From 0000000000000000000000000000000000000000 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Lum=C3=ADr=20Balhar?= <lbalhar@redhat.com>
Date: Fri, 10 Jul 2026 08:12:50 +0000
Subject: 00490: CVE-2026-15308
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gh-153030: Fix quadratic complexity in incremental parsing in HTMLParser (GH-153031) (GH-153038)

When an unterminated construct (e.g. a tag or comment) spanned many
feed() calls, rescanning the growing buffer and concatenating new data
onto it were both quadratic.  New data is now accumulated in a list and
only joined and parsed once enough has piled up.
(cherry picked from commit bcf98ddbc40ec9b3ee87da0124a5660b19b7e606)

Co-authored-by: Serhiy Storchaka <storchaka@gmail.com>
Co-authored-by: Claude Opus 4.8 <noreply@anthropic.com>
Co-authored-by: Lumír Balhar <lbalhar@redhat.com>
---
 Lib/html/parser.py          | 32 ++++++++++++++++++++++++++++++--
 Lib/test/test_htmlparser.py | 21 +++++++++++++++++++++
 2 files changed, 51 insertions(+), 2 deletions(-)

diff --git a/Lib/html/parser.py b/Lib/html/parser.py
index ef869bc72d..37bbadcc05 100644
--- a/Lib/html/parser.py
+++ b/Lib/html/parser.py
@@ -99,6 +99,9 @@ class HTMLParser(_markupbase.ParserBase):
         self.lasttag = '???'
         self.interesting = interesting_normal
         self.cdata_elem = None
+        self._pending = []
+        self._pending_len = 0
+        self._parse_threshold = 1
         _markupbase.ParserBase.reset(self)
 
     def feed(self, data):
@@ -107,11 +110,36 @@ class HTMLParser(_markupbase.ParserBase):
         Call this as often as you want, with as little or as much text
         as you want (may include '\n').
         """
-        self.rawdata = self.rawdata + data
-        self.goahead(0)
+        # Accumulate new data in a list and only join and parse it once
+        # enough has piled up.  Rescanning an unparsed buffer (e.g. an
+        # unterminated tag) and concatenating onto it on every call would
+        # both be quadratic in the input size.
+        self._pending_len += len(data)
+        if self._pending_len < self._parse_threshold:
+            self._pending.append(data)
+        else:
+            if not self._pending:
+                self.rawdata += data
+            else:
+                self._pending.append(data)
+                self.rawdata += ''.join(self._pending)
+                self._pending.clear()
+            self._pending_len = 0
+            n = len(self.rawdata)
+            self.goahead(0)
+            if len(self.rawdata) < n:
+                # Some data was parsed; resume on the next call.
+                self._parse_threshold = 1
+            else:
+                # Nothing was parsed; wait until the buffer doubles.
+                self._parse_threshold = len(self.rawdata)
 
     def close(self):
         """Handle any buffered data."""
+        if self._pending:
+            self.rawdata += ''.join(self._pending)
+            self._pending.clear()
+            self._pending_len = 0
         self.goahead(1)
 
     __starttag_text = None
diff --git a/Lib/test/test_htmlparser.py b/Lib/test/test_htmlparser.py
index 326e34290f..a2916b9511 100644
--- a/Lib/test/test_htmlparser.py
+++ b/Lib/test/test_htmlparser.py
@@ -3,6 +3,7 @@
 import html.parser
 import pprint
 import unittest
+from test import support
 
 
 class EventCollector(html.parser.HTMLParser):
@@ -641,6 +642,26 @@ text
              ('endtag', 'a'), ('data', ' bar & baz')]
         )
 
+    @support.requires_resource('cpu')
+    def test_incremental_no_quadratic_complexity(self):
+        # An unterminated construct fed in many small chunks used to take
+        # quadratic time, both to rescan and to concatenate the buffer.
+        # Now it takes a fraction of a second.
+        def check(prefix, chunk, suffix):
+            parser = html.parser.HTMLParser()
+            parser.feed(prefix)
+            for _ in range(200_000):
+                parser.feed(chunk)
+            parser.feed(suffix)
+            parser.close()
+        chunk = "a" * 64
+        check("<!--", chunk, "-->")       # comment
+        check("<?", chunk, ">")           # processing instruction
+        check("<!doctype ", chunk, ">")   # doctype
+        check("<![CDATA[", chunk, "]]>")  # CDATA section
+        check("<a href='", chunk, "'>")   # start tag
+        check("<script>", chunk, "</script>")  # RAWTEXT element
+
 
 class AttributesTestCase(TestCaseBase):