From bbc74a7f7eb7335e913bdb4787f22e83a9be947e Mon Sep 17 00:00:00 2001
From: Sarah Boyce <42296566+sarahboyce@users.noreply.github.com>
Date: Wed, 13 Nov 2024 15:06:23 +0100
Subject: [PATCH] [5.1.x] Fixed CVE-2024-53907 -- Mitigated potential DoS in
 strip_tags().

Origin: https://github.com/django/django/commit/bbc74a7f7eb7335e913bdb4787f22e83a9be947e

Thanks to jiangniao for the report, and Shai Berger and Natalia Bidart
for the reviews.
---
 django/utils/html.py           | 10 ++++++++--
 tests/utils_tests/test_html.py |  7 +++++++
 5 files changed, 63 insertions(+), 2 deletions(-)

diff --git a/django/utils/html.py b/django/utils/html.py
index d9513fc75848..ff8684f5a974 100644
--- a/django/utils/html.py
+++ b/django/utils/html.py
@@ -7,6 +7,7 @@
 from html.parser import HTMLParser
 from urllib.parse import parse_qsl, quote, unquote, urlencode, urlsplit, urlunsplit
 
+from django.core.exceptions import SuspiciousOperation
 from django.utils.deprecation import RemovedInDjango60Warning
 from django.utils.encoding import punycode
 from django.utils.functional import Promise, cached_property, keep_lazy, keep_lazy_text
@@ -37,6 +38,7 @@
 }
 
 MAX_URL_LENGTH = 2048
+MAX_STRIP_TAGS_DEPTH = 50
 
 
 @keep_lazy(SafeString)
@@ -205,15 +207,19 @@ def _strip_once(value):
 @keep_lazy_text
 def strip_tags(value):
     """Return the given HTML with all tags stripped."""
-    # Note: in typical case this loop executes _strip_once once. Loop condition
-    # is redundant, but helps to reduce number of executions of _strip_once.
     value = str(value)
+    # Note: in typical case this loop executes _strip_once twice (the second
+    # execution does not remove any more tags).
+    strip_tags_depth = 0
     while "<" in value and ">" in value:
+        if strip_tags_depth >= MAX_STRIP_TAGS_DEPTH:
+            raise SuspiciousOperation
         new_value = _strip_once(value)
         if value.count("<") == new_value.count("<"):
             # _strip_once wasn't able to detect more tags.
             break
         value = new_value
+        strip_tags_depth += 1
     return value
 
 
diff --git a/tests/utils_tests/test_html.py b/tests/utils_tests/test_html.py
index 9bee483dc7ff..75873061de41 100644
--- a/tests/utils_tests/test_html.py
+++ b/tests/utils_tests/test_html.py
@@ -1,6 +1,7 @@
 import os
 from datetime import datetime
 
+from django.core.exceptions import SuspiciousOperation
 from django.core.serializers.json import DjangoJSONEncoder
 from django.test import SimpleTestCase
 from django.utils.deprecation import RemovedInDjango60Warning
@@ -123,12 +124,18 @@ def test_strip_tags(self):
             ("<script>alert()</script>&h", "alert()h"),
             ("><!" + ("&" * 16000) + "D", "><!" + ("&" * 16000) + "D"),
             ("X<<<<br>br>br>br>X", "XX"),
+            ("<" * 50 + "a>" * 50, ""),
         )
         for value, output in items:
             with self.subTest(value=value, output=output):
                 self.check_output(strip_tags, value, output)
                 self.check_output(strip_tags, lazystr(value), output)
 
+    def test_strip_tags_suspicious_operation(self):
+        value = "<" * 51 + "a>" * 51, "<a>"
+        with self.assertRaises(SuspiciousOperation):
+            strip_tags(value)
+
     def test_strip_tags_files(self):
         # Test with more lengthy content (also catching performance regressions)
         for filename in ("strip_tags1.html", "strip_tags2.txt"):