anonymousland-synapse/synapse/handlers/oidc_handler.py

# -*- coding: utf-8 -*-
# Copyright 2020 Quentin Gliech
#
# 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 inspect
import logging
from typing import TYPE_CHECKING, Dict, Generic, List, Optional, TypeVar
from urllib.parse import urlencode

import attr
import pymacaroons
from authlib.common.security import generate_token
from authlib.jose import JsonWebToken
from authlib.oauth2.auth import ClientAuth
from authlib.oauth2.rfc6749.parameters import prepare_grant_uri
from authlib.oidc.core import CodeIDToken, ImplicitIDToken, UserInfo
from authlib.oidc.discovery import OpenIDProviderMetadata, get_well_known_url
from jinja2 import Environment, Template
from pymacaroons.exceptions import (
    MacaroonDeserializationException,
    MacaroonInvalidSignatureException,
)
from typing_extensions import TypedDict

from twisted.web.client import readBody

from synapse.config import ConfigError
from synapse.config.oidc_config import OidcProviderConfig
from synapse.handlers.sso import MappingException, UserAttributes
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
from synapse.util import json_decoder
from synapse.util.caches.cached_call import RetryOnExceptionCachedCall

if TYPE_CHECKING:
    from synapse.server import HomeServer

logger = logging.getLogger(__name__)

# we want the cookie to be returned to us even when the request is the POSTed
# result of a form on another domain, as is used with `response_mode=form_post`.
#
# Modern browsers will not do so unless we set SameSite=None; however *older*
# browsers (including all versions of Safari on iOS 12?) don't support
# SameSite=None, and interpret it as SameSite=Strict:
# https://bugs.webkit.org/show_bug.cgi?id=198181
#
# As a rather painful workaround, we set *two* cookies, one with SameSite=None
# and one with no SameSite, in the hope that at least one of them will get
# back to us.
#
# Secure is necessary for SameSite=None (and, empirically, also breaks things
# on iOS 12.)
#
# Here we have the names of the cookies, and the options we use to set them.
_SESSION_COOKIES = [
    (b"oidc_session", b"Path=/_synapse/client/oidc; HttpOnly; Secure; SameSite=None"),
    (b"oidc_session_no_samesite", b"Path=/_synapse/client/oidc; HttpOnly"),
]

#: A token exchanged from the token endpoint, as per RFC6749 sec 5.1. and
#: OpenID.Core sec 3.1.3.3.
Token = TypedDict(
    "Token",
    {
        "access_token": str,
        "token_type": str,
        "id_token": Optional[str],
        "refresh_token": Optional[str],
        "expires_in": int,
        "scope": Optional[str],
    },
)

#: A JWK, as per RFC7517 sec 4. The type could be more precise than that, but
#: there is no real point of doing this in our case.
JWK = Dict[str, str]

#: A JWK Set, as per RFC7517 sec 5.
JWKS = TypedDict("JWKS", {"keys": List[JWK]})


class OidcHandler:
    """Handles requests related to the OpenID Connect login flow."""

    def __init__(self, hs: "HomeServer"):
        self._sso_handler = hs.get_sso_handler()

        provider_confs = hs.config.oidc.oidc_providers
        # we should not have been instantiated if there is no configured provider.
        assert provider_confs

        self._token_generator = OidcSessionTokenGenerator(hs)
        self._providers = {
            p.idp_id: OidcProvider(hs, self._token_generator, p) for p in provider_confs
        }  # type: Dict[str, OidcProvider]

    async def load_metadata(self) -> None:
        """Validate the config and load the metadata from the remote endpoint.

        Called at startup to ensure we have everything we need.
        """
        for idp_id, p in self._providers.items():
            try:
                await p.load_metadata()
                await p.load_jwks()
            except Exception as e:
                raise Exception(
                    "Error while initialising OIDC provider %r" % (idp_id,)
                ) from e

    async def handle_oidc_callback(self, request: SynapseRequest) -> None:
        """Handle an incoming request to /_synapse/client/oidc/callback

        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._sso_handler.render_error`` which displays an HTML page for the error.

        Most of the OpenID Connect logic happens here:

          - first, we check if there was any error returned by the provider and
            display it
          - then we fetch the session cookie, decode and verify it
          - the ``state`` query parameter should match with the one stored in the
            session cookie

        Once we know the session is legit, we then delegate to the OIDC Provider
        implementation, which will exchange the code with the provider and complete the
        login/authentication.

        Args:
            request: the incoming request from the browser.
        """
        # The provider might redirect with an error.
        # In that case, just display it as-is.
        if b"error" in request.args:
            # error response from the auth server. see:
            #  https://tools.ietf.org/html/rfc6749#section-4.1.2.1
            #  https://openid.net/specs/openid-connect-core-1_0.html#AuthError
            error = request.args[b"error"][0].decode()
            description = request.args.get(b"error_description", [b""])[0].decode()

            # Most of the errors returned by the provider could be due by
            # either the provider misbehaving or Synapse being misconfigured.
            # The only exception of that is "access_denied", where the user
            # probably cancelled the login flow. In other cases, log those errors.
            logger.log(
                logging.INFO if error == "access_denied" else logging.ERROR,
                "Received OIDC callback with error: %s %s",
                error,
                description,
            )

            self._sso_handler.render_error(request, error, description)
            return

        # otherwise, it is presumably a successful response. see:
        #   https://tools.ietf.org/html/rfc6749#section-4.1.2

        # Fetch the session cookie. See the comments on SESSION_COOKIES for why there
        # are two.

        for cookie_name, _ in _SESSION_COOKIES:
            session = request.getCookie(cookie_name)  # type: Optional[bytes]
            if session is not None:
                break
        else:
            logger.info("Received OIDC callback, with no session cookie")
            self._sso_handler.render_error(
                request, "missing_session", "No session cookie found"
            )
            return

        # Remove the cookies. There is a good chance that if the callback failed
        # once, it will fail next time and the code will already be exchanged.
        # Removing the cookies early avoids spamming the provider with token requests.
        #
        # we have to build the header by hand rather than calling request.addCookie
        # because the latter does not support SameSite=None
        # (https://twistedmatrix.com/trac/ticket/10088)

        for cookie_name, options in _SESSION_COOKIES:
            request.cookies.append(
                b"%s=; Expires=Thu, Jan 01 1970 00:00:00 UTC; %s"
                % (cookie_name, options)
            )

        # Check for the state query parameter
        if b"state" not in request.args:
            logger.info("Received OIDC callback, with no state parameter")
            self._sso_handler.render_error(
                request, "invalid_request", "State parameter is missing"
            )
            return

        state = request.args[b"state"][0].decode()

        # Deserialize the session token and verify it.
        try:
            session_data = self._token_generator.verify_oidc_session_token(
                session, state
            )
        except (MacaroonDeserializationException, ValueError) as e:
            logger.exception("Invalid session for OIDC callback")
            self._sso_handler.render_error(request, "invalid_session", str(e))
            return
        except MacaroonInvalidSignatureException as e:
            logger.exception("Could not verify session for OIDC callback")
            self._sso_handler.render_error(request, "mismatching_session", str(e))
            return

        logger.info("Received OIDC callback for IdP %s", session_data.idp_id)

        oidc_provider = self._providers.get(session_data.idp_id)
        if not oidc_provider:
            logger.error("OIDC session uses unknown IdP %r", oidc_provider)
            self._sso_handler.render_error(request, "unknown_idp", "Unknown IdP")
            return

        if b"code" not in request.args:
            logger.info("Code parameter is missing")
            self._sso_handler.render_error(
                request, "invalid_request", "Code parameter is missing"
            )
            return

        code = request.args[b"code"][0].decode()

        await oidc_provider.handle_oidc_callback(request, session_data, code)


class OidcError(Exception):
    """Used to catch errors when calling the token_endpoint"""

    def __init__(self, error, error_description=None):
        self.error = error
        self.error_description = error_description

    def __str__(self):
        if self.error_description:
            return "{}: {}".format(self.error, self.error_description)
        return self.error


class OidcProvider:
    """Wraps the config for a single OIDC IdentityProvider

    Provides methods for handling redirect requests and callbacks via that particular
    IdP.
    """

    def __init__(
        self,
        hs: "HomeServer",
        token_generator: "OidcSessionTokenGenerator",
        provider: OidcProviderConfig,
    ):
        self._store = hs.get_datastore()

        self._token_generator = token_generator

        self._config = provider
        self._callback_url = hs.config.oidc_callback_url  # type: str

        self._scopes = provider.scopes
        self._user_profile_method = provider.user_profile_method
        self._client_auth = ClientAuth(
            provider.client_id,
            provider.client_secret,
            provider.client_auth_method,
        )  # type: ClientAuth
        self._client_auth_method = provider.client_auth_method

        # cache of metadata for the identity provider (endpoint uris, mostly). This is
        # loaded on-demand from the discovery endpoint (if discovery is enabled), with
        # possible overrides from the config.  Access via `load_metadata`.
        self._provider_metadata = RetryOnExceptionCachedCall(self._load_metadata)

        # cache of JWKs used by the identity provider to sign tokens. Loaded on demand
        # from the IdP's jwks_uri, if required.
        self._jwks = RetryOnExceptionCachedCall(self._load_jwks)

        self._user_mapping_provider = provider.user_mapping_provider_class(
            provider.user_mapping_provider_config
        )
        self._skip_verification = provider.skip_verification
        self._allow_existing_users = provider.allow_existing_users

        self._http_client = hs.get_proxied_http_client()
        self._server_name = hs.config.server_name  # type: str

        # identifier for the external_ids table
        self.idp_id = provider.idp_id

        # user-facing name of this auth provider
        self.idp_name = provider.idp_name

        # MXC URI for icon for this auth provider
        self.idp_icon = provider.idp_icon

        # optional brand identifier for this auth provider
        self.idp_brand = provider.idp_brand

        self._sso_handler = hs.get_sso_handler()

        self._sso_handler.register_identity_provider(self)

    def _validate_metadata(self, m: OpenIDProviderMetadata) -> None:
        """Verifies the provider metadata.

        This checks the validity of the currently loaded provider. Not
        everything is checked, only:

          - ``issuer``
          - ``authorization_endpoint``
          - ``token_endpoint``
          - ``response_types_supported`` (checks if "code" is in it)
          - ``jwks_uri``

        Raises:
            ValueError: if something in the provider is not valid
        """
        # Skip verification to allow non-compliant providers (e.g. issuers not running on a secure origin)
        if self._skip_verification is True:
            return

        m.validate_issuer()
        m.validate_authorization_endpoint()
        m.validate_token_endpoint()

        if m.get("token_endpoint_auth_methods_supported") is not None:
            m.validate_token_endpoint_auth_methods_supported()
            if (
                self._client_auth_method
                not in m["token_endpoint_auth_methods_supported"]
            ):
                raise ValueError(
                    '"{auth_method}" not in "token_endpoint_auth_methods_supported" ({supported!r})'.format(
                        auth_method=self._client_auth_method,
                        supported=m["token_endpoint_auth_methods_supported"],
                    )
                )

        if m.get("response_types_supported") is not None:
            m.validate_response_types_supported()

            if "code" not in m["response_types_supported"]:
                raise ValueError(
                    '"code" not in "response_types_supported" (%r)'
                    % (m["response_types_supported"],)
                )

        # Ensure there's a userinfo endpoint to fetch from if it is required.
        if self._uses_userinfo:
            if m.get("userinfo_endpoint") is None:
                raise ValueError(
                    'provider has no "userinfo_endpoint", even though it is required'
                )
        else:
            # If we're not using userinfo, we need a valid jwks to validate the ID token
            m.validate_jwks_uri()

    @property
    def _uses_userinfo(self) -> bool:
        """Returns True if the ``userinfo_endpoint`` should be used.

        This is based on the requested scopes: if the scopes include
        ``openid``, the provider should give use an ID token containing the
        user information. If not, we should fetch them using the
        ``access_token`` with the ``userinfo_endpoint``.
        """

        return (
            "openid" not in self._scopes
            or self._user_profile_method == "userinfo_endpoint"
        )

    async def load_metadata(self, force: bool = False) -> OpenIDProviderMetadata:
        """Return the provider metadata.

        If this is the first call, the metadata is built from the config and from the
        metadata discovery endpoint (if enabled), and then validated. If the metadata
        is successfully validated, it is then cached for future use.

        Args:
            force: If true, any cached metadata is discarded to force a reload.

        Raises:
            ValueError: if something in the provider is not valid

        Returns:
            The provider's metadata.
        """
        if force:
            # reset the cached call to ensure we get a new result
            self._provider_metadata = RetryOnExceptionCachedCall(self._load_metadata)

        return await self._provider_metadata.get()

    async def _load_metadata(self) -> OpenIDProviderMetadata:
        # start out with just the issuer (unlike the other settings, discovered issuer
        # takes precedence over configured issuer, because configured issuer is
        # required for discovery to take place.)
        #
        metadata = OpenIDProviderMetadata(issuer=self._config.issuer)

        # load any data from the discovery endpoint, if enabled
        if self._config.discover:
            url = get_well_known_url(self._config.issuer, external=True)
            metadata_response = await self._http_client.get_json(url)
            metadata.update(metadata_response)

        # override any discovered data with any settings in our config
        if self._config.authorization_endpoint:
            metadata["authorization_endpoint"] = self._config.authorization_endpoint

        if self._config.token_endpoint:
            metadata["token_endpoint"] = self._config.token_endpoint

        if self._config.userinfo_endpoint:
            metadata["userinfo_endpoint"] = self._config.userinfo_endpoint

        if self._config.jwks_uri:
            metadata["jwks_uri"] = self._config.jwks_uri

        self._validate_metadata(metadata)

        return metadata

    async def load_jwks(self, force: bool = False) -> JWKS:
        """Load the JSON Web Key Set used to sign ID tokens.

        If we're not using the ``userinfo_endpoint``, user infos are extracted
        from the ID token, which is a JWT signed by keys given by the provider.
        The keys are then cached.

        Args:
            force: Force reloading the keys.

        Returns:
            The key set

            Looks like this::

                {
                    'keys': [
                        {
                            'kid': 'abcdef',
                            'kty': 'RSA',
                            'alg': 'RS256',
                            'use': 'sig',
                            'e': 'XXXX',
                            'n': 'XXXX',
                        }
                    ]
                }
        """
        if force:
            # reset the cached call to ensure we get a new result
            self._jwks = RetryOnExceptionCachedCall(self._load_jwks)
        return await self._jwks.get()

    async def _load_jwks(self) -> JWKS:
        if self._uses_userinfo:
            # We're not using jwt signing, return an empty jwk set
            return {"keys": []}

        metadata = await self.load_metadata()

        # Load the JWKS using the `jwks_uri` metadata.
        uri = metadata.get("jwks_uri")
        if not uri:
            # this should be unreachable: load_metadata validates that
            # there is a jwks_uri in the metadata if _uses_userinfo is unset
            raise RuntimeError('Missing "jwks_uri" in metadata')

        jwk_set = await self._http_client.get_json(uri)

        return jwk_set

    async def _exchange_code(self, code: str) -> Token:
        """Exchange an authorization code for a token.

        This calls the ``token_endpoint`` with the authorization code we
        received in the callback to exchange it for a token. The call uses the
        ``ClientAuth`` to authenticate with the client with its ID and secret.

        See:
           https://tools.ietf.org/html/rfc6749#section-3.2
           https://openid.net/specs/openid-connect-core-1_0.html#TokenEndpoint

        Args:
            code: The authorization code we got from the callback.

        Returns:
            A dict containing various tokens.

            May look like this::

                {
                    'token_type': 'bearer',
                    'access_token': 'abcdef',
                    'expires_in': 3599,
                    'id_token': 'ghijkl',
                    'refresh_token': 'mnopqr',
                }

        Raises:
            OidcError: when the ``token_endpoint`` returned an error.
        """
        metadata = await self.load_metadata()
        token_endpoint = metadata.get("token_endpoint")
        headers = {
            "Content-Type": "application/x-www-form-urlencoded",
            "User-Agent": self._http_client.user_agent,
            "Accept": "application/json",
        }

        args = {
            "grant_type": "authorization_code",
            "code": code,
            "redirect_uri": self._callback_url,
        }
        body = urlencode(args, True)

        # Fill the body/headers with credentials
        uri, headers, body = self._client_auth.prepare(
            method="POST", uri=token_endpoint, headers=headers, body=body
        )
        headers = {k: [v] for (k, v) in headers.items()}

        # Do the actual request
        # We're not using the SimpleHttpClient util methods as we don't want to
        # check the HTTP status code and we do the body encoding ourself.
        response = await self._http_client.request(
            method="POST",
            uri=uri,
            data=body.encode("utf-8"),
            headers=headers,
        )

        # This is used in multiple error messages below
        status = "{code} {phrase}".format(
            code=response.code, phrase=response.phrase.decode("utf-8")
        )

        resp_body = await make_deferred_yieldable(readBody(response))

        if response.code >= 500:
            # In case of a server error, we should first try to decode the body
            # and check for an error field. If not, we respond with a generic
            # error message.
            try:
                resp = json_decoder.decode(resp_body.decode("utf-8"))
                error = resp["error"]
                description = resp.get("error_description", error)
            except (ValueError, KeyError):
                # Catch ValueError for the JSON decoding and KeyError for the "error" field
                error = "server_error"
                description = (
                    (
                        'Authorization server responded with a "{status}" error '
                        "while exchanging the authorization code."
                    ).format(status=status),
                )

            raise OidcError(error, description)

        # Since it is a not a 5xx code, body should be a valid JSON. It will
        # raise if not.
        resp = json_decoder.decode(resp_body.decode("utf-8"))

        if "error" in resp:
            error = resp["error"]
            # In case the authorization server responded with an error field,
            # it should be a 4xx code. If not, warn about it but don't do
            # anything special and report the original error message.
            if response.code < 400:
                logger.debug(
                    "Invalid response from the authorization server: "
                    'responded with a "{status}" '
                    "but body has an error field: {error!r}".format(
                        status=status, error=resp["error"]
                    )
                )

            description = resp.get("error_description", error)
            raise OidcError(error, description)

        # Now, this should not be an error. According to RFC6749 sec 5.1, it
        # should be a 200 code. We're a bit more flexible than that, and will
        # only throw on a 4xx code.
        if response.code >= 400:
            description = (
                'Authorization server responded with a "{status}" error '
                'but did not include an "error" field in its response.'.format(
                    status=status
                )
            )
            logger.warning(description)
            # Body was still valid JSON. Might be useful to log it for debugging.
            logger.warning("Code exchange response: {resp!r}".format(resp=resp))
            raise OidcError("server_error", description)

        return resp

    async def _fetch_userinfo(self, token: Token) -> UserInfo:
        """Fetch user information from the ``userinfo_endpoint``.

        Args:
            token: the token given by the ``token_endpoint``.
                Must include an ``access_token`` field.

        Returns:
            UserInfo: an object representing the user.
        """
        logger.debug("Using the OAuth2 access_token to request userinfo")
        metadata = await self.load_metadata()

        resp = await self._http_client.get_json(
            metadata["userinfo_endpoint"],
            headers={"Authorization": ["Bearer {}".format(token["access_token"])]},
        )

        logger.debug("Retrieved user info from userinfo endpoint: %r", resp)

        return UserInfo(resp)

    async def _parse_id_token(self, token: Token, nonce: str) -> UserInfo:
        """Return an instance of UserInfo from token's ``id_token``.

        Args:
            token: the token given by the ``token_endpoint``.
                Must include an ``id_token`` field.
            nonce: the nonce value originally sent in the initial authorization
                request. This value should match the one inside the token.

        Returns:
            An object representing the user.
        """
        metadata = await self.load_metadata()
        claims_params = {
            "nonce": nonce,
            "client_id": self._client_auth.client_id,
        }
        if "access_token" in token:
            # If we got an `access_token`, there should be an `at_hash` claim
            # in the `id_token` that we can check against.
            claims_params["access_token"] = token["access_token"]
            claims_cls = CodeIDToken
        else:
            claims_cls = ImplicitIDToken

        alg_values = metadata.get("id_token_signing_alg_values_supported", ["RS256"])
        jwt = JsonWebToken(alg_values)

        claim_options = {"iss": {"values": [metadata["issuer"]]}}

        id_token = token["id_token"]
        logger.debug("Attempting to decode JWT id_token %r", id_token)

        # Try to decode the keys in cache first, then retry by forcing the keys
        # to be reloaded
        jwk_set = await self.load_jwks()
        try:
            claims = jwt.decode(
                id_token,
                key=jwk_set,
                claims_cls=claims_cls,
                claims_options=claim_options,
                claims_params=claims_params,
            )
        except ValueError:
            logger.info("Reloading JWKS after decode error")
            jwk_set = await self.load_jwks(force=True)  # try reloading the jwks
            claims = jwt.decode(
                id_token,
                key=jwk_set,
                claims_cls=claims_cls,
                claims_options=claim_options,
                claims_params=claims_params,
            )

        logger.debug("Decoded id_token JWT %r; validating", claims)

        claims.validate(leeway=120)  # allows 2 min of clock skew
        return UserInfo(claims)

    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

        It returns a redirect to the authorization endpoint with a few
        parameters:

          - ``client_id``: the client ID set in ``oidc_config.client_id``
          - ``response_type``: ``code``
          - ``redirect_uri``: the callback URL ; ``{base url}/_synapse/client/oidc/callback``
          - ``scope``: the list of scopes set in ``oidc_config.scopes``
          - ``state``: a random string
          - ``nonce``: a random string

        In addition generating a redirect URL, we are setting a cookie with
        a signed macaroon token containing the state, the nonce and the
        client_redirect_url params. Those are then checked when the client
        comes back from the provider.

        Args:
            request: the incoming request from the browser.
                We'll respond to it with a redirect and a cookie.
            client_redirect_url: the URL that we should redirect the client to
                when everything is done (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:
            The redirect URL to the authorization endpoint.

        """

        state = generate_token()
        nonce = generate_token()

        if not client_redirect_url:
            client_redirect_url = b""

        cookie = self._token_generator.generate_oidc_session_token(
            state=state,
            session_data=OidcSessionData(
                idp_id=self.idp_id,
                nonce=nonce,
                client_redirect_url=client_redirect_url.decode(),
                ui_auth_session_id=ui_auth_session_id,
            ),
        )

        # Set the cookies. See the comments on _SESSION_COOKIES for why there are two.
        #
        # we have to build the header by hand rather than calling request.addCookie
        # because the latter does not support SameSite=None
        # (https://twistedmatrix.com/trac/ticket/10088)

        for cookie_name, options in _SESSION_COOKIES:
            request.cookies.append(
                b"%s=%s; Max-Age=3600; %s"
                % (cookie_name, cookie.encode("utf-8"), options)
            )

        metadata = await self.load_metadata()
        authorization_endpoint = metadata.get("authorization_endpoint")
        return prepare_grant_uri(
            authorization_endpoint,
            client_id=self._client_auth.client_id,
            response_type="code",
            redirect_uri=self._callback_url,
            scope=self._scopes,
            state=state,
            nonce=nonce,
        )

    async def handle_oidc_callback(
        self, request: SynapseRequest, session_data: "OidcSessionData", code: str
    ) -> None:
        """Handle an incoming request to /_synapse/client/oidc/callback

        By this time we have already validated the session on the synapse side, and
        now need to do the provider-specific operations. This includes:

          - exchange the code with the provider using the ``token_endpoint`` (see
            ``_exchange_code``)
          - once we have the token, use it to either extract the UserInfo from
            the ``id_token`` (``_parse_id_token``), or use the ``access_token``
            to fetch UserInfo from the ``userinfo_endpoint``
            (``_fetch_userinfo``)
          - map those UserInfo to a Matrix user (``_map_userinfo_to_user``) and
            finish the login

        Args:
            request: the incoming request from the browser.
            session_data: the session data, extracted from our cookie
            code: The authorization code we got from the callback.
        """
        # Exchange the code with the provider
        try:
            logger.debug("Exchanging OAuth2 code for a token")
            token = await self._exchange_code(code)
        except OidcError as e:
            logger.exception("Could not exchange OAuth2 code")
            self._sso_handler.render_error(request, e.error, e.error_description)
            return

        logger.debug("Successfully obtained OAuth2 token data: %r", token)

        # Now that we have a token, get the userinfo, either by decoding the
        # `id_token` or by fetching the `userinfo_endpoint`.
        if self._uses_userinfo:
            try:
                userinfo = await self._fetch_userinfo(token)
            except Exception as e:
                logger.exception("Could not fetch userinfo")
                self._sso_handler.render_error(request, "fetch_error", str(e))
                return
        else:
            try:
                userinfo = await self._parse_id_token(token, nonce=session_data.nonce)
            except Exception as e:
                logger.exception("Invalid id_token")
                self._sso_handler.render_error(request, "invalid_token", str(e))
                return

        # first check if we're doing a UIA
        if session_data.ui_auth_session_id:
            try:
                remote_user_id = self._remote_id_from_userinfo(userinfo)
            except Exception as e:
                logger.exception("Could not extract remote user id")
                self._sso_handler.render_error(request, "mapping_error", str(e))
                return

            return await self._sso_handler.complete_sso_ui_auth_request(
                self.idp_id, remote_user_id, session_data.ui_auth_session_id, request
            )

        # otherwise, it's a login

        # Call the mapper to register/login the user
        try:
            await self._complete_oidc_login(
                userinfo, token, request, session_data.client_redirect_url
            )
        except MappingException as e:
            logger.exception("Could not map user")
            self._sso_handler.render_error(request, "mapping_error", str(e))

    async def _complete_oidc_login(
        self,
        userinfo: UserInfo,
        token: Token,
        request: SynapseRequest,
        client_redirect_url: str,
    ) -> None:
        """Given a UserInfo response, complete the login flow

        UserInfo should have a claim that uniquely identifies users. This claim
        is usually `sub`, but can be configured with `oidc_config.subject_claim`.
        It is then used as an `external_id`.

        If we don't find the user that way, we should register the user,
        mapping the localpart and the display name from the UserInfo.

        If a user already exists with the mxid we've mapped and allow_existing_users
        is disabled, raise an exception.

        Otherwise, render a redirect back to the client_redirect_url with a loginToken.

        Args:
            userinfo: an object representing the user
            token: a dict with the tokens obtained from the provider
            request: The request to respond to
            client_redirect_url: The redirect URL passed in by the client.

        Raises:
            MappingException: if there was an error while mapping some properties
        """
        try:
            remote_user_id = self._remote_id_from_userinfo(userinfo)
        except Exception as e:
            raise MappingException(
                "Failed to extract subject from OIDC response: %s" % (e,)
            )

        # Older mapping providers don't accept the `failures` argument, so we
        # try and detect support.
        mapper_signature = inspect.signature(
            self._user_mapping_provider.map_user_attributes
        )
        supports_failures = "failures" in mapper_signature.parameters

        async def oidc_response_to_user_attributes(failures: int) -> UserAttributes:
            """
            Call the mapping provider to map the OIDC userinfo and token to user attributes.

            This is backwards compatibility for abstraction for the SSO handler.
            """
            if supports_failures:
                attributes = await self._user_mapping_provider.map_user_attributes(
                    userinfo, token, failures
                )
            else:
                # If the mapping provider does not support processing failures,
                # do not continually generate the same Matrix ID since it will
                # continue to already be in use. Note that the error raised is
                # arbitrary and will get turned into a MappingException.
                if failures:
                    raise MappingException(
                        "Mapping provider does not support de-duplicating Matrix IDs"
                    )

                attributes = await self._user_mapping_provider.map_user_attributes(  # type: ignore
                    userinfo, token
                )

            return UserAttributes(**attributes)

        async def grandfather_existing_users() -> Optional[str]:
            if self._allow_existing_users:
                # If allowing existing users we want to generate a single localpart
                # and attempt to match it.
                attributes = await oidc_response_to_user_attributes(failures=0)

                user_id = UserID(attributes.localpart, self._server_name).to_string()
                users = await self._store.get_users_by_id_case_insensitive(user_id)
                if users:
                    # If an existing matrix ID is returned, then use it.
                    if len(users) == 1:
                        previously_registered_user_id = next(iter(users))
                    elif user_id in users:
                        previously_registered_user_id = user_id
                    else:
                        # Do not attempt to continue generating Matrix IDs.
                        raise MappingException(
                            "Attempted to login as '{}' but it matches more than one user inexactly: {}".format(
                                user_id, users
                            )
                        )

                    return previously_registered_user_id

            return None

        # Mapping providers might not have get_extra_attributes: only call this
        # method if it exists.
        extra_attributes = None
        get_extra_attributes = getattr(
            self._user_mapping_provider, "get_extra_attributes", None
        )
        if get_extra_attributes:
            extra_attributes = await get_extra_attributes(userinfo, token)

        await self._sso_handler.complete_sso_login_request(
            self.idp_id,
            remote_user_id,
            request,
            client_redirect_url,
            oidc_response_to_user_attributes,
            grandfather_existing_users,
            extra_attributes,
        )

    def _remote_id_from_userinfo(self, userinfo: UserInfo) -> str:
        """Extract the unique remote id from an OIDC UserInfo block

        Args:
            userinfo: An object representing the user given by the OIDC provider
        Returns:
            remote user id
        """
        remote_user_id = self._user_mapping_provider.get_remote_user_id(userinfo)
        # Some OIDC providers use integer IDs, but Synapse expects external IDs
        # to be strings.
        return str(remote_user_id)


class OidcSessionTokenGenerator:
    """Methods for generating and checking OIDC Session cookies."""

    def __init__(self, hs: "HomeServer"):
        self._clock = hs.get_clock()
        self._server_name = hs.hostname
        self._macaroon_secret_key = hs.config.key.macaroon_secret_key

    def generate_oidc_session_token(
        self,
        state: str,
        session_data: "OidcSessionData",
        duration_in_ms: int = (60 * 60 * 1000),
    ) -> str:
        """Generates a signed token storing data about an OIDC session.

        When Synapse initiates an authorization flow, it creates a random state
        and a random nonce. Those parameters are given to the provider and
        should be verified when the client comes back from the provider.
        It is also used to store the client_redirect_url, which is used to
        complete the SSO login flow.

        Args:
            state: The ``state`` parameter passed to the OIDC provider.
            session_data: data to include in the session token.
            duration_in_ms: An optional duration for the token in milliseconds.
                Defaults to an hour.

        Returns:
            A signed macaroon token with the session information.
        """
        macaroon = pymacaroons.Macaroon(
            location=self._server_name,
            identifier="key",
            key=self._macaroon_secret_key,
        )
        macaroon.add_first_party_caveat("gen = 1")
        macaroon.add_first_party_caveat("type = session")
        macaroon.add_first_party_caveat("state = %s" % (state,))
        macaroon.add_first_party_caveat("idp_id = %s" % (session_data.idp_id,))
        macaroon.add_first_party_caveat("nonce = %s" % (session_data.nonce,))
        macaroon.add_first_party_caveat(
            "client_redirect_url = %s" % (session_data.client_redirect_url,)
        )
        if session_data.ui_auth_session_id:
            macaroon.add_first_party_caveat(
                "ui_auth_session_id = %s" % (session_data.ui_auth_session_id,)
            )
        now = self._clock.time_msec()
        expiry = now + duration_in_ms
        macaroon.add_first_party_caveat("time < %d" % (expiry,))

        return macaroon.serialize()

    def verify_oidc_session_token(
        self, session: bytes, state: str
    ) -> "OidcSessionData":
        """Verifies and extract an OIDC session token.

        This verifies that a given session token was issued by this homeserver
        and extract the nonce and client_redirect_url caveats.

        Args:
            session: The session token to verify
            state: The state the OIDC provider gave back

        Returns:
            The data extracted from the session cookie

        Raises:
            ValueError if an expected caveat is missing from the macaroon.
        """
        macaroon = pymacaroons.Macaroon.deserialize(session)

        v = pymacaroons.Verifier()
        v.satisfy_exact("gen = 1")
        v.satisfy_exact("type = session")
        v.satisfy_exact("state = %s" % (state,))
        v.satisfy_general(lambda c: c.startswith("nonce = "))
        v.satisfy_general(lambda c: c.startswith("idp_id = "))
        v.satisfy_general(lambda c: c.startswith("client_redirect_url = "))
        # Sometimes there's a UI auth session ID, it seems to be OK to attempt
        # to always satisfy this.
        v.satisfy_general(lambda c: c.startswith("ui_auth_session_id = "))
        v.satisfy_general(self._verify_expiry)

        v.verify(macaroon, self._macaroon_secret_key)

        # Extract the session data from the token.
        nonce = self._get_value_from_macaroon(macaroon, "nonce")
        idp_id = self._get_value_from_macaroon(macaroon, "idp_id")
        client_redirect_url = self._get_value_from_macaroon(
            macaroon, "client_redirect_url"
        )
        try:
            ui_auth_session_id = self._get_value_from_macaroon(
                macaroon, "ui_auth_session_id"
            )  # type: Optional[str]
        except ValueError:
            ui_auth_session_id = None

        return OidcSessionData(
            nonce=nonce,
            idp_id=idp_id,
            client_redirect_url=client_redirect_url,
            ui_auth_session_id=ui_auth_session_id,
        )

    def _get_value_from_macaroon(self, macaroon: pymacaroons.Macaroon, key: str) -> str:
        """Extracts a caveat value from a macaroon token.

        Args:
            macaroon: the token
            key: the key of the caveat to extract

        Returns:
            The extracted value

        Raises:
            ValueError: if the caveat was not in the macaroon
        """
        prefix = key + " = "
        for caveat in macaroon.caveats:
            if caveat.caveat_id.startswith(prefix):
                return caveat.caveat_id[len(prefix) :]
        raise ValueError("No %s caveat in macaroon" % (key,))

    def _verify_expiry(self, caveat: str) -> bool:
        prefix = "time < "
        if not caveat.startswith(prefix):
            return False
        expiry = int(caveat[len(prefix) :])
        now = self._clock.time_msec()
        return now < expiry


@attr.s(frozen=True, slots=True)
class OidcSessionData:
    """The attributes which are stored in a OIDC session cookie"""

    # the Identity Provider being used
    idp_id = attr.ib(type=str)

    # The `nonce` parameter passed to the OIDC provider.
    nonce = attr.ib(type=str)

    # The URL the client gave when it initiated the flow. ("" if this is a UI Auth)
    client_redirect_url = attr.ib(type=str)

    # The session ID of the ongoing UI Auth (None if this is a login)
    ui_auth_session_id = attr.ib(type=Optional[str], default=None)


UserAttributeDict = TypedDict(
    "UserAttributeDict",
    {"localpart": Optional[str], "display_name": Optional[str], "emails": List[str]},
)
C = TypeVar("C")


class OidcMappingProvider(Generic[C]):
    """A mapping provider maps a UserInfo object to user attributes.

    It should provide the API described by this class.
    """

    def __init__(self, config: C):
        """
        Args:
            config: A custom config object from this module, parsed by ``parse_config()``
        """

    @staticmethod
    def parse_config(config: dict) -> C:
        """Parse the dict provided by the homeserver's config

        Args:
            config: A dictionary containing configuration options for this provider

        Returns:
            A custom config object for this module
        """
        raise NotImplementedError()

    def get_remote_user_id(self, userinfo: UserInfo) -> str:
        """Get a unique user ID for this user.

        Usually, in an OIDC-compliant scenario, it should be the ``sub`` claim from the UserInfo object.

        Args:
            userinfo: An object representing the user given by the OIDC provider

        Returns:
            A unique user ID
        """
        raise NotImplementedError()

    async def map_user_attributes(
        self, userinfo: UserInfo, token: Token, failures: int
    ) -> UserAttributeDict:
        """Map a `UserInfo` object into user attributes.

        Args:
            userinfo: An object representing the user given by the OIDC provider
            token: A dict with the tokens returned by the provider
            failures: How many times a call to this function with this
                UserInfo has resulted in a failure.

        Returns:
            A dict containing the ``localpart`` and (optionally) the ``display_name``
        """
        raise NotImplementedError()

    async def get_extra_attributes(self, userinfo: UserInfo, token: Token) -> JsonDict:
        """Map a `UserInfo` object into additional attributes passed to the client during login.

        Args:
            userinfo: An object representing the user given by the OIDC provider
            token: A dict with the tokens returned by the provider

        Returns:
            A dict containing additional attributes. Must be JSON serializable.
        """
        return {}


# Used to clear out "None" values in templates
def jinja_finalize(thing):
    return thing if thing is not None else ""


env = Environment(finalize=jinja_finalize)


@attr.s(slots=True, frozen=True)
class JinjaOidcMappingConfig:
    subject_claim = attr.ib(type=str)
    localpart_template = attr.ib(type=Optional[Template])
    display_name_template = attr.ib(type=Optional[Template])
    email_template = attr.ib(type=Optional[Template])
    extra_attributes = attr.ib(type=Dict[str, Template])


class JinjaOidcMappingProvider(OidcMappingProvider[JinjaOidcMappingConfig]):
    """An implementation of a mapping provider based on Jinja templates.

    This is the default mapping provider.
    """

    def __init__(self, config: JinjaOidcMappingConfig):
        self._config = config

    @staticmethod
    def parse_config(config: dict) -> JinjaOidcMappingConfig:
        subject_claim = config.get("subject_claim", "sub")

        def parse_template_config(option_name: str) -> Optional[Template]:
            if option_name not in config:
                return None
            try:
                return env.from_string(config[option_name])
            except Exception as e:
                raise ConfigError("invalid jinja template", path=[option_name]) from e

        localpart_template = parse_template_config("localpart_template")
        display_name_template = parse_template_config("display_name_template")
        email_template = parse_template_config("email_template")

        extra_attributes = {}  # type Dict[str, Template]
        if "extra_attributes" in config:
            extra_attributes_config = config.get("extra_attributes") or {}
            if not isinstance(extra_attributes_config, dict):
                raise ConfigError("must be a dict", path=["extra_attributes"])

            for key, value in extra_attributes_config.items():
                try:
                    extra_attributes[key] = env.from_string(value)
                except Exception as e:
                    raise ConfigError(
                        "invalid jinja template", path=["extra_attributes", key]
                    ) from e

        return JinjaOidcMappingConfig(
            subject_claim=subject_claim,
            localpart_template=localpart_template,
            display_name_template=display_name_template,
            email_template=email_template,
            extra_attributes=extra_attributes,
        )

    def get_remote_user_id(self, userinfo: UserInfo) -> str:
        return userinfo[self._config.subject_claim]

    async def map_user_attributes(
        self, userinfo: UserInfo, token: Token, failures: int
    ) -> UserAttributeDict:
        localpart = None

        if self._config.localpart_template:
            localpart = self._config.localpart_template.render(user=userinfo).strip()

            # Ensure only valid characters are included in the MXID.
            localpart = map_username_to_mxid_localpart(localpart)

            # Append suffix integer if last call to this function failed to produce
            # a usable mxid.
            localpart += str(failures) if failures else ""

        def render_template_field(template: Optional[Template]) -> Optional[str]:
            if template is None:
                return None
            return template.render(user=userinfo).strip()

        display_name = render_template_field(self._config.display_name_template)
        if display_name == "":
            display_name = None

        emails = []  # type: List[str]
        email = render_template_field(self._config.email_template)
        if email:
            emails.append(email)

        return UserAttributeDict(
            localpart=localpart, display_name=display_name, emails=emails
        )

    async def get_extra_attributes(self, userinfo: UserInfo, token: Token) -> JsonDict:
        extras = {}  # type: Dict[str, str]
        for key, template in self._config.extra_attributes.items():
            try:
                extras[key] = template.render(user=userinfo).strip()
            except Exception as e:
                # Log an error and skip this value (don't break login for this).
                logger.error("Failed to render OIDC extra attribute %s: %s" % (key, e))
        return extras