Rename handler and config modules which end in handler/config. (#9816)

1 files changed, 1384 insertions, 0 deletions

diff --git a/synapse/handlers/oidc.py b/synapse/handlers/oidc.py
new file mode 100644
index 0000000000..45514be50f
--- /dev/null
+++ b/synapse/handlers/oidc.py
@@ -0,0 +1,1384 @@
+# Copyright 2020 Quentin Gliech
+# Copyright 2021 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 inspect
+import logging
+from typing import TYPE_CHECKING, Dict, Generic, List, Optional, TypeVar, Union
+from urllib.parse import urlencode
+
+import attr
+import pymacaroons
+from authlib.common.security import generate_token
+from authlib.jose import JsonWebToken, jwt
+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,
+    MacaroonInitException,
+    MacaroonInvalidSignatureException,
+)
+from typing_extensions import TypedDict
+
+from twisted.web.client import readBody
+from twisted.web.http_headers import Headers
+
+from synapse.config import ConfigError
+from synapse.config.oidc import OidcProviderClientSecretJwtKey, 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 Clock, json_decoder
+from synapse.util.caches.cached_call import RetryOnExceptionCachedCall
+from synapse.util.macaroons import get_value_from_macaroon, satisfy_expiry
+
+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.
+        """
+        # This will always be set by the time Twisted calls us.
+        assert request.args is not None
+
+        # 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 (MacaroonInitException, MacaroonDeserializationException, KeyError) 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._oidc_attribute_requirements = provider.attribute_requirements
+        self._scopes = provider.scopes
+        self._user_profile_method = provider.user_profile_method
+
+        client_secret = None  # type: Union[None, str, JwtClientSecret]
+        if provider.client_secret:
+            client_secret = provider.client_secret
+        elif provider.client_secret_jwt_key:
+            client_secret = JwtClientSecret(
+                provider.client_secret_jwt_key,
+                provider.client_id,
+                provider.issuer,
+                hs.get_clock(),
+            )
+
+        self._client_auth = ClientAuth(
+            provider.client_id,
+            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
+
+        # Optional brand identifier for the unstable API (see MSC2858).
+        self.unstable_idp_brand = provider.unstable_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")
+        raw_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, raw_headers, body = self._client_auth.prepare(
+            method="POST", uri=token_endpoint, headers=raw_headers, body=body
+        )
+        headers = Headers({k: [v] for (k, v) in raw_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 or "",
+            ),
+        )
+
+        # 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
+        logger.debug("Userinfo for OIDC login: %s", userinfo)
+
+        # Ensure that the attributes of the logged in user meet the required
+        # attributes by checking the userinfo against attribute_requirements
+        # In order to deal with the fact that OIDC userinfo can contain many
+        # types of data, we wrap non-list values in lists.
+        if not self._sso_handler.check_required_attributes(
+            request,
+            {k: v if isinstance(v, list) else [v] for k, v in userinfo.items()},
+            self._oidc_attribute_requirements,
+        ):
+            return
+
+        # 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)
+
+
+# number of seconds a newly-generated client secret should be valid for
+CLIENT_SECRET_VALIDITY_SECONDS = 3600
+
+# minimum remaining validity on a client secret before we should generate a new one
+CLIENT_SECRET_MIN_VALIDITY_SECONDS = 600
+
+
+class JwtClientSecret:
+    """A class which generates a new client secret on demand, based on a JWK
+
+    This implementation is designed to comply with the requirements for Apple Sign in:
+    https://developer.apple.com/documentation/sign_in_with_apple/generate_and_validate_tokens#3262048
+
+    It looks like those requirements are based on https://tools.ietf.org/html/rfc7523,
+    but it's worth noting that we still put the generated secret in the "client_secret"
+    field (or rather, whereever client_auth_method puts it) rather than in a
+    client_assertion field in the body as that RFC seems to require.
+    """
+
+    def __init__(
+        self,
+        key: OidcProviderClientSecretJwtKey,
+        oauth_client_id: str,
+        oauth_issuer: str,
+        clock: Clock,
+    ):
+        self._key = key
+        self._oauth_client_id = oauth_client_id
+        self._oauth_issuer = oauth_issuer
+        self._clock = clock
+        self._cached_secret = b""
+        self._cached_secret_replacement_time = 0
+
+    def __str__(self):
+        # if client_auth_method is client_secret_basic, then ClientAuth.prepare calls
+        # encode_client_secret_basic, which calls "{}".format(secret), which ends up
+        # here.
+        return self._get_secret().decode("ascii")
+
+    def __bytes__(self):
+        # if client_auth_method is client_secret_post, then ClientAuth.prepare calls
+        # encode_client_secret_post, which ends up here.
+        return self._get_secret()
+
+    def _get_secret(self) -> bytes:
+        now = self._clock.time()
+
+        # if we have enough validity on our existing secret, use it
+        if now < self._cached_secret_replacement_time:
+            return self._cached_secret
+
+        issued_at = int(now)
+        expires_at = issued_at + CLIENT_SECRET_VALIDITY_SECONDS
+
+        # we copy the configured header because jwt.encode modifies it.
+        header = dict(self._key.jwt_header)
+
+        # see https://tools.ietf.org/html/rfc7523#section-3
+        payload = {
+            "sub": self._oauth_client_id,
+            "aud": self._oauth_issuer,
+            "iat": issued_at,
+            "exp": expires_at,
+            **self._key.jwt_payload,
+        }
+        logger.info(
+            "Generating new JWT for %s: %s %s", self._oauth_issuer, header, payload
+        )
+        self._cached_secret = jwt.encode(header, payload, self._key.key)
+        self._cached_secret_replacement_time = (
+            expires_at - CLIENT_SECRET_MIN_VALIDITY_SECONDS
+        )
+        return self._cached_secret
+
+
+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,)
+        )
+        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:
+            KeyError 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 = "))
+        v.satisfy_general(lambda c: c.startswith("ui_auth_session_id = "))
+        satisfy_expiry(v, self._clock.time_msec)
+
+        v.verify(macaroon, self._macaroon_secret_key)
+
+        # Extract the session data from the token.
+        nonce = get_value_from_macaroon(macaroon, "nonce")
+        idp_id = get_value_from_macaroon(macaroon, "idp_id")
+        client_redirect_url = get_value_from_macaroon(macaroon, "client_redirect_url")
+        ui_auth_session_id = get_value_from_macaroon(macaroon, "ui_auth_session_id")
+        return OidcSessionData(
+            nonce=nonce,
+            idp_id=idp_id,
+            client_redirect_url=client_redirect_url,
+            ui_auth_session_id=ui_auth_session_id,
+        )
+
+
+@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 ("" if this is a login)
+    ui_auth_session_id = attr.ib(type=str)
+
+
+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