From b70438a2cfe0e454ea31984b6f344e2ae7f05571 Mon Sep 17 00:00:00 2001
From: Cursor Agent <cursoragent@cursor.com>
Date: Sun, 17 May 2026 11:15:31 +0000
Subject: [PATCH] fix: guard chat completion error writes

Co-authored-by: mkatwi <mkatwi@users.noreply.github.com>
---
 backend/open_webui/main.py                    | 34 +++++++++++++-
 .../test/apps/webui/routers/test_chats.py     | 46 +++++++++++++++++++
 2 files changed, 78 insertions(+), 2 deletions(-)

diff --git a/backend/open_webui/main.py b/backend/open_webui/main.py
index 544756a6e8f4..9663ad6e852e 100644
--- a/backend/open_webui/main.py
+++ b/backend/open_webui/main.py
@@ -457,6 +457,16 @@
 from open_webui.utils.redis import get_sentinels_from_env
 
 
+def can_update_chat(chat_id: Optional[str], user: UserModel) -> bool:
+    if not chat_id:
+        return False
+
+    if user.role == "admin" and ENABLE_ADMIN_CHAT_ACCESS:
+        return Chats.get_chat_by_id(chat_id) is not None
+
+    return Chats.get_chat_by_id_and_user_id(chat_id, user.id) is not None
+
+
 if SAFE_MODE:
     print("SAFE MODE ENABLED")
     Functions.deactivate_all_functions()
@@ -1351,6 +1361,12 @@ async def chat_completion(
             ),
         }
 
+        if metadata.get("chat_id") and not can_update_chat(metadata["chat_id"], user):
+            raise HTTPException(
+                status_code=status.HTTP_403_FORBIDDEN,
+                detail="Chat not found",
+            )
+
         request.state.metadata = metadata
         form_data["metadata"] = metadata
 
@@ -1360,7 +1376,11 @@ async def chat_completion(
 
     except Exception as e:
         log.debug(f"Error processing chat payload: {e}")
-        if metadata.get("chat_id") and metadata.get("message_id"):
+        if (
+            metadata.get("chat_id")
+            and metadata.get("message_id")
+            and can_update_chat(metadata["chat_id"], user)
+        ):
             # Update the chat message with the error
             Chats.upsert_message_to_chat_by_id_and_message_id(
                 metadata["chat_id"],
@@ -1370,6 +1390,9 @@ async def chat_completion(
                 },
             )
 
+        if isinstance(e, HTTPException):
+            raise e
+
         raise HTTPException(
             status_code=status.HTTP_400_BAD_REQUEST,
             detail=str(e),
@@ -1383,7 +1406,11 @@ async def chat_completion(
         )
     except Exception as e:
         log.debug(f"Error in chat completion: {e}")
-        if metadata.get("chat_id") and metadata.get("message_id"):
+        if (
+            metadata.get("chat_id")
+            and metadata.get("message_id")
+            and can_update_chat(metadata["chat_id"], user)
+        ):
             # Update the chat message with the error
             Chats.upsert_message_to_chat_by_id_and_message_id(
                 metadata["chat_id"],
@@ -1393,6 +1420,9 @@ async def chat_completion(
                 },
             )
 
+        if isinstance(e, HTTPException):
+            raise e
+
         raise HTTPException(
             status_code=status.HTTP_400_BAD_REQUEST,
             detail=str(e),
diff --git a/backend/open_webui/test/apps/webui/routers/test_chats.py b/backend/open_webui/test/apps/webui/routers/test_chats.py
index a36a01fb1497..1c666aa2ae4a 100644
--- a/backend/open_webui/test/apps/webui/routers/test_chats.py
+++ b/backend/open_webui/test/apps/webui/routers/test_chats.py
@@ -234,3 +234,49 @@ def test_delete_shared_chat_by_id(self):
 
         chat = self.chats.get_chat_by_id(chat_id)
         assert chat.share_id is None
+
+    def test_chat_completion_error_does_not_update_other_users_chat(self):
+        from open_webui.models.chats import ChatForm
+
+        victim_message_id = "victim-message"
+        victim_chat = self.chats.insert_new_chat(
+            "2",
+            ChatForm(
+                chat={
+                    "title": "Victim chat",
+                    "history": {
+                        "currentId": victim_message_id,
+                        "messages": {
+                            victim_message_id: {
+                                "id": victim_message_id,
+                                "role": "assistant",
+                                "content": "do not overwrite",
+                            }
+                        },
+                    },
+                }
+            ),
+        )
+
+        with mock_webui_user(id="3", role="user"):
+            response = self.fast_api_client.post(
+                "/api/chat/completions",
+                json={
+                    "model": "attacker-direct",
+                    "model_item": {
+                        "id": "attacker-direct",
+                        "direct": True,
+                        "owned_by": "openai",
+                        "info": {"meta": {}},
+                    },
+                    "chat_id": victim_chat.id,
+                    "id": victim_message_id,
+                },
+            )
+
+        assert response.status_code == 403
+        message = self.chats.get_message_by_id_and_message_id(
+            victim_chat.id, victim_message_id
+        )
+        assert message["content"] == "do not overwrite"
+        assert "error" not in message