Add a confirmation step to the SSO login flow

docs/sample_config.yaml

@@ -1360,6 +1360,40 @@ saml2_config:
 #   #    name: value
 
 
+# Additional settings to use with single-sign on systems such as SAML2 and CAS.
+#
+sso:
+    # Directory in which Synapse will try to find the template files below.
+    # If not set, default templates from within the Synapse package will be used.
+    #
+    # DO NOT UNCOMMENT THIS SETTING unless you want to customise the templates.
+    # If you *do* uncomment it, you will need to make sure that all the templates
+    # below are in the directory.
+    #
+    # Synapse will look for the following templates in this directory:
+    #
+    # * HTML page for confirmation of redirect during authentication:
+    #   'sso_redirect_confirm.html'.
+    #
+    #   When rendering, this template is given three variables:
+    #     * redirect_url: the URL the user is about to be redirected to. Needs
+    #                     manual escaping (see
+    #                     https://jinja.palletsprojects.com/en/2.11.x/templates/#html-escaping).
+    #
+    #     * display_url: the same as `redirect_url`, but with the query
+    #                    parameters stripped. The intention is to have a 
+    #                    human-readable URL to show to users, not to use it as
+    #                    the final address to redirect to. Needs manual escaping
+    #                    (see https://jinja.palletsprojects.com/en/2.11.x/templates/#html-escaping).
+    #
+    #     * server_name: the homeserver's name.
+    #
+    # You can see the default templates at:
+    # https://github.com/matrix-org/synapse/tree/master/synapse/res/templates
+    #
+    #template_dir: "res/templates"
+
+
 # The JWT needs to contain a globally unique "sub" (subject) claim.
 #
 #jwt_config:

synapse/config/_base.pyi

@@ -24,6 +24,7 @@ from synapse.config import (
     server,
     server_notices_config,
     spam_checker,
+    sso,
     stats,
     third_party_event_rules,
     tls,
@@ -57,6 +58,7 @@ class RootConfig:
     key: key.KeyConfig
     saml2: saml2_config.SAML2Config
     cas: cas.CasConfig
+    sso: sso.SSOConfig
     jwt: jwt_config.JWTConfig
     password: password.PasswordConfig
     email: emailconfig.EmailConfig

synapse/config/homeserver.py

@@ -38,6 +38,7 @@ from .saml2_config import SAML2Config
 from .server import ServerConfig
 from .server_notices_config import ServerNoticesConfig
 from .spam_checker import SpamCheckerConfig
+from .sso import SSOConfig
 from .stats import StatsConfig
 from .third_party_event_rules import ThirdPartyRulesConfig
 from .tls import TlsConfig
@@ -65,6 +66,7 @@ class HomeServerConfig(RootConfig):
         KeyConfig,
         SAML2Config,
         CasConfig,
+        SSOConfig,
         JWTConfig,
         PasswordConfig,
         EmailConfig,

synapse/config/sso.py

@@ -0,0 +1,74 @@
+# -*- coding: utf-8 -*-
+# Copyright 2020 The Matrix.org Foundation C.I.C.
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+from typing import Any, Dict
+
+import pkg_resources
+
+from ._base import Config, ConfigError
+
+
+class SSOConfig(Config):
+    """SSO Configuration
+    """
+
+    section = "sso"
+
+    def read_config(self, config, **kwargs):
+        sso_config = config.get("sso") or {}  # type: Dict[str, Any]
+
+        # Pick a template directory in order of:
+        # * The sso-specific template_dir
+        # * /path/to/synapse/install/res/templates
+        template_dir = sso_config.get("template_dir")
+        if not template_dir:
+            template_dir = pkg_resources.resource_filename("synapse", "res/templates",)
+
+        self.sso_redirect_confirm_template_dir = template_dir
+
+    def generate_config_section(self, **kwargs):
+        return """\
+        # Additional settings to use with single-sign on systems such as SAML2 and CAS.
+        #
+        sso:
+            # Directory in which Synapse will try to find the template files below.
+            # If not set, default templates from within the Synapse package will be used.
+            #
+            # DO NOT UNCOMMENT THIS SETTING unless you want to customise the templates.
+            # If you *do* uncomment it, you will need to make sure that all the templates
+            # below are in the directory.
+            #
+            # Synapse will look for the following templates in this directory:
+            #
+            # * HTML page for a confirmation step before redirecting back to the client
+            #   with the login token: 'sso_redirect_confirm.html'.
+            #
+            #   When rendering, this template is given three variables:
+            #     * redirect_url: the URL the user is about to be redirected to. Needs
+            #                     manual escaping (see
+            #                     https://jinja.palletsprojects.com/en/2.11.x/templates/#html-escaping).
+            #
+            #     * display_url: the same as `redirect_url`, but with the query
+            #                    parameters stripped. The intention is to have a
+            #                    human-readable URL to show to users, not to use it as
+            #                    the final address to redirect to. Needs manual escaping
+            #                    (see https://jinja.palletsprojects.com/en/2.11.x/templates/#html-escaping).
+            #
+            #     * server_name: the homeserver's name.
+            #
+            # You can see the default templates at:
+            # https://github.com/matrix-org/synapse/tree/master/synapse/res/templates
+            #
+            #template_dir: "res/templates"
+        """

synapse/res/templates/sso_redirect_confirm.html

@@ -0,0 +1,14 @@
+<!DOCTYPE html>
+<html lang="en">
+<head>
+    <meta charset="UTF-8">
+    <title>SSO redirect confirmation</title>
+</head>
+    <body>
+        <p>The application at <span style="font-weight:bold">{{ display_url | e }}</span> is requesting full access to your <span style="font-weight:bold">{{ server_name }}</span> Matrix account.</p>
+        <p>If you don't recognise this address, you should ignore this and close this tab.</p>
+        <p>
+            <a href="{{ redirect_url | e }}">I trust this address</a>
+        </p>
+    </body>
+</html>

synapse/rest/client/v1/login.py

@@ -29,6 +29,7 @@ from synapse.http.servlet import (
     parse_string,
 )
 from synapse.http.site import SynapseRequest
+from synapse.push.mailer import load_jinja2_templates
 from synapse.rest.client.v2_alpha._base import client_patterns
 from synapse.rest.well_known import WellKnownBuilder
 from synapse.types import UserID, map_username_to_mxid_localpart
@@ -548,6 +549,13 @@ class SSOAuthHandler(object):
         self._registration_handler = hs.get_registration_handler()
         self._macaroon_gen = hs.get_macaroon_generator()
 
+        # Load the redirect page HTML template
+        self._template = load_jinja2_templates(
+            hs.config.sso_redirect_confirm_template_dir, ["sso_redirect_confirm.html"],
+        )[0]
+
+        self._server_name = hs.config.server_name
+
     async def on_successful_auth(
         self, username, request, client_redirect_url, user_display_name=None
     ):
@@ -592,21 +600,41 @@ class SSOAuthHandler(object):
             request:
             client_redirect_url:
         """
-
+        # Create a login token
         login_token = self._macaroon_gen.generate_short_term_login_token(
             registered_user_id
         )
-        redirect_url = self._add_login_token_to_redirect_url(
+
-            client_redirect_url, login_token
+        # Remove the query parameters from the redirect URL to get a shorter version of
+        # it. This is only to display a human-readable URL in the template, but not the
+        # URL we redirect users to.
+        redirect_url_no_params = client_redirect_url.split("?")[0]
+
+        # Append the login token to the original redirect URL (i.e. with its query
+        # parameters kept intact) to build the URL to which the template needs to
+        # redirect the users once they have clicked on the confirmation link.
+        redirect_url = self._add_query_param_to_url(
+            client_redirect_url, "loginToken", login_token
         )
-        request.redirect(redirect_url)
+
+        # Serve the redirect confirmation page
+        html = self._template.render(
+            display_url=redirect_url_no_params,
+            redirect_url=redirect_url,
+            server_name=self._server_name,
+        )
+
+        request.setResponseCode(200)
+        request.setHeader(b"Content-Type", b"text/html; charset=utf-8")
+        request.setHeader(b"Content-Length", b"%d" % (len(html),))
+        request.write(html.encode("utf8"))
         finish_request(request)
 
     @staticmethod
-    def _add_login_token_to_redirect_url(url, token):
+    def _add_query_param_to_url(url, param_name, param):
         url_parts = list(urllib.parse.urlparse(url))
         query = dict(urllib.parse.parse_qsl(url_parts[4]))
-        query.update({"loginToken": token})
+        query.update({param_name: param})
         url_parts[4] = urllib.parse.urlencode(query)
         return urllib.parse.urlunparse(url_parts)
 

tests/rest/client/v1/test_login.py

@@ -1,4 +1,7 @@
 import json
+import urllib.parse
+
+from mock import Mock
 
 import synapse.rest.admin
 from synapse.rest.client.v1 import login
@@ -252,3 +255,85 @@ class LoginRestServletTestCase(unittest.HomeserverTestCase):
         )
         self.render(request)
         self.assertEquals(channel.code, 200, channel.result)
+
+
+class CASRedirectConfirmTestCase(unittest.HomeserverTestCase):
+
+    servlets = [
+        login.register_servlets,
+    ]
+
+    def make_homeserver(self, reactor, clock):
+        self.base_url = "https://matrix.goodserver.com/"
+        self.redirect_path = "_synapse/client/login/sso/redirect/confirm"
+
+        config = self.default_config()
+        config["enable_registration"] = True
+        config["cas_config"] = {
+            "enabled": True,
+            "server_url": "https://fake.test",
+            "service_url": "https://matrix.goodserver.com:8448",
+        }
+        config["public_baseurl"] = self.base_url
+
+        async def get_raw(uri, args):
+            """Return an example response payload from a call to the `/proxyValidate`
+            endpoint of a CAS server, copied from
+            https://apereo.github.io/cas/5.0.x/protocol/CAS-Protocol-V2-Specification.html#26-proxyvalidate-cas-20
+
+            This needs to be returned by an async function (as opposed to set as the
+            mock's return value) because the corresponding Synapse code awaits on it.
+            """
+            return """
+                <cas:serviceResponse xmlns:cas='http://www.yale.edu/tp/cas'>
+                  <cas:authenticationSuccess>
+                      <cas:user>username</cas:user>
+                      <cas:proxyGrantingTicket>PGTIOU-84678-8a9d...</cas:proxyGrantingTicket>
+                      <cas:proxies>
+                          <cas:proxy>https://proxy2/pgtUrl</cas:proxy>
+                          <cas:proxy>https://proxy1/pgtUrl</cas:proxy>
+                      </cas:proxies>
+                  </cas:authenticationSuccess>
+                </cas:serviceResponse>
+            """
+
+        mocked_http_client = Mock(spec=["get_raw"])
+        mocked_http_client.get_raw.side_effect = get_raw
+
+        self.hs = self.setup_test_homeserver(
+            config=config, proxied_http_client=mocked_http_client,
+        )
+
+        return self.hs
+
+    def test_cas_redirect_confirm(self):
+        """Tests that the SSO login flow serves a confirmation page before redirecting a
+        user to the redirect URL.
+        """
+        base_url = "/login/cas/ticket?redirectUrl"
+        redirect_url = "https://dodgy-site.com/"
+
+        url_parts = list(urllib.parse.urlparse(base_url))
+        query = dict(urllib.parse.parse_qsl(url_parts[4]))
+        query.update({"redirectUrl": redirect_url})
+        query.update({"ticket": "ticket"})
+        url_parts[4] = urllib.parse.urlencode(query)
+        cas_ticket_url = urllib.parse.urlunparse(url_parts)
+
+        # Get Synapse to call the fake CAS and serve the template.
+        request, channel = self.make_request("GET", cas_ticket_url)
+        self.render(request)
+
+        # Test that the response is HTML.
+        content_type_header_value = ""
+        for header in channel.result.get("headers", []):
+            if header[0] == b"Content-Type":
+                content_type_header_value = header[1].decode("utf8")
+
+        self.assertTrue(content_type_header_value.startswith("text/html"))
+
+        # Test that the body isn't empty.
+        self.assertTrue(len(channel.result["body"]) > 0)
+
+        # And that it contains our redirect link
+        self.assertIn(redirect_url, channel.result["body"].decode("UTF-8"))