Handle "registration_enabled" parameter for CAS (#16262)

Similar to OIDC, CAS providers can now disable registration such
that only existing users are able to login via SSO.

changelog.d/16262.feature

@@ -0,0 +1 @@
+Add the ability to enable/disable registrations when in the CAS flow. Contributed by Aurélien Grimpard.

docs/usage/configuration/config_documentation.md

@@ -3430,6 +3430,12 @@ Has the following sub-options:
    and the values must match the given value. Alternately if the given value
    is `None` then any value is allowed (the attribute just must exist).
    All of the listed attributes must match for the login to be permitted.
+* `enable_registration`: set to 'false' to disable automatic registration of new
+   users. This allows the CAS SSO flow to be limited to sign in only, rather than
+   automatically registering users that have a valid SSO login but do not have
+   a pre-registered account. Defaults to true.
+
+   *Added in Synapse 1.93.0.*
 
 Example configuration:
 ```yaml
@@ -3441,6 +3447,7 @@ cas_config:
   required_attributes:
     userGroup: "staff"
     department: None
+  enable_registration: true
 ```
 ---
 ### `sso`

synapse/config/cas.py

@@ -57,6 +57,8 @@ def read_config(self, config: JsonDict, **kwargs: Any) -> None:
                 required_attributes
             )
 
+            self.cas_enable_registration = cas_config.get("enable_registration", True)
+
             self.idp_name = cas_config.get("idp_name", "CAS")
             self.idp_icon = cas_config.get("idp_icon")
             self.idp_brand = cas_config.get("idp_brand")
@@ -67,6 +69,7 @@ def read_config(self, config: JsonDict, **kwargs: Any) -> None:
             self.cas_protocol_version = None
             self.cas_displayname_attribute = None
             self.cas_required_attributes = []
+            self.cas_enable_registration = False
 
 
 # CAS uses a legacy required attributes mapping, not the one provided by

synapse/handlers/cas.py

@@ -70,6 +70,7 @@ def __init__(self, hs: "HomeServer"):
         self._cas_protocol_version = hs.config.cas.cas_protocol_version
         self._cas_displayname_attribute = hs.config.cas.cas_displayname_attribute
         self._cas_required_attributes = hs.config.cas.cas_required_attributes
+        self._cas_enable_registration = hs.config.cas.cas_enable_registration
 
         self._http_client = hs.get_proxied_http_client()
 
@@ -395,4 +396,5 @@ async def grandfather_existing_users() -> Optional[str]:
             client_redirect_url,
             cas_response_to_user_attributes,
             grandfather_existing_users,
+            registration_enabled=self._cas_enable_registration,
         )

tests/handlers/test_cas.py

@@ -197,6 +197,23 @@ def test_required_attributes(self) -> None:
             auth_provider_session_id=None,
         )
 
+    @override_config({"cas_config": {"enable_registration": False}})
+    def test_map_cas_user_does_not_register_new_user(self) -> None:
+        """Ensures new users are not registered if the enabled registration flag is disabled."""
+
+        # stub out the auth handler
+        auth_handler = self.hs.get_auth_handler()
+        auth_handler.complete_sso_login = AsyncMock()  # type: ignore[method-assign]
+
+        cas_response = CasResponse("test_user", {})
+        request = _mock_request()
+        self.get_success(
+            self.handler._handle_cas_response(request, cas_response, "redirect_uri", "")
+        )
+
+        # check that the auth handler was not called as expected
+        auth_handler.complete_sso_login.assert_not_called()
+
 
 def _mock_request() -> Mock:
     """Returns a mock which will stand in as a SynapseRequest"""