True
            elif self._sso_update_profile_information:
                attributes = await self._call_attribute_mapper(sso_to_matrix_id_mapper)
                if attributes.display_name:
                    user_id_obj = UserID.from_string(user_id)
                    profile_display_name = await self._profile_handler.get_displayname(
                        user_id_obj
                    )
                    if profile_display_name != attributes.display_name:
                        requester = create_requester(
                            user_id,
                            authenticated_entity=user_id,
                        )
                        await self._profile_handler.set_displayname(
                            user_id_obj, requester, attributes.display_name, True
                        )

        await self._auth_handler.complete_sso_login(
            user_id,
            auth_provider_id,
            request,
            client_redirect_url,
            extra_login_attributes,
            new_user=new_user,
        )

    async def _call_attribute_mapper(
        self,
        sso_to_matrix_id_mapper: Callable[[int], Awaitable[UserAttributes]],
    ) -> UserAttributes:
        """Call the attribute mapper function in a loop, until we get a unique userid"""
        for i in range(self._MAP_USERNAME_RETRIES):
            try:
                attributes = await sso_to_matrix_id_mapper(i)
            except (RedirectException, MappingException):
                # Mapping providers are allowed to issue a redirect (e.g. to ask
                # the user for more information) and can issue a mapping exception
                # if a name cannot be generated.
                raise
            except Exception as e:
                # Any other exception is unexpected.
                raise MappingException(
                    "Could not extract user attributes from SSO response."
                ) from e

            logger.debug(
                "Retrieved user attributes from user mapping provider: %r (attempt %d)",
                attributes,
                i,
            )

            if not attributes.localpart:
                # the mapper has not picked a localpart
                return attributes

            # Check if this mxid already exists
            user_id = UserID(attributes.localpart, self._server_name).to_string()
            if not await self._store.get_users_by_id_case_insensitive(user_id):
                # This mxid is free
                break
        else:
            # Unable to generate a username in 1000 iterations
            # Break and return error to the user
            raise MappingException(
                "Unable to generate a Matrix ID from the SSO response"
            )
        return attributes

    async def _redirect_to_username_picker(
        self,
        auth_provider_id: str,
        remote_user_id: str,
        attributes: UserAttributes,
        client_redirect_url: str,
        extra_login_attributes: Optional[JsonDict],
    ) -> NoReturn:
        """Creates a UsernameMappingSession and redirects the browser

        Called if the user mapping provider doesn't return a localpart for a new user.
        Raises a RedirectException which redirects the browser to the username picker.

        Args:
            auth_provider_id: A unique identifier for this SSO provider, e.g.
                "oidc" or "saml".

            remote_user_id: The unique identifier from the SSO provider.

            attributes: the user attributes returned by the user mapping provider.

            client_redirect_url: The redirect URL passed in by the client, which we
                will eventually redirect back to.

            extra_login_attributes: An optional dictionary of extra
                attributes to be provided to the client in the login response.

        Raises:
            RedirectException
        """
        session_id = random_string(16)
        now = self._clock.time_msec()
        session = UsernameMappingSession(
            auth_provider_id=auth_provider_id,
            remote_user_id=remote_user_id,
            display_name=attributes.display_name,
            emails=attributes.emails,
            client_redirect_url=client_redirect_url,
            expiry_time_ms=now + self._MAPPING_SESSION_VALIDITY_PERIOD_MS,
            extra_login_attributes=extra_login_attributes,
        )

        self._username_mapping_sessions[session_id] = session
        logger.info("Recorded registration session id %s", session_id)

        # Set the cookie and redirect to the username picker
        e = RedirectException(b"/_synapse/client/pick_username/account_details")
        e.cookies.append(
            b"%s=%s; path=/"
            % (USERNAME_MAPPING_SESSION_COOKIE_NAME, session_id.encode("ascii"))
        )
        raise e

    async def _register_mapped_user(
        self,
        attributes: UserAttributes,
        auth_provider_id: str,
        remote_user_id: str,
        user_agent: str,
        ip_address: str,
    ) -> str:
        """Register a new SSO user.

        This is called once we have successfully mapped the remote user id onto a local
        user id, one way or another.

        Args:
             attributes: user attributes returned by the user mapping provider,
                including a non-empty localpart.

            auth_provider_id: A unique identifier for this SSO provider, e.g.
                "oidc" or "saml".

            remote_user_id: The unique identifier from the SSO provider.

            user_agent: The user-agent in the HTTP request (used for potential
                shadow-banning.)

            ip_address: The IP address of the requester (used for potential
                shadow-banning.)

        Raises:
            a MappingException if the localpart is invalid.

            a SynapseError with code 400 and errcode Codes.USER_IN_USE if the localpart
            is already taken.
        """

        # Since the localpart is provided via a potentially untrusted module,
        # ensure the MXID is valid before registering.
        if not attributes.localpart or contains_invalid_mxid_characters(
            attributes.localpart
        ):
            raise MappingException("localpart is invalid: %s" % (attributes.localpart,))

        logger.debug("Mapped SSO user to local part %s", attributes.localpart)
        registered_user_id = await self._registration_handler.register_user(
            localpart=attributes.localpart,
            default_display_name=attributes.display_name,
            bind_emails=attributes.emails,
            user_agent_ips=[(user_agent, ip_address)],
            auth_provider_id=auth_provider_id,
        )

        await self._store.record_user_external_id(
            auth_provider_id, remote_user_id, registered_user_id
        )
        return registered_user_id

    async def complete_sso_ui_auth_request(
        self,
        auth_provider_id: str,
        remote_user_id: str,
        ui_auth_session_id: str,
        request: Request,
    ) -> None:
        """
        Given an SSO ID, retrieve the user ID for it and complete UIA.

        Note that this requires that the user is mapped in the "user_external_ids"
        table. This will be the case if they have ever logged in via SAML or OIDC in
        recentish synapse versions, but may not be for older users.

        Args:
            auth_provider_id: A unique identifier for this SSO provider, e.g.
                "oidc" or "saml".
            remote_user_id: The unique identifier from the SSO provider.
            ui_auth_session_id: The ID of the user-interactive auth session.
            request: The request to complete.
        """

        user_id = await self.get_sso_user_by_remote_user_id(
            auth_provider_id,
            remote_user_id,
        )

        user_id_to_verify: str = await self._auth_handler.get_session_data(
            ui_auth_session_id, UIAuthSessionDataConstants.REQUEST_USER_ID
        )

        if not user_id:
            logger.warning(
                "Remote user %s/%s has not previously logged in here: UIA will fail",
                auth_provider_id,
                remote_user_id,
            )
        elif user_id != user_id_to_verify:
            logger.warning(
                "Remote user %s/%s mapped onto incorrect user %s: UIA will fail",
                auth_provider_id,
                remote_user_id,
                user_id,
            )
        else:
            # success!
            # Mark the stage of the authentication as successful.
            await self._store.mark_ui_auth_stage_complete(
                ui_auth_session_id, LoginType.SSO, user_id
            )

            # Render the HTML confirmation page and return.
            html = self._sso_auth_success_template
            respond_with_html(request, 200, html)
            return

        # the user_id didn't match: mark the stage of the authentication as unsuccessful
        await self._store.mark_ui_auth_stage_complete(
            ui_auth_session_id, LoginType.SSO, ""
        )

        # render an error page.
        html = self._bad_user_template.render(
            server_name=self._server_name,
            user_id_to_verify=user_id_to_verify,
        )
        respond_with_html(request, 200, html)

    def get_mapping_session(self, session_id: str) -> UsernameMappingSession:
        """Look up the given username mapping session

        If it is not found, raises a SynapseError with an http code of 400

        Args:
            session_id: session to look up
        Returns:
            active mapping session
        Raises:
            SynapseError if the session is not found/has expired
        """
        self._expire_old_sessions()
        session = self._username_mapping_sessions.get(session_id)
        if session:
            return session
        logger.info("Couldn't find session id %s", session_id)
        raise SynapseError(400, "unknown session")

    async def check_username_availability(
        self,
        localpart: str,
        session_id: str,
    ) -> bool:
        """Handle an "is username available" callback check

        Args:
            localpart: desired localpart
            session_id: the session id for the username picker
        Returns:
            True if the username is available
        Raises:
            SynapseError if the localpart is invalid or the session is unknown
        """

        # make sure that there is a valid mapping session, to stop people dictionary-
        # scanning for accounts
        self.get_mapping_session(session_id)

        logger.info(
            "[session %s] Checking for availability of username %s",
            session_id,
            localpart,
        )

        if contains_invalid_mxid_characters(localpart):
            raise SynapseError(400, "localpart is invalid: %s" % (localpart,))
        user_id = UserID(localpart, self._server_name).to_string()
        user_infos = await self._store.get_users_by_id_case_insensitive(user_id)

        logger.info("[session %s] users: %s", session_id, user_infos)
        return not user_infos

    async def handle_submit_username_request(
        self,
        request: SynapseRequest,
        session_id: str,
        localpart: str,
        use_display_name: bool,
        emails_to_use: Iterable[str],
    ) -> None:
        """Handle a request to the username-picker 'submit' endpoint

        Will serve an HTTP response to the request.

        Args:
            request: HTTP request
            localpart: localpart requested by the user
            session_id: ID of the username mapping session, extracted from a cookie
            use_display_name: whether the user wants to use the suggested display name
            emails_to_use: emails that the user would like to use
        """
        try:
            session = self.get_mapping_session(session_id)
        except SynapseError as e:
            self.render_error(request, "bad_session", e.msg, code=e.code)
            return

        # update the session with the user's choices
        session.chosen_localpart = localpart
        session.use_display_name = use_display_name

        emails_from_idp = set(session.emails)
        filtered_emails: Set[str] = set()

        # we iterate through the list rather than just building a set conjunction, so
        # that we can log attempts to use unknown addresses
        for email in emails_to_use:
            if email in emails_from_idp:
                filtered_emails.add(email)
            else:
                logger.warning(
                    "[session %s] ignoring user request to use unknown email address %r",
                    session_id,
                    email,
                )
        session.emails_to_use = filtered_emails

        # we may now need to collect consent from the user, in which case, redirect
        # to the consent-extraction-unit
        if self._consent_at_registration:
            redirect_url = b"/_synapse/client/new_user_consent"

        # otherwise, redirect to the completion page
        else:
            redirect_url = b"/_synapse/client/sso_register"

        respond_with_redirect(request, redirect_url)

    async def handle_terms_accepted(
        self, request: Request, session_id: str, terms_version: str
    ):
        """Handle a request to the new-user 'consent' endpoint

        Will serve an HTTP response to the request.

        Args:
            request: HTTP request
            session_id: ID of the username mapping session, extracted from a cookie
            terms_version: the version of the terms which the user viewed and consented
                to
        """
        logger.info(
            "[session %s] User consented to terms version %s",
            session_id,
            terms_version,
        )
        try:
            session = self.get_mapping_session(session_id)
        except SynapseError as e:
            self.render_error(request, "bad_session", e.msg, code=e.code)
            return

        session.terms_accepted_version = terms_version

        # we're done; now we can register the user
        respond_with_redirect(request, b"/_synapse/client/sso_register")

    async def register_sso_user(self, request: Request, session_id: str) -> None:
        """Called once we have all the info we need to register a new user.

        Does so and serves an HTTP response

        Args:
            request: HTTP request
            session_id: ID of the username mapping session, extracted from a cookie
        """
        try:
            session = self.get_mapping_session(session_id)
        except SynapseError as e:
            self.render_error(request, "bad_session", e.msg, code=e.code)
            return

        logger.info(
            "[session %s] Registering localpart %s",
            session_id,
            session.chosen_localpart,
        )

        attributes = UserAttributes(
            localpart=session.chosen_localpart,
            emails=session.emails_to_use,
        )

        if session.use_display_name:
            attributes.display_name = session.display_name

        # the following will raise a 400 error if the username has been taken in the
        # meantime.
        user_id = await self._register_mapped_user(
            attributes,
            session.auth_provider_id,
            session.remote_user_id,
            get_request_user_agent(request),
            request.getClientIP(),
        )

        logger.info(
            "[session %s] Registered userid %s with attributes %s",
            session_id,
            user_id,
            attributes,
        )

        # delete the mapping session and the cookie
        del self._username_mapping_sessions[session_id]

        # delete the cookie
        request.addCookie(
            USERNAME_MAPPING_SESSION_COOKIE_NAME,
            b"",
            expires=b"Thu, 01 Jan 1970 00:00:00 GMT",
            path=b"/",
        )

        auth_result = {}
        if session.terms_accepted_version:
            # TODO: make this less awful.
            auth_result[LoginType.TERMS] = True

        await self._registration_handler.post_registration_actions(
            user_id, auth_result, access_token=None
        )

        await self._auth_handler.complete_sso_login(
            user_id,
            session.auth_provider_id,
            request,
            session.client_redirect_url,
            session.extra_login_attributes,
            new_user=True,
        )

    def _expire_old_sessions(self):
        to_expire = []
        now = int(self._clock.time_msec())

        for session_id, session in self._username_mapping_sessions.items():
            if session.expiry_time_ms <= now:
                to_expire.append(session_id)

        for session_id in to_expire:
            logger.info("Expiring mapping session %s", session_id)
            del self._username_mapping_sessions[session_id]

    def check_required_attributes(
        self,
        request: SynapseRequest,
        attributes: Mapping[str, List[Any]],
        attribute_requirements: Iterable[SsoAttributeRequirement],
    ) -> bool:
        """
        Confirm that the required attributes were present in the SSO response.

        If all requirements are met, this will return True.

        If any requirement is not met, then the request will be finalized by
        showing an error page to the user and False will be returned.

        Args:
            request: The request to (potentially) respond to.
            attributes: The attributes from the SSO IdP.
            attribute_requirements: The requirements that attributes must meet.

        Returns:
            True if all requirements are met, False if any attribute fails to
            meet the requirement.

        """
        # Ensure that the attributes of the logged in user meet the required
        # attributes.
        for requirement in attribute_requirements:
            if not _check_attribute_requirement(attributes, requirement):
                self.render_error(
                    request, "unauthorised", "You are not authorised to log in here."
                )
                return False

        return True


def get_username_mapping_session_cookie_from_request(request: IRequest) -> str:
    """Extract the session ID from the cookie

    Raises a SynapseError if the cookie isn't found
    """
    session_id = request.getCookie(USERNAME_MAPPING_SESSION_COOKIE_NAME)
    if not session_id:
        raise SynapseError(code=400, msg="missing session_id")
    return session_id.decode("ascii", errors="replace")


def _check_attribute_requirement(
    attributes: Mapping[str, List[Any]], req: SsoAttributeRequirement
) -> bool:
    """Check if SSO attributes meet the proper requirements.

    Args:
        attributes: A mapping of attributes to an iterable of one or more values.
        requirement: The configured requirement to check.

    Returns:
        True if the required attribute was found and had a proper value.
    """
    if req.attribute not in attributes:
        logger.info("SSO attribute missing: %s", req.attribute)
        return False

    # If the requirement is None, the attribute existing is enough.
    if req.value is None:
        return True

    values = attributes[req.attribute]
    if req.value in values:
        return True

    logger.info(
        "SSO attribute %s did not match required value '%s' (was '%s')",
        req.attribute,
        req.value,
        values,
    )
    return False