1 files changed, 7 insertions, 1 deletions
diff --git a/synapse/handlers/auth.py b/synapse/handlers/auth.py
index d001f2fb2f..4f986d90cb 100644
--- a/synapse/handlers/auth.py
+++ b/synapse/handlers/auth.py
@@ -274,6 +274,8 @@ class AuthHandler:
         # response.
         self._extra_attributes: Dict[str, SsoLoginExtraAttributes] = {}
 
+        self.msc3861_oauth_delegation_enabled = hs.config.experimental.msc3861.enabled
+
     async def validate_user_via_ui_auth(
         self,
         requester: Requester,
@@ -322,8 +324,12 @@ class AuthHandler:
 
             LimitExceededError if the ratelimiter's failed request count for this
                 user is too high to proceed
-
         """
+        if self.msc3861_oauth_delegation_enabled:
+            raise SynapseError(
+                HTTPStatus.INTERNAL_SERVER_ERROR, "UIA shouldn't be used with MSC3861"
+            )
+
         if not requester.access_token_id:
             raise ValueError("Cannot validate a user without an access token")
         if can_skip_ui_auth and self._ui_auth_session_timeout: