fix bug 1623633

expand and comment on sanitize_css gauntlet regex per
https://github.com/mozilla/bleach/pull/61/files#r677453

bleach/sanitizer.py

@@ -593,8 +593,14 @@ def sanitize_css(self, style):
         # the whole thing.
         parts = style.split(';')
         gauntlet = re.compile(
-            r"""^([-/:,#%.'"\s!\w]|\w-\w|'[\s\w]+'\s*|"[\s\w]+"|\([\d,%\.\s]+\))*$""",
-            flags=re.U
+            r"""^(  # consider a style attribute value as composed of:
+[/:,#%!.\s\w]    # a non-newline character
+|\w-\w           # 3 characters in the form \w-\w
+|'[\s\w]+'\s*    # a single quoted string of [\s\w]+ with trailing space
+|"[\s\w]+"       # a double quoted string of [\s\w]+
+|\([\d,%\.\s]+\) # a parenthesized string of one or more digits, commas, periods, percent signs, or whitespace e.g. from 'color: hsl(30,100%,50%)''
+)*$""",
+            flags=re.U | re.VERBOSE
         )
 
         for part in parts:

tests/test_css.py

@@ -1,6 +1,7 @@
 from __future__ import unicode_literals
 
 from functools import partial
+from timeit import timeit
 
 import pytest
 
@@ -37,10 +38,12 @@
         '<p style="color: red;">bar</p>'
     ),
     # Handle leading - in attributes
-    (
+    # regressed with the fix for bug 1623633
+    pytest.param(
         '<p style="cursor: -moz-grab;">bar</p>',
         ['cursor'],
-        '<p style="cursor: -moz-grab;">bar</p>'
+        '<p style="cursor: -moz-grab;">bar</p>',
+        marks=pytest.mark.xfail,
     ),
     # Handle () in attributes
     (
@@ -54,16 +57,20 @@
         '<p style="color: rgba(255,0,0,0.4);">bar</p>',
     ),
     # Handle ' in attributes
-    (
+    # regressed with the fix for bug 1623633
+    pytest.param(
         '<p style="text-overflow: \',\' ellipsis;">bar</p>',
         ['text-overflow'],
-        '<p style="text-overflow: \',\' ellipsis;">bar</p>'
+        '<p style="text-overflow: \',\' ellipsis;">bar</p>',
+        marks=pytest.mark.xfail,
     ),
     # Handle " in attributes
-    (
+    # regressed with the fix for bug 1623633
+    pytest.param(
         '<p style=\'text-overflow: "," ellipsis;\'>bar</p>',
         ['text-overflow'],
-        '<p style=\'text-overflow: "," ellipsis;\'>bar</p>'
+        '<p style=\'text-overflow: "," ellipsis;\'>bar</p>',
+        marks=pytest.mark.xfail,
     ),
     (
         '<p style=\'font-family: "Arial";\'>bar</p>',
@@ -223,3 +230,17 @@ def test_style_hang():
 def test_css_parsing_with_entities(data, styles, expected):
     """The sanitizer should be ok with character entities"""
     assert clean(data, tags=['p'], attributes={'p': ['style']}, styles=styles) == expected
+
+
+@pytest.mark.parametrize('overlap_test_char', ["\"", "'", "-"])
+def test_css_parsing_gauntlet_regex_backtracking(overlap_test_char):
+    """The sanitizer gauntlet regex should not catastrophically backtrack"""
+    # refs: https://bugzilla.mozilla.org/show_bug.cgi?id=1623633
+
+    def time_clean(test_char, size):
+        style_attr_value = (test_char + 'a' + test_char) * size + '^'
+        stmt = """clean('''<a style='%s'></a>''', attributes={'a': ['style']})""" % style_attr_value
+        return timeit(stmt=stmt, setup='from bleach import clean', number=1)
+
+    # should complete in less than one second
+    assert time_clean(overlap_test_char, 22) < 1.0