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; will open a pull request shortly.

pull request with fix at https://github.com/python/cpython/pull/27946

New changeset 989f6a3800f06b2bd31cfef7c3269a443ad94fac by wouter bolsterlee in branch 'main':
bpo-45001: Make email date parsing more robust against malformed input (GH-27946)
https://github.com/python/cpython/commit/989f6a3800f06b2bd31cfef7c3269a443ad94fac

New changeset 9a79242567d79f42ad1a953cce2b1c4a94df23ea by Miss Islington (bot) in branch '3.10':
bpo-45001: Make email date parsing more robust against malformed input (GH-27946)
https://github.com/python/cpython/commit/9a79242567d79f42ad1a953cce2b1c4a94df23ea

New changeset 2cdbd3b8b2bb4fa0dbcc04ce305fbafaeedd9e67 by Miss Islington (bot) in branch '3.9':
bpo-45001: Make email date parsing more robust against malformed input (GH-27946) (GH-27973)
https://github.com/python/cpython/commit/2cdbd3b8b2bb4fa0dbcc04ce305fbafaeedd9e67

New changeset 81148c6f91092c3aa207a53b657b2548a20b230c by Miss Islington (bot) in branch '3.8':
bpo-45001: Make email date parsing more robust against malformed input (GH-27946) (GH-27974)
https://github.com/python/cpython/commit/81148c6f91092c3aa207a53b657b2548a20b230c

New changeset e9b85afd7dc004460f6d914375ab67d617a8a7ff by Miss Islington (bot) in branch '3.7':
bpo-45001: Make email date parsing more robust against malformed input (GH-27946) (GH-27975)
https://github.com/python/cpython/commit/e9b85afd7dc004460f6d914375ab67d617a8a7ff

New changeset da9d6c554697414b1d275c8502e00a07c2ce06e6 by Miss Islington (bot) in branch '3.6':
bpo-45001: Make email date parsing more robust against malformed input (GH-27946) (GH-27976)
https://github.com/python/cpython/commit/da9d6c554697414b1d275c8502e00a07c2ce06e6

Thanks for the PR!