fix: enforce filter_allowed_access_grants on channel create and update (#23638)

Unlike all other resource routers (knowledge, models, notes, prompts, tools, skills), the channel router did not call filter_allowed_access_grants. This allowed any user to set wildcard access grants on group channels, bypassing the admin's public sharing permission framework.

Adds filter_allowed_access_grants with the sharing.public_channels permission key to both create and update endpoints, matching the pattern used by all other resource routers.

backend/open_webui/routers/channels.py

@@ -61,7 +61,7 @@ from open_webui.utils.chat import generate_chat_completion
 
 
 from open_webui.utils.auth import get_admin_user, get_verified_user
-from open_webui.utils.access_control import has_permission
+from open_webui.utils.access_control import has_permission, filter_allowed_access_grants
 from open_webui.utils.webhook import post_webhook
 from open_webui.utils.channels import extract_mentions, replace_mentions
 from open_webui.internal.db import get_async_session
@@ -303,6 +303,14 @@ async def create_new_channel(
             detail=ERROR_MESSAGES.UNAUTHORIZED,
         )
 
+    form_data.access_grants = filter_allowed_access_grants(
+        request.app.state.config.USER_PERMISSIONS,
+        user.id,
+        user.role,
+        form_data.access_grants,
+        'sharing.public_channels',
+    )
+
     try:
         if form_data.type == 'dm':
             existing_channel = await Channels.get_dm_channel_by_user_ids([user.id, *form_data.user_ids], db=db)
@@ -633,6 +641,14 @@ async def update_channel_by_id(
     if channel.user_id != user.id and user.role != 'admin':
         raise HTTPException(status_code=status.HTTP_403_FORBIDDEN, detail=ERROR_MESSAGES.DEFAULT())
 
+    form_data.access_grants = filter_allowed_access_grants(
+        request.app.state.config.USER_PERMISSIONS,
+        user.id,
+        user.role,
+        form_data.access_grants,
+        'sharing.public_channels',
+    )
+
     try:
         channel = await Channels.update_channel_by_id(id, form_data, db=db)
         return ChannelModel(**channel.model_dump())