diff --git a/backend/open_webui/models/messages.py b/backend/open_webui/models/messages.py
index 304276b3d9..0b6c4e080b 100644
--- a/backend/open_webui/models/messages.py
+++ b/backend/open_webui/models/messages.py
@@ -328,7 +328,8 @@ class MessageTable:
         async with get_async_db_context(db) as db:
             message = await db.get(Message, parent_id)
 
-            if not message:
+            # Thread parent must belong to the requested channel; never disclose a foreign-channel message.
+            if not message or message.channel_id != channel_id:
                 return []
 
             result = await db.execute(
diff --git a/backend/open_webui/routers/channels.py b/backend/open_webui/routers/channels.py
index 7a509b6b09..f3f6fd2885 100644
--- a/backend/open_webui/routers/channels.py
+++ b/backend/open_webui/routers/channels.py
@@ -1129,6 +1129,13 @@ async def new_message_handler(request: Request, id: str, form_data: MessageForm,
         ):
             raise HTTPException(status_code=status.HTTP_403_FORBIDDEN, detail=ERROR_MESSAGES.DEFAULT())
 
+    # Thread parent / reply target must belong to this channel (no cross-channel binding).
+    for ref_id in (form_data.parent_id, form_data.reply_to_id):
+        if ref_id:
+            ref = await Messages.get_message_by_id(ref_id, include_thread_replies=False, db=db)
+            if not ref or ref.channel_id != channel.id:
+                raise HTTPException(status_code=status.HTTP_400_BAD_REQUEST, detail=ERROR_MESSAGES.DEFAULT())
+
     try:
         message = await Messages.insert_new_message(form_data, channel.id, user.id, db=db)
         if message: