bpo45001: Make email date parsing more robust against malformed input

wbolster · wbolster · commit d75760c23a9e · 2021-08-25T16:33:58.000+02:00
See https://bugs.python.org/issue45001 Various date parsing utilities in the email module, such as email.utils.parsedate(), are supposed to gracefully handle invalid input, typically by raising an appropriate exception or by returning None. The internal email._parseaddr._parsedate_tz() helper used by some of these date parsing routines tries to be robust against malformed input, but unfortunately it can still crash ungracefully when a non-empty but whitespace-only input is passed. This manifests as an unexpected IndexError. In practice, this can happen when parsing an email with only a newline inside a ‘Date:’ header, which unfortunately happens occasionally in the real world. Here's a minimal example: $ python Python 3.9.6 (default, Jun 30 2021, 10:22:16) [GCC 11.1.0] on linux Type "help", "copyright", "credits" or "license" for more information. >>> import email.utils >>> email.utils.parsedate('foo') >>> email.utils.parsedate(' ') Traceback (most recent call last): File "<stdin>", line 1, in <module> File "/usr/lib/python3.9/email/_parseaddr.py", line 176, in parsedate t = parsedate_tz(data) File "/usr/lib/python3.9/email/_parseaddr.py", line 50, in parsedate_tz res = _parsedate_tz(data) File "/usr/lib/python3.9/email/_parseaddr.py", line 72, in _parsedate_tz if data[0].endswith(',') or data[0].lower() in _daynames: IndexError: list index out of range The fix is rather straight-forward: guard against empty lists, after splitting on whitespace, but before accessing the first element.
diff --git a/Lib/email/_parseaddr.py b/Lib/email/_parseaddr.py
@@ -67,6 +67,8 @@ def _parsedate_tz(data):
     if not data:
         return None
     data = data.split()
+    if not data:  # This happens for whitespace-only input.
+        return None
     # The FWS after the comma after the day-of-week is optional, so search and
     # adjust for this.
     if data[0].endswith(',') or data[0].lower() in _daynames:
diff --git a/Lib/test/test_email/test_email.py b/Lib/test/test_email/test_email.py
@@ -3003,6 +3003,8 @@ def test_formatdate_usegmt(self):
     def test_parsedate_returns_None_for_invalid_strings(self):
         self.assertIsNone(utils.parsedate(''))
         self.assertIsNone(utils.parsedate_tz(''))
+        self.assertIsNone(utils.parsedate(' '))
+        self.assertIsNone(utils.parsedate_tz(' '))
         self.assertIsNone(utils.parsedate('0'))
         self.assertIsNone(utils.parsedate_tz('0'))
         self.assertIsNone(utils.parsedate('A Complete Waste of Time'))
