# UNDF: UNDF-2026-000000298
diff --git a/lib/sqlalchemy/orm/bulk_persistence.py b/lib/sqlalchemy/orm/bulk_persistence.py
--- a/lib/sqlalchemy/orm/bulk_persistence.py
+++ b/lib/sqlalchemy/orm/bulk_persistence.py
@@ -1870,8 +1870,9 @@ class BulkORMUpdate(BulkUDCompileState, BulkORMSelectAndUpdateMixin):
             else:
                 value_evaluators[key] = _evaluator

-        evaluated_keys = list(value_evaluators.keys())
+        # CWE-407 fix: use a set for O(1) membership test in to_prefetch
+        # comprehension and .difference() call below.
+        evaluated_keys = set(value_evaluators.keys())
         attrib = {k for k, v in resolved_keys_as_propnames}