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Fix quadratic behavior in the minidom node ID cache (CVE-2025-12084) #471

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Fix quadratic behavior in the minidom node ID cache (CVE-2025-12084) #471

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e64387a
Remove quadratic behavior in node ID cache (CVE-2025-12084)
sethmlarson Dec 3, 2025
67dc5be
Test the absence of quadratic behavior in appendChild()
colesbury Dec 24, 2025
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40 changes: 40 additions & 0 deletions Lib/test/test_minidom.py
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Original file line number Diff line number Diff line change
Expand Up @@ -2,6 +2,7 @@

import copy
import pickle
import sys
from StringIO import StringIO
from test.test_support import verbose, run_unittest, findfile
import unittest
Expand Down Expand Up @@ -151,6 +152,45 @@ def testAppendChild(self):
self.confirm(dom.documentElement.childNodes[-1].data == "Hello")
dom.unlink()

def testAppendChildNoQuadraticComplexity(self):
impl = getDOMImplementation()

def work(n):
doc = impl.createDocument(None, "some_tag", None)
element = doc.documentElement
total_lines = [0]

# Count the lines executed in minidom as a proxy for work done
def line_counter(frame, event, arg):
total_lines[0] += 1
return line_counter

def tracer(frame, event, arg):
if frame.f_globals.get('__name__') == 'xml.dom.minidom':
return line_counter
return None

sys.settrace(tracer)
try:
for _ in range(n):
child = doc.createElement("child")
element.appendChild(child)
element = child
finally:
sys.settrace(None)
return total_lines[0]

# Doubling N should not ~quadruple the work.
w1 = work(256)
w2 = work(512)
w3 = work(1024)

self.assertGreater(w1, 0)
r1 = float(w2) / w1
r2 = float(w3) / w2
self.assertLess(max(r1, r2), 3.2,
"Possible quadratic behavior: work=%r ratios=%r" % ((w1, w2, w3), (r1, r2)))

def testAppendChildFragment(self):
dom, orig, c1, c2, c3, frag = self._create_fragment_test_nodes()
dom.documentElement.appendChild(frag)
Expand Down
9 changes: 1 addition & 8 deletions Lib/xml/dom/minidom.py
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Original file line number Diff line number Diff line change
Expand Up @@ -283,13 +283,6 @@ def _append_child(self, node):
childNodes.append(node)
node.__dict__["parentNode"] = self

def _in_document(node):
# return True iff node is part of a document tree
while node is not None:
if node.nodeType == Node.DOCUMENT_NODE:
return True
node = node.parentNode
return False

def _write_data(writer, data):
"Writes datachars to writer."
Expand Down Expand Up @@ -1468,7 +1461,7 @@ def _clear_id_cache(node):
if node.nodeType == Node.DOCUMENT_NODE:
node._id_cache.clear()
node._id_search_stack = None
elif _in_document(node):
elif node.ownerDocument:
node.ownerDocument._id_cache.clear()
node.ownerDocument._id_search_stack= None

Expand Down