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.
(cherry picked from commit 989f6a3800f06b2bd31cfef7c3269a443ad94fac)
Co-authored-by: wouter bolsterlee <wouter@bolsterl.ee>