# UNDF: UNDF-2026-000000046
diff --git a/distlib/util.py b/distlib/util.py
index 0d5bd7a..f5f3c83 100644
--- a/distlib/util.py
+++ b/distlib/util.py
@@ -1154,6 +1154,8 @@ class Sequencer(object):
         # http://en.wikipedia.org/wiki/Tarjan%27s_strongly_connected_components_algorithm
         index_counter = [0]
         stack = []
+        # CWE-407 fix: set for O(1) stack membership test (list `in` is O(n))
+        stack_set = set()
         lowlinks = {}
         index = {}
         result = []
@@ -1166,6 +1168,7 @@ class Sequencer(object):
             lowlinks[node] = index_counter[0]
             index_counter[0] += 1
             stack.append(node)
+            stack_set.add(node)
 
             # Consider successors
             try:
@@ -1177,7 +1180,7 @@ class Sequencer(object):
                     # Successor has not yet been visited
                     strongconnect(successor)
                     lowlinks[node] = min(lowlinks[node], lowlinks[successor])
-                elif successor in stack:
+                elif successor in stack_set:
                     # the successor is in the stack and hence in the current
                     # strongly connected component (SCC)
                     lowlinks[node] = min(lowlinks[node], index[successor])
@@ -1188,6 +1191,7 @@ class Sequencer(object):
 
                 while True:
                     successor = stack.pop()
+                    stack_set.discard(successor)
                     connected_component.append(successor)
                     if successor == node:
                         break