Fixed CVE-2018-7536 -- Fixed catastrophic backtracking in urlize and urlizetrunc template filters.

Thanks Florian Apolloner for assisting with the patch.

django/utils/html.py

@@ -13,12 +13,7 @@ from django.utils.safestring import SafeData, SafeText, mark_safe
 from django.utils.text import normalize_newlines
 
 # Configuration for urlize() function.
-TRAILING_PUNCTUATION_RE = re.compile(
+TRAILING_PUNCTUATION_CHARS = '.,:;!'
-    '^'           # Beginning of word
-    '(.*?)'       # The URL in word
-    '([.,:;!]+)'  # Allowed non-wrapping, trailing punctuation
-    '$'           # End of word
-)
 WRAPPING_PUNCTUATION = [('(', ')'), ('<', '>'), ('[', ']'), ('&lt;', '&gt;'), ('"', '"'), ('\'', '\'')]
 
 # List of possible strings used for bullets in bulleted lists.
@@ -28,7 +23,6 @@ unencoded_ampersands_re = re.compile(r'&(?!(\w+|#\d+);)')
 word_split_re = re.compile(r'''([\s<>"']+)''')
 simple_url_re = re.compile(r'^https?://\[?\w', re.IGNORECASE)
 simple_url_2_re = re.compile(r'^www\.|^(?!http)\w[^@]+\.(com|edu|gov|int|mil|net|org)($|/.*)$', re.IGNORECASE)
-simple_email_re = re.compile(r'^\S+@\S+\.\S+$')
 
 _html_escapes = {
     ord('&'): '&amp;',
@@ -293,10 +287,10 @@ def urlize(text, trim_url_limit=None, nofollow=False, autoescape=False):
             trimmed_something = False
 
             # Trim trailing punctuation.
-            match = TRAILING_PUNCTUATION_RE.match(middle)
+            stripped = middle.rstrip(TRAILING_PUNCTUATION_CHARS)
-            if match:
+            if middle != stripped:
-                middle = match.group(1)
+                trail = middle[len(stripped):] + trail
-                trail = match.group(2) + trail
+                middle = stripped
                 trimmed_something = True
 
             # Trim wrapping punctuation.
@@ -313,6 +307,21 @@ def urlize(text, trim_url_limit=None, nofollow=False, autoescape=False):
                     trimmed_something = True
         return lead, middle, trail
 
+    def is_email_simple(value):
+        """Return True if value looks like an email address."""
+        # An @ must be in the middle of the value.
+        if '@' not in value or value.startswith('@') or value.endswith('@'):
+            return False
+        try:
+            p1, p2 = value.split('@')
+        except ValueError:
+            # value contains more than one @.
+            return False
+        # Dot must be in p2 (e.g. example.com)
+        if '.' not in p2 or p2.startswith('.'):
+            return False
+        return True
+
     words = word_split_re.split(str(text))
     for i, word in enumerate(words):
         if '.' in word or '@' in word or ':' in word:
@@ -332,7 +341,7 @@ def urlize(text, trim_url_limit=None, nofollow=False, autoescape=False):
             elif simple_url_2_re.match(middle):
                 middle, middle_unescaped, trail = unescape(middle, trail)
                 url = smart_urlquote('http://%s' % middle_unescaped)
-            elif ':' not in middle and simple_email_re.match(middle):
+            elif ':' not in middle and is_email_simple(middle):
                 local, domain = middle.rsplit('@', 1)
                 try:
                     domain = domain.encode('idna').decode('ascii')

docs/releases/1.11.11.txt

@@ -5,3 +5,14 @@ Django 1.11.11 release notes
 *March 6, 2018*
 
 Django 1.11.11 fixes two security issues in 1.11.10.
+
+CVE-2018-7536: Denial-of-service possibility in ``urlize`` and ``urlizetrunc`` template filters
+===============================================================================================
+
+The ``django.utils.html.urlize()`` function was extremely slow to evaluate
+certain inputs due to catastrophic backtracking vulnerabilities in two regular
+expressions. The ``urlize()`` function is used to implement the ``urlize`` and
+``urlizetrunc`` template filters, which were thus vulnerable.
+
+The problematic regular expressions are replaced with parsing logic that
+behaves similarly.

docs/releases/1.8.19.txt

@@ -5,3 +5,14 @@ Django 1.8.19 release notes
 *March 6, 2018*
 
 Django 1.8.19 fixes two security issues in 1.18.18.
+
+CVE-2018-7536: Denial-of-service possibility in ``urlize`` and ``urlizetrunc`` template filters
+===============================================================================================
+
+The ``django.utils.html.urlize()`` function was extremely slow to evaluate
+certain inputs due to a catastrophic backtracking vulnerability in a regular
+expression. The ``urlize()`` function is used to implement the ``urlize`` and
+``urlizetrunc`` template filters, which were thus vulnerable.
+
+The problematic regular expression is replaced with parsing logic that behaves
+similarly.

docs/releases/2.0.3.txt

@@ -7,6 +7,17 @@ Django 2.0.3 release notes
 Django 2.0.3 fixes two security issues and several bugs in 2.0.2. Also, the
 latest string translations from Transifex are incorporated.
 
+CVE-2018-7536: Denial-of-service possibility in ``urlize`` and ``urlizetrunc`` template filters
+===============================================================================================
+
+The ``django.utils.html.urlize()`` function was extremely slow to evaluate
+certain inputs due to catastrophic backtracking vulnerabilities in two regular
+expressions. The ``urlize()`` function is used to implement the ``urlize`` and
+``urlizetrunc`` template filters, which were thus vulnerable.
+
+The problematic regular expressions are replaced with parsing logic that
+behaves similarly.
+
 Bugfixes
 ========
 

tests/utils_tests/test_html.py

@@ -253,3 +253,12 @@ class TestUtilsHtml(SimpleTestCase):
         for value, output in tests:
             with self.subTest(value=value):
                 self.assertEqual(urlize(value), output)
+
+    def test_urlize_unchanged_inputs(self):
+        tests = (
+            ('a' + '@a' * 50000) + 'a',  # simple_email_re catastrophic test
+            ('a' + '.' * 1000000) + 'a',  # trailing_punctuation catastrophic test
+        )
+        for value in tests:
+            with self.subTest(value=value):
+                self.assertEqual(urlize(value), value)