gh-146333: Fix quadratic regex backtracking in configparser option parsing

Author: joshuaswanson

Lib/configparser.py

@@ -1145,6 +1145,26 @@ def _handle_option(self, st, line, fpname):
         # an option line?
         st.indent_level = st.cur_indent_level
 
+        # Fast path: if no delimiter is present, skip the regex to avoid
+        # quadratic backtracking (gh-146333). When allow_no_value is True,
+        # treat the whole line as an option name with no value.
+        if not any(d in line.clean for d in self._delimiters):
+            if self._allow_no_value:
+                st.optname = self.optionxform(line.clean.strip())
+                if not st.optname:
+                    st.errors.append(ParsingError(fpname, st.lineno, line))
+                    return
+                if (self._strict and
+                    (st.sectname, st.optname) in st.elements_added):
+                    raise DuplicateOptionError(st.sectname, st.optname,
+                                            fpname, st.lineno)
+                st.elements_added.add((st.sectname, st.optname))
+                st.cursect[st.optname] = None
+                return
+            else:
+                st.errors.append(ParsingError(fpname, st.lineno, line))
+                return
+
         mo = self._optcre.match(line.clean)
         if not mo:
             # a non-fatal parsing error occurred. set up the

Lib/test/test_configparser.py

@@ -2270,6 +2270,26 @@ def test_section_bracket_in_key(self):
         output.close()
 
 
+class ReDoSTestCase(unittest.TestCase):
+    """Regression tests for quadratic regex backtracking (gh-146333)."""
+
+    def test_option_regex_does_not_backtrack(self):
+        # A line with many spaces between non-delimiter characters
+        # should be parsed in linear time, not quadratic.
+        parser = configparser.RawConfigParser()
+        content = "[section]\n" + "x" + " " * 40000 + "y" + "\n"
+        # This should complete almost instantly. Before the fix,
+        # it would take over a minute due to catastrophic backtracking.
+        with self.assertRaises(configparser.ParsingError):
+            parser.read_string(content)
+
+    def test_option_regex_no_value_does_not_backtrack(self):
+        parser = configparser.RawConfigParser(allow_no_value=True)
+        content = "[section]\n" + "x" + " " * 40000 + "y" + "\n"
+        parser.read_string(content)
+        self.assertTrue(parser.has_option("section", "x" + " " * 40000 + "y"))
+
+
 class MiscTestCase(unittest.TestCase):
     def test__all__(self):
         support.check__all__(self, configparser, not_exported={"Error"})

Misc/NEWS.d/next/Security/2026-03-25-00-51-03.gh-issue-146333.LqdL__bn.rst

@@ -0,0 +1,3 @@
+Fix quadratic backtracking in :class:`configparser.RawConfigParser` option
+parsing regexes (``OPTCRE`` and ``OPTCRE_NV``). A crafted configuration line
+with many whitespace characters could cause excessive CPU usage.