# UNDF: UNDF-2026-000000713
diff --git a/src/compiler/scala/tools/nsc/typechecker/RefChecks.scala b/src/compiler/scala/tools/nsc/typechecker/RefChecks.scala
index abcdef00..cwe407fix 100644
--- a/src/compiler/scala/tools/nsc/typechecker/RefChecks.scala
+++ b/src/compiler/scala/tools/nsc/typechecker/RefChecks.scala
@@ -464,7 +464,11 @@ trait RefChecks {
         //Console.println(infoString(member) + " overrides " + infoString(other) + " in " + clazz);//DEBUG

         /* Is the intersection between given two lists of overridden symbols empty? */
-        def intersectionIsEmpty(syms1: List[Symbol], syms2: List[Symbol]) = !syms1.exists(syms2.contains)
+        // CWE-407 fix: syms2.contains on List[Symbol] is O(|syms2|) per call.
+        // extendedOverriddenSymbols returns O(D) symbols (D = hierarchy depth).
+        // Original: O(|syms1| x |syms2|) = O(D^2) per pair checked in checkAllOverrides.
+        // Fix: convert syms2 to a Set[Symbol] once — O(|syms2|) — then O(1) per lookup.
+        def intersectionIsEmpty(syms1: List[Symbol], syms2: List[Symbol]) = { val s2 = syms2.toSet; !syms1.exists(s2.contains) }