Do not validate that the client dict is stable during UI Auth. (#7483)

This backs out some of the validation for the client dictionary and logs if
this changes during a user interactive authentication session instead.

changelog.d/7483.bugfix

@@ -0,0 +1 @@
+Restore compatibility with non-compliant clients during the user interactive authentication process.

synapse/handlers/auth.py

@@ -252,7 +252,6 @@ async def check_auth(
         clientdict: Dict[str, Any],
         clientip: str,
         description: str,
-        validate_clientdict: bool = True,
     ) -> Tuple[dict, dict, str]:
         """
         Takes a dictionary sent by the client in the login / registration
@@ -278,10 +277,6 @@ async def check_auth(
             description: A human readable string to be displayed to the user that
                          describes the operation happening on their account.
 
-            validate_clientdict: Whether to validate that the operation happening
-                                 on the account has not changed. If this is false,
-                                 the client dict is persisted instead of validated.
-
         Returns:
             A tuple of (creds, params, session_id).
 
@@ -346,26 +341,30 @@ async def check_auth(
 
             # Ensure that the queried operation does not vary between stages of
             # the UI authentication session. This is done by generating a stable
-            # comparator based on the URI, method, and client dict (minus the
-            # auth dict) and storing it during the initial query. Subsequent
+            # comparator and storing it during the initial query. Subsequent
             # queries ensure that this comparator has not changed.
-            if validate_clientdict:
-                session_comparator = (session.uri, session.method, session.clientdict)
-                comparator = (uri, method, clientdict)
-            else:
-                session_comparator = (session.uri, session.method)  # type: ignore
-                comparator = (uri, method)  # type: ignore
-
-            if session_comparator != comparator:
+            #
+            # The comparator is based on the requested URI and HTTP method. The
+            # client dict (minus the auth dict) should also be checked, but some
+            # clients are not spec compliant, just warn for now if the client
+            # dict changes.
+            if (session.uri, session.method) != (uri, method):
                 raise SynapseError(
                     403,
                     "Requested operation has changed during the UI authentication session.",
                 )
 
-            # For backwards compatibility the registration endpoint persists
-            # changes to the client dict instead of validating them.
-            if not validate_clientdict:
-                await self.store.set_ui_auth_clientdict(sid, clientdict)
+            if session.clientdict != clientdict:
+                logger.warning(
+                    "Requested operation has changed during the UI "
+                    "authentication session. A future version of Synapse "
+                    "will remove this capability."
+                )
+
+            # For backwards compatibility, changes to the client dict are
+            # persisted as clients modify them throughout their user interactive
+            # authentication flow.
+            await self.store.set_ui_auth_clientdict(sid, clientdict)
 
         if not authdict:
             raise InteractiveAuthIncompleteError(

synapse/rest/client/v2_alpha/register.py

@@ -516,7 +516,6 @@ async def on_POST(self, request):
             body,
             self.hs.get_ip_from_request(request),
             "register a new account",
-            validate_clientdict=False,
         )
 
         # Check that we're not trying to register a denied 3pid.

tests/rest/client/v2_alpha/test_auth.py

@@ -133,47 +133,6 @@ def test_fallback_captcha(self):
         # We're given a registered user.
         self.assertEqual(channel.json_body["user_id"], "@user:test")
 
-    def test_legacy_registration(self):
-        """
-        Registration allows the parameters to vary through the process.
-        """
-
-        # Make the initial request to register. (Later on a different password
-        # will be used.)
-        # Returns a 401 as per the spec
-        channel = self.register(
-            401, {"username": "user", "type": "m.login.password", "password": "bar"},
-        )
-
-        # Grab the session
-        session = channel.json_body["session"]
-        # Assert our configured public key is being given
-        self.assertEqual(
-            channel.json_body["params"]["m.login.recaptcha"]["public_key"], "brokencake"
-        )
-
-        # Complete the recaptcha step.
-        self.recaptcha(session, 200)
-
-        # also complete the dummy auth
-        self.register(200, {"auth": {"session": session, "type": "m.login.dummy"}})
-
-        # Now we should have fulfilled a complete auth flow, including
-        # the recaptcha fallback step. Make the initial request again, but
-        # with a changed password. This still completes.
-        channel = self.register(
-            200,
-            {
-                "username": "user",
-                "type": "m.login.password",
-                "password": "foo",  # Note that this is different.
-                "auth": {"session": session},
-            },
-        )
-
-        # We're given a registered user.
-        self.assertEqual(channel.json_body["user_id"], "@user:test")
-
     def test_complete_operation_unknown_session(self):
         """
         Attempting to mark an invalid session as complete should error.
@@ -282,9 +241,15 @@ def test_ui_auth(self):
             },
         )
 
-    def test_cannot_change_body(self):
+    def test_can_change_body(self):
         """
-        The initial requested client dict cannot be modified during the user interactive authentication session.
+        The client dict can be modified during the user interactive authentication session.
+
+        Note that it is not spec compliant to modify the client dict during a
+        user interactive authentication session, but many clients currently do.
+
+        When Synapse is updated to be spec compliant, the call to re-use the
+        session ID should be rejected.
         """
         # Create a second login.
         self.login("test", self.user_pass)
@@ -302,9 +267,9 @@ def test_cannot_change_body(self):
         self.assertIn({"stages": ["m.login.password"]}, channel.json_body["flows"])
 
         # Make another request providing the UI auth flow, but try to delete the
-        # second device. This results in an error.
+        # second device.
         self.delete_devices(
-            403,
+            200,
             {
                 "devices": [device_ids[1]],
                 "auth": {