fix: prevent KB collaborators from permanently deleting files they do not own

The file/remove endpoint checked KB write access but not file ownership before calling Files.delete_file_by_id. A collaborator with KB write access could permanently destroy any file linked to the shared KB, affecting all other KBs and chats that reference it.

Collaborators can still unlink files from the KB. Only the file owner or an admin can permanently delete the underlying file.

backend/open_webui/routers/knowledge.py

@@ -380,8 +380,18 @@ async def reindex_knowledge_base_metadata_embeddings(
 
 
 class KnowledgeFilesResponse(KnowledgeResponse):
+    """Response for knowledge base endpoints that include file lists.
+
+    file_deleted semantics (only set by the /file/remove endpoint):
+      - None:  delete_file=false was requested (unlink only, no deletion attempted)
+      - True:  file was permanently deleted (caller is the file owner or admin)
+      - False: permanent deletion was denied (caller has KB write access but
+               does not own the file); the file was unlinked from the KB only
+    """
+
     files: Optional[list[FileMetadataResponse]] = None
     write_access: Optional[bool] = False
+    file_deleted: Optional[bool] = None
 
 
 @router.get('/{id}', response_model=Optional[KnowledgeFilesResponse])
@@ -827,24 +837,37 @@ def remove_file_from_knowledge_by_id(
         log.debug(e)
         pass
 
+    file_deleted = None
     if delete_file:
-        try:
-            # Remove the file's collection from vector database
-            file_collection = f'file-{form_data.file_id}'
-            if VECTOR_DB_CLIENT.has_collection(collection_name=file_collection):
-                VECTOR_DB_CLIENT.delete_collection(collection_name=file_collection)
-        except Exception as e:
-            log.debug('This was most likely caused by bypassing embedding processing')
-            log.debug(e)
-            pass
+        # Only the file owner or an admin can permanently delete the file.
+        # Collaborators with KB write access can unlink files from the KB
+        # (handled above) but must not be able to destroy another user's
+        # file globally.
+        if file.user_id != user.id and user.role != 'admin':
+            log.warning(
+                f'User {user.id} attempted to delete file {form_data.file_id} '
+                f'owned by {file.user_id} via KB {id}, skipping file deletion'
+            )
+            file_deleted = False
+        else:
+            try:
+                # Remove the file's collection from vector database
+                file_collection = f'file-{form_data.file_id}'
+                if VECTOR_DB_CLIENT.has_collection(collection_name=file_collection):
+                    VECTOR_DB_CLIENT.delete_collection(collection_name=file_collection)
+            except Exception as e:
+                log.debug('This was most likely caused by bypassing embedding processing')
+                log.debug(e)
+                pass
 
-        # Delete file from database
-        Files.delete_file_by_id(form_data.file_id, db=db)
+            # Delete file from database
+            file_deleted = Files.delete_file_by_id(form_data.file_id, db=db)
 
     if knowledge:
         return KnowledgeFilesResponse(
             **knowledge.model_dump(),
             files=Knowledges.get_file_metadatas_by_id(knowledge.id, db=db),
+            file_deleted=file_deleted,
         )
     else:
         raise HTTPException(