[Python-checkins] bpo-45001: Make email date parsing more robust against malformed input (GH-27946)

[miss-islington]

https://github.com/python/cpython/commit/9a79242567d79f42ad1a953cce2b1c4a94df23ea
commit: 9a79242567d79f42ad1a953cce2b1c4a94df23ea
branch: 3.10
author: Miss Islington (bot) <31488909+miss-islington at users.noreply.github.com>
committer: miss-islington <31488909+miss-islington at users.noreply.github.com>
date: 2021-08-26T08:47:27-07:00
summary:

bpo-45001: Make email date parsing more robust against malformed input (GH-27946)


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 at bolsterl.ee>

files:
A Misc/NEWS.d/next/Library/2021-08-26-16-25-48.bpo-45001.tn_dKp.rst
M Lib/email/_parseaddr.py
M Lib/test/test_email/test_email.py

diff --git a/Lib/email/_parseaddr.py b/Lib/email/_parseaddr.py
index 4d27f87974b20..977fedf67b159 100644
--- 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
index 0154bbad1f63f..4001f716471dc 100644
--- 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'))
diff --git a/Misc/NEWS.d/next/Library/2021-08-26-16-25-48.bpo-45001.tn_dKp.rst b/Misc/NEWS.d/next/Library/2021-08-26-16-25-48.bpo-45001.tn_dKp.rst
new file mode 100644
index 0000000000000..55cc409d0da30
--- /dev/null
+++ b/Misc/NEWS.d/next/Library/2021-08-26-16-25-48.bpo-45001.tn_dKp.rst
@@ -0,0 +1,2 @@
+Made email date parsing more robust against malformed input, namely a
+whitespace-only ``Date:`` header. Patch by Wouter Bolsterlee.