Fixed CVE-2023-43665 -- Mitigated potential DoS in django.utils.text.Truncator when truncating HTML text.

Thanks Wenchao Li of Alibaba Group for the report.

django/utils/text.py

@@ -83,8 +83,14 @@ def add_truncation_text(text, truncate=None):
 class Truncator(SimpleLazyObject):
     """
     An object used to truncate text, either by characters or words.
+
+    When truncating HTML text (either chars or words), input will be limited to
+    at most `MAX_LENGTH_HTML` characters.
     """
 
+    # 5 million characters are approximately 4000 text pages or 3 web pages.
+    MAX_LENGTH_HTML = 5_000_000
+
     def __init__(self, text):
         super().__init__(lambda: str(text))
 
@@ -165,6 +171,11 @@ class Truncator(SimpleLazyObject):
         if words and length <= 0:
             return ""
 
+        size_limited = False
+        if len(text) > self.MAX_LENGTH_HTML:
+            text = text[: self.MAX_LENGTH_HTML]
+            size_limited = True
+
         html4_singlets = (
             "br",
             "col",
@@ -221,10 +232,14 @@ class Truncator(SimpleLazyObject):
                 # Add it to the start of the open tags list
                 open_tags.insert(0, tagname)
 
-        if current_len <= length:
-            return text
-        out = text[:end_text_pos]
         truncate_text = add_truncation_text("", truncate)
+
+        if current_len <= length:
+            if size_limited and truncate_text:
+                text += truncate_text
+            return text
+
+        out = text[:end_text_pos]
         if truncate_text:
             out += truncate_text
         # Close any tags still open

docs/ref/templates/builtins.txt

@@ -2648,6 +2648,16 @@ If ``value`` is ``"<p>Joel is a slug</p>"``, the output will be
 
 Newlines in the HTML content will be preserved.
 
+.. admonition:: Size of input string
+
+    Processing large, potentially malformed HTML strings can be
+    resource-intensive and impact service performance. ``truncatechars_html``
+    limits input to the first five million characters.
+
+.. versionchanged:: 3.2.22
+
+    In older versions, strings over five million characters were processed.
+
 .. templatefilter:: truncatewords
 
 ``truncatewords``
@@ -2690,6 +2700,16 @@ If ``value`` is ``"<p>Joel is a slug</p>"``, the output will be
 
 Newlines in the HTML content will be preserved.
 
+.. admonition:: Size of input string
+
+    Processing large, potentially malformed HTML strings can be
+    resource-intensive and impact service performance. ``truncatewords_html``
+    limits input to the first five million characters.
+
+.. versionchanged:: 3.2.22
+
+    In older versions, strings over five million characters were processed.
+
 .. templatefilter:: unordered_list
 
 ``unordered_list``

docs/releases/3.2.22.txt

@@ -6,4 +6,20 @@ Django 3.2.22 release notes
 
 Django 3.2.22 fixes a security issue with severity "moderate" in 3.2.21.
 
-...
+CVE-2023-43665: Denial-of-service possibility in ``django.utils.text.Truncator``
+================================================================================
+
+Following the fix for :cve:`2019-14232`, the regular expressions used in the
+implementation of ``django.utils.text.Truncator``'s ``chars()`` and ``words()``
+methods (with ``html=True``) were revised and improved. However, these regular
+expressions still exhibited linear backtracking complexity, so when given a
+very long, potentially malformed HTML input, the evaluation would still be
+slow, leading to a potential denial of service vulnerability.
+
+The ``chars()`` and ``words()`` methods are used to implement the
+:tfilter:`truncatechars_html` and :tfilter:`truncatewords_html` template
+filters, which were thus also vulnerable.
+
+The input processed by ``Truncator``, when operating in HTML mode, has been
+limited to the first five million characters in order to avoid potential
+performance and memory issues.

docs/releases/4.1.12.txt

@@ -6,4 +6,20 @@ Django 4.1.12 release notes
 
 Django 4.1.12 fixes a security issue with severity "moderate" in 4.1.11.
 
-...
+CVE-2023-43665: Denial-of-service possibility in ``django.utils.text.Truncator``
+================================================================================
+
+Following the fix for :cve:`2019-14232`, the regular expressions used in the
+implementation of ``django.utils.text.Truncator``'s ``chars()`` and ``words()``
+methods (with ``html=True``) were revised and improved. However, these regular
+expressions still exhibited linear backtracking complexity, so when given a
+very long, potentially malformed HTML input, the evaluation would still be
+slow, leading to a potential denial of service vulnerability.
+
+The ``chars()`` and ``words()`` methods are used to implement the
+:tfilter:`truncatechars_html` and :tfilter:`truncatewords_html` template
+filters, which were thus also vulnerable.
+
+The input processed by ``Truncator``, when operating in HTML mode, has been
+limited to the first five million characters in order to avoid potential
+performance and memory issues.

docs/releases/4.2.6.txt

@@ -7,6 +7,24 @@ Django 4.2.6 release notes
 Django 4.2.6 fixes a security issue with severity "moderate" and several bugs
 in 4.2.5.
 
+CVE-2023-43665: Denial-of-service possibility in ``django.utils.text.Truncator``
+================================================================================
+
+Following the fix for :cve:`2019-14232`, the regular expressions used in the
+implementation of ``django.utils.text.Truncator``'s ``chars()`` and ``words()``
+methods (with ``html=True``) were revised and improved. However, these regular
+expressions still exhibited linear backtracking complexity, so when given a
+very long, potentially malformed HTML input, the evaluation would still be
+slow, leading to a potential denial of service vulnerability.
+
+The ``chars()`` and ``words()`` methods are used to implement the
+:tfilter:`truncatechars_html` and :tfilter:`truncatewords_html` template
+filters, which were thus also vulnerable.
+
+The input processed by ``Truncator``, when operating in HTML mode, has been
+limited to the first five million characters in order to avoid potential
+performance and memory issues.
+
 Bugfixes
 ========
 

tests/utils_tests/test_text.py

@@ -1,5 +1,6 @@
 import json
 import sys
+from unittest.mock import patch
 
 from django.core.exceptions import SuspiciousFileOperation
 from django.test import SimpleTestCase
@@ -94,11 +95,17 @@ class TestUtilsText(SimpleTestCase):
             text.Truncator(lazystr("The quick brown fox")).chars(10), "The quick…"
         )
 
-    def test_truncate_chars_html(self):
+    @patch("django.utils.text.Truncator.MAX_LENGTH_HTML", 10_000)
+    def test_truncate_chars_html_size_limit(self):
+        max_len = text.Truncator.MAX_LENGTH_HTML
+        bigger_len = text.Truncator.MAX_LENGTH_HTML + 1
+        valid_html = "<p>Joel is a slug</p>"  # 14 chars
         perf_test_values = [
-            (("</a" + "\t" * 50000) + "//>", None),
-            ("&" * 50000, "&" * 9 + "…"),
+            ("</a" + "\t" * (max_len - 6) + "//>", None),
+            ("</p" + "\t" * bigger_len + "//>", "</p" + "\t" * 6 + "…"),
+            ("&" * bigger_len, "&" * 9 + "…"),
             ("_X<<<<<<<<<<<>", None),
+            (valid_html * bigger_len, "<p>Joel is a…</p>"),  # 10 chars
         ]
         for value, expected in perf_test_values:
             with self.subTest(value=value):
@@ -176,15 +183,25 @@ class TestUtilsText(SimpleTestCase):
         truncator = text.Truncator("<p>I &lt;3 python, what about you?</p>")
         self.assertEqual("<p>I &lt;3 python,…</p>", truncator.words(3, html=True))
 
+    @patch("django.utils.text.Truncator.MAX_LENGTH_HTML", 10_000)
+    def test_truncate_words_html_size_limit(self):
+        max_len = text.Truncator.MAX_LENGTH_HTML
+        bigger_len = text.Truncator.MAX_LENGTH_HTML + 1
+        valid_html = "<p>Joel is a slug</p>"  # 4 words
         perf_test_values = [
-            ("</a" + "\t" * 50000) + "//>",
-            "&" * 50000,
-            "_X<<<<<<<<<<<>",
+            ("</a" + "\t" * (max_len - 6) + "//>", None),
+            ("</p" + "\t" * bigger_len + "//>", "</p" + "\t" * (max_len - 3) + "…"),
+            ("&" * max_len, None),  # no change
+            ("&" * bigger_len, "&" * max_len + "…"),
+            ("_X<<<<<<<<<<<>", None),
+            (valid_html * bigger_len, valid_html * 12 + "<p>Joel is…</p>"),  # 50 words
         ]
-        for value in perf_test_values:
+        for value, expected in perf_test_values:
             with self.subTest(value=value):
                 truncator = text.Truncator(value)
-                self.assertEqual(value, truncator.words(50, html=True))
+                self.assertEqual(
+                    expected if expected else value, truncator.words(50, html=True)
+                )
 
     def test_wrap(self):
         digits = "1234 67 9"