msc3861_delegated.py

#
# This file is licensed under the Affero General Public License (AGPL) version 3.
#
# Copyright 2023 The Matrix.org Foundation.
# Copyright (C) 2023 New Vector, Ltd
#
# This program is free software: you can redistribute it and/or modify
# it under the terms of the GNU Affero General Public License as
# published by the Free Software Foundation, either version 3 of the
# License, or (at your option) any later version.
#
# See the GNU Affero General Public License for more details:
# <https://www.gnu.org/licenses/agpl-3.0.html>.
#
# Originally licensed under the Apache License, Version 2.0:
# <http://www.apache.org/licenses/LICENSE-2.0>.
#
# [This file includes modifications made by New Vector Limited]
#
#
import logging
from typing import TYPE_CHECKING, Any, Dict, List, Optional
from urllib.parse import urlencode

from authlib.oauth2 import ClientAuth
from authlib.oauth2.auth import encode_client_secret_basic, encode_client_secret_post
from authlib.oauth2.rfc7523 import ClientSecretJWT, PrivateKeyJWT, private_key_jwt_sign
from authlib.oauth2.rfc7662 import IntrospectionToken
from authlib.oidc.discovery import OpenIDProviderMetadata, get_well_known_url
from prometheus_client import Histogram

from twisted.web.client import readBody
from twisted.web.http_headers import Headers

from synapse.api.auth.base import BaseAuth
from synapse.api.errors import (
    AuthError,
    HttpResponseException,
    InvalidClientTokenError,
    OAuthInsufficientScopeError,
    StoreError,
    SynapseError,
    UnrecognizedRequestError,
)
from synapse.http.site import SynapseRequest
from synapse.logging.context import make_deferred_yieldable
from synapse.types import Requester, UserID, create_requester
from synapse.util import json_decoder
from synapse.util.caches.cached_call import RetryOnExceptionCachedCall

if TYPE_CHECKING:
    from synapse.rest.admin.experimental_features import ExperimentalFeature
    from synapse.server import HomeServer

logger = logging.getLogger(__name__)

introspection_response_timer = Histogram(
    "synapse_api_auth_delegated_introspection_response",
    "Time taken to get a response for an introspection request",
    ["code"],
)


# Scope as defined by MSC2967
# https://github.com/matrix-org/matrix-spec-proposals/pull/2967
SCOPE_MATRIX_API = "urn:matrix:org.matrix.msc2967.client:api:*"
SCOPE_MATRIX_GUEST = "urn:matrix:org.matrix.msc2967.client:api:guest"
SCOPE_MATRIX_DEVICE_PREFIX = "urn:matrix:org.matrix.msc2967.client:device:"

# Scope which allows access to the Synapse admin API
SCOPE_SYNAPSE_ADMIN = "urn:synapse:admin:*"


def scope_to_list(scope: str) -> List[str]:
    """Convert a scope string to a list of scope tokens"""
    return scope.strip().split(" ")


class PrivateKeyJWTWithKid(PrivateKeyJWT):  # type: ignore[misc]
    """An implementation of the private_key_jwt client auth method that includes a kid header.

    This is needed because some providers (Keycloak) require the kid header to figure
    out which key to use to verify the signature.
    """

    def sign(self, auth: Any, token_endpoint: str) -> bytes:
        return private_key_jwt_sign(
            auth.client_secret,
            client_id=auth.client_id,
            token_endpoint=token_endpoint,
            claims=self.claims,
            header={"kid": auth.client_secret["kid"]},
        )


class MSC3861DelegatedAuth(BaseAuth):
    AUTH_METHODS = {
        "client_secret_post": encode_client_secret_post,
        "client_secret_basic": encode_client_secret_basic,
        "client_secret_jwt": ClientSecretJWT(),
        "private_key_jwt": PrivateKeyJWTWithKid(),
    }

    EXTERNAL_ID_PROVIDER = "oauth-delegated"

    def __init__(self, hs: "HomeServer"):
        super().__init__(hs)

        self._config = hs.config.experimental.msc3861
        auth_method = MSC3861DelegatedAuth.AUTH_METHODS.get(
            self._config.client_auth_method.value, None
        )
        # Those assertions are already checked when parsing the config
        assert self._config.enabled, "OAuth delegation is not enabled"
        assert self._config.issuer, "No issuer provided"
        assert self._config.client_id, "No client_id provided"
        assert auth_method is not None, "Invalid client_auth_method provided"

        self._clock = hs.get_clock()
        self._http_client = hs.get_proxied_http_client()
        self._hostname = hs.hostname
        self._admin_token = self._config.admin_token

        self._issuer_metadata = RetryOnExceptionCachedCall[OpenIDProviderMetadata](
            self._load_metadata
        )

        if isinstance(auth_method, PrivateKeyJWTWithKid):
            # Use the JWK as the client secret when using the private_key_jwt method
            assert self._config.jwk, "No JWK provided"
            self._client_auth = ClientAuth(
                self._config.client_id, self._config.jwk, auth_method
            )
        else:
            # Else use the client secret
            assert self._config.client_secret, "No client_secret provided"
            self._client_auth = ClientAuth(
                self._config.client_id, self._config.client_secret, auth_method
            )

    async def _load_metadata(self) -> OpenIDProviderMetadata:
        if self._config.issuer_metadata is not None:
            return OpenIDProviderMetadata(**self._config.issuer_metadata)
        url = get_well_known_url(self._config.issuer, external=True)
        response = await self._http_client.get_json(url)
        metadata = OpenIDProviderMetadata(**response)
        # metadata.validate_introspection_endpoint()
        return metadata

    async def issuer(self) -> str:
        """
        Get the configured issuer

        This will use the issuer value set in the metadata,
        falling back to the one set in the config if not set in the metadata
        """
        metadata = await self._issuer_metadata.get()
        return metadata.issuer or self._config.issuer

    async def account_management_url(self) -> Optional[str]:
        """
        Get the configured account management URL

        This will discover the account management URL from the issuer if it's not set in the config
        """
        if self._config.account_management_url is not None:
            return self._config.account_management_url

        try:
            metadata = await self._issuer_metadata.get()
            return metadata.get("account_management_uri", None)
        # We don't want to raise here if we can't load the metadata
        except Exception:
            logger.warning("Failed to load metadata:", exc_info=True)
            return None

    async def _introspection_endpoint(self) -> str:
        """
        Returns the introspection endpoint of the issuer

        It uses the config option if set, otherwise it will use OIDC discovery to get it
        """
        if self._config.introspection_endpoint is not None:
            return self._config.introspection_endpoint

        metadata = await self._issuer_metadata.get()
        return metadata.get("introspection_endpoint")

    async def _introspect_token(self, token: str) -> IntrospectionToken:
        """
        Send a token to the introspection endpoint and returns the introspection response

        Parameters:
            token: The token to introspect

        Raises:
            HttpResponseException: If the introspection endpoint returns a non-2xx response
            ValueError: If the introspection endpoint returns an invalid JSON response
            JSONDecodeError: If the introspection endpoint returns a non-JSON response
            Exception: If the HTTP request fails

        Returns:
            The introspection response
        """
        introspection_endpoint = await self._introspection_endpoint()
        raw_headers: Dict[str, str] = {
            "Content-Type": "application/x-www-form-urlencoded",
            "User-Agent": str(self._http_client.user_agent, "utf-8"),
            "Accept": "application/json",
        }

        args = {"token": token, "token_type_hint": "access_token"}
        body = urlencode(args, True)

        # Fill the body/headers with credentials
        uri, raw_headers, body = self._client_auth.prepare(
            method="POST", uri=introspection_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 ourselves.

        start_time = self._clock.time()
        try:
            response = await self._http_client.request(
                method="POST",
                uri=uri,
                data=body.encode("utf-8"),
                headers=headers,
            )

            resp_body = await make_deferred_yieldable(readBody(response))
        except Exception:
            end_time = self._clock.time()
            introspection_response_timer.labels("ERR").observe(end_time - start_time)
            raise

        end_time = self._clock.time()
        introspection_response_timer.labels(response.code).observe(
            end_time - start_time
        )

        if response.code < 200 or response.code >= 300:
            raise HttpResponseException(
                response.code,
                response.phrase.decode("ascii", errors="replace"),
                resp_body,
            )

        resp = json_decoder.decode(resp_body.decode("utf-8"))

        if not isinstance(resp, dict):
            raise ValueError(
                "The introspection endpoint returned an invalid JSON response."
            )

        return IntrospectionToken(**resp)

    async def is_server_admin(self, requester: Requester) -> bool:
        return "urn:synapse:admin:*" in requester.scope

    async def get_user_by_req(
        self,
        request: SynapseRequest,
        allow_guest: bool = False,
        allow_expired: bool = False,
        allow_locked: bool = False,
    ) -> Requester:
        access_token = self.get_access_token_from_request(request)

        requester = await self.get_appservice_user(request, access_token)
        if not requester:
            # TODO: we probably want to assert the allow_guest inside this call
            # so that we don't provision the user if they don't have enough permission:
            requester = await self.get_user_by_access_token(access_token, allow_expired)

        # Do not record requests from MAS using the virtual `__oidc_admin` user.
        if access_token != self._admin_token:
            await self._record_request(request, requester)

        if not allow_guest and requester.is_guest:
            raise OAuthInsufficientScopeError([SCOPE_MATRIX_API])

        request.requester = requester

        return requester

    async def get_user_by_req_experimental_feature(
        self,
        request: SynapseRequest,
        feature: "ExperimentalFeature",
        allow_guest: bool = False,
        allow_expired: bool = False,
        allow_locked: bool = False,
    ) -> Requester:
        try:
            requester = await self.get_user_by_req(
                request,
                allow_guest=allow_guest,
                allow_expired=allow_expired,
                allow_locked=allow_locked,
            )
            if await self.store.is_feature_enabled(requester.user.to_string(), feature):
                return requester

            raise UnrecognizedRequestError(code=404)
        except (AuthError, InvalidClientTokenError):
            if feature.is_globally_enabled(self.hs.config):
                # If its globally enabled then return the auth error
                raise

            raise UnrecognizedRequestError(code=404)

    async def get_user_by_access_token(
        self,
        token: str,
        allow_expired: bool = False,
    ) -> Requester:
        if self._admin_token is not None and token == self._admin_token:
            # XXX: This is a temporary solution so that the admin API can be called by
            # the OIDC provider. This will be removed once we have OIDC client
            # credentials grant support in matrix-authentication-service.
            logging.info("Admin toked used")
            # XXX: that user doesn't exist and won't be provisioned.
            # This is mostly fine for admin calls, but we should also think about doing
            # requesters without a user_id.
            admin_user = UserID("__oidc_admin", self._hostname)
            return create_requester(
                user_id=admin_user,
                scope=["urn:synapse:admin:*"],
            )

        try:
            introspection_result = await self._introspect_token(token)
        except Exception:
            logger.exception("Failed to introspect token")
            raise SynapseError(503, "Unable to introspect the access token")

        logger.debug("Introspection result: %r", introspection_result)

        # TODO: introspection verification should be more extensive, especially:
        #   - verify the audience
        if not introspection_result.get("active"):
            raise InvalidClientTokenError("Token is not active")

        # Let's look at the scope
        scope: List[str] = scope_to_list(introspection_result.get("scope", ""))

        # Determine type of user based on presence of particular scopes
        has_user_scope = SCOPE_MATRIX_API in scope
        has_guest_scope = SCOPE_MATRIX_GUEST in scope

        if not has_user_scope and not has_guest_scope:
            raise InvalidClientTokenError("No scope in token granting user rights")

        # Match via the sub claim
        sub: Optional[str] = introspection_result.get("sub")
        if sub is None:
            raise InvalidClientTokenError(
                "Invalid sub claim in the introspection result"
            )

        user_id_str = await self.store.get_user_by_external_id(
            MSC3861DelegatedAuth.EXTERNAL_ID_PROVIDER, sub
        )
        if user_id_str is None:
            # If we could not find a user via the external_id, it either does not exist,
            # or the external_id was never recorded

            # TODO: claim mapping should be configurable
            username: Optional[str] = introspection_result.get("username")
            if username is None or not isinstance(username, str):
                raise AuthError(
                    500,
                    "Invalid username claim in the introspection result",
                )
            user_id = UserID(username, self._hostname)

            # First try to find a user from the username claim
            user_info = await self.store.get_user_by_id(user_id=user_id.to_string())
            if user_info is None:
                # If the user does not exist, we should create it on the fly
                # TODO: we could use SCIM to provision users ahead of time and listen
                # for SCIM SET events if those ever become standard:
                # https://datatracker.ietf.org/doc/html/draft-hunt-scim-notify-00

                # TODO: claim mapping should be configurable
                # If present, use the name claim as the displayname
                name: Optional[str] = introspection_result.get("name")

                await self.store.register_user(
                    user_id=user_id.to_string(), create_profile_with_displayname=name
                )

            # And record the sub as external_id
            await self.store.record_user_external_id(
                MSC3861DelegatedAuth.EXTERNAL_ID_PROVIDER, sub, user_id.to_string()
            )
        else:
            user_id = UserID.from_string(user_id_str)

        # Find device_ids in scope
        # We only allow a single device_id in the scope, so we find them all in the
        # scope list, and raise if there are more than one. The OIDC server should be
        # the one enforcing valid scopes, so we raise a 500 if we find an invalid scope.
        device_ids = [
            tok[len(SCOPE_MATRIX_DEVICE_PREFIX) :]
            for tok in scope
            if tok.startswith(SCOPE_MATRIX_DEVICE_PREFIX)
        ]

        if len(device_ids) > 1:
            raise AuthError(
                500,
                "Multiple device IDs in scope",
            )

        device_id = device_ids[0] if device_ids else None
        if device_id is not None:
            # Sanity check the device_id
            if len(device_id) > 255 or len(device_id) < 1:
                raise AuthError(
                    500,
                    "Invalid device ID in scope",
                )

            # Create the device on the fly if it does not exist
            try:
                await self.store.get_device(
                    user_id=user_id.to_string(), device_id=device_id
                )
            except StoreError:
                await self.store.store_device(
                    user_id=user_id.to_string(),
                    device_id=device_id,
                    initial_device_display_name="OIDC-native client",
                )

        # TODO: there is a few things missing in the requester here, which still need
        # to be figured out, like:
        #   - impersonation, with the `authenticated_entity`, which is used for
        #     rate-limiting, MAU limits, etc.
        #   - shadow-banning, with the `shadow_banned` flag
        #   - a proper solution for appservices, which still needs to be figured out in
        #     the context of MSC3861
        return create_requester(
            user_id=user_id,
            device_id=device_id,
            scope=scope,
            is_guest=(has_guest_scope and not has_user_scope),
        )