Give the user a better error when they present bad SSO creds

If a user tries to do UI Auth via SSO, but uses the wrong account on the SSO
IdP, try to give them a better error.

Previously, the UIA would claim to be successful, but then the operation in
question would simply fail with "auth fail". Instead, serve up an error page
which explains the failure.

changelog.d/9091.feature

@@ -0,0 +1 @@
+During user-interactive authentication via single-sign-on, give a better error if the user uses the wrong account on the SSO IdP.

docs/sample_config.yaml

@@ -1969,6 +1969,14 @@ sso:
     #
     #   This template has no additional variables.
     #
+    # * HTML page shown after a user-interactive authentication session which
+    #   does not map correctly onto the expected user: 'sso_auth_bad_user.html'.
+    #
+    #   When rendering, this template is given the following variables:
+    #     * server_name: the homeserver's name.
+    #     * user_id_to_verify: the MXID of the user that we are trying to
+    #       validate.
+    #
     # * HTML page shown during single sign-on if a deactivated user (according to Synapse's database)
     #   attempts to login: 'sso_account_deactivated.html'.
     #

synapse/config/sso.py

@@ -37,6 +37,7 @@ def read_config(self, config, **kwargs):
             self.sso_error_template,
             sso_account_deactivated_template,
             sso_auth_success_template,
+            self.sso_auth_bad_user_template,
         ) = self.read_templates(
             [
                 "sso_login_idp_picker.html",
@@ -45,6 +46,7 @@ def read_config(self, config, **kwargs):
                 "sso_error.html",
                 "sso_account_deactivated.html",
                 "sso_auth_success.html",
+                "sso_auth_bad_user.html",
             ],
             template_dir,
         )
@@ -160,6 +162,14 @@ def generate_config_section(self, **kwargs):
             #
             #   This template has no additional variables.
             #
+            # * HTML page shown after a user-interactive authentication session which
+            #   does not map correctly onto the expected user: 'sso_auth_bad_user.html'.
+            #
+            #   When rendering, this template is given the following variables:
+            #     * server_name: the homeserver's name.
+            #     * user_id_to_verify: the MXID of the user that we are trying to
+            #       validate.
+            #
             # * HTML page shown during single sign-on if a deactivated user (according to Synapse's database)
             #   attempts to login: 'sso_account_deactivated.html'.
             #

synapse/handlers/sso.py

@@ -23,6 +23,7 @@
 from twisted.web.http import Request
 
 from synapse.api.errors import Codes, RedirectException, SynapseError
+from synapse.handlers.ui_auth import UIAuthSessionDataConstants
 from synapse.http import get_request_user_agent
 from synapse.http.server import respond_with_html
 from synapse.http.site import SynapseRequest
@@ -147,6 +148,7 @@ def __init__(self, hs: "HomeServer"):
         self._server_name = hs.hostname
         self._registration_handler = hs.get_registration_handler()
         self._error_template = hs.config.sso_error_template
+        self._bad_user_template = hs.config.sso_auth_bad_user_template
         self._auth_handler = hs.get_auth_handler()
 
         # a lock on the mappings
@@ -577,19 +579,35 @@ async def complete_sso_ui_auth_request(
             auth_provider_id, remote_user_id,
         )
 
+        user_id_to_verify = await self._auth_handler.get_session_data(
+            ui_auth_session_id, UIAuthSessionDataConstants.REQUEST_USER_ID
+        )  # type: str
+
         if not user_id:
             logger.warning(
                 "Remote user %s/%s has not previously logged in here: UIA will fail",
                 auth_provider_id,
                 remote_user_id,
             )
-            # Let the UIA flow handle this the same as if they presented creds for a
-            # different user.
-            user_id = ""
+        elif user_id != user_id_to_verify:
+            logger.warning(
+                "Remote user %s/%s mapped onto incorrect user %s: UIA will fail",
+                auth_provider_id,
+                remote_user_id,
+                user_id,
+            )
+        else:
+            # success!
+            await self._auth_handler.complete_sso_ui_auth(
+                user_id, ui_auth_session_id, request
+            )
+            return
 
-        await self._auth_handler.complete_sso_ui_auth(
-            user_id, ui_auth_session_id, request
+        # the user_id didn't match: render an error page.
+        html = self._bad_user_template.render(
+            server_name=self._server_name, user_id_to_verify=user_id_to_verify,
         )
+        respond_with_html(request, 200, html)
 
     async def check_username_availability(
         self, localpart: str, session_id: str,

synapse/res/templates/sso_auth_bad_user.html

@@ -0,0 +1,18 @@
+<html>
+<head>
+    <title>Authentication Failed</title>
+</head>
+    <body>
+        <div>
+            <p>
+                We were unable to validate your <tt>{{server_name | e}}</tt> account via
+                single-sign-on (SSO), because the SSO Identity Provider returned
+                different details than when you logged in.
+            </p>
+            <p>
+                Try the operation again, and ensure that you use the same details on
+                the Identity Provider as when you log into your account.
+            </p>
+        </div>
+    </body>
+</html>