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gh-121284: Fix email address header folding with parsed encoded-word (GH-122754)

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Email generators using email.policy.default may convert an RFC 2047
encoded-word to unencoded form during header refolding. In a structured
header, this could allow 'specials' chars outside a quoted-string,
leading to invalid address headers and enabling spoofing. This change
ensures a parsed encoded-word that contains specials is kept as an
encoded-word while the header is refolded.

[Better fix from @bitdancer.]

---------

Co-authored-by: R David Murray <rdmurray@bitdance.com>
Co-authored-by: Petr Viktorin <encukou@gmail.com>
This commit is contained in:
Mike Edmunds 2025-03-18 04:07:17 -07:00 • committed by GitHub
parent ab6333f7f5
commit 295b53df2a
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3 changed files with 37 additions and 5 deletions
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10
Lib/email/_header_value_parser.py
View file

@ -1053,7 +1053,7 @@ def get_fws(value):
fws = WhiteSpaceTerminal(value[:len(value)-len(newvalue)], 'fws')
return fws, newvalue
def get_encoded_word(value):
def get_encoded_word(value, terminal_type='vtext'):
""" encoded-word = "=?" charset "?" encoding "?" encoded-text "?="
"""
@ -1092,7 +1092,7 @@ def get_encoded_word(value):
ew.append(token)
continue
chars, *remainder = _wsp_splitter(text, 1)
vtext = ValueTerminal(chars, 'vtext')
vtext = ValueTerminal(chars, terminal_type)
_validate_xtext(vtext)
ew.append(vtext)
text = ''.join(remainder)
@ -1134,7 +1134,7 @@ def get_unstructured(value):
valid_ew = True
if value.startswith('=?'):
try:
token, value = get_encoded_word(value)
token, value = get_encoded_word(value, 'utext')
except _InvalidEwError:
valid_ew = False
except errors.HeaderParseError:
@ -1163,7 +1163,7 @@ def get_unstructured(value):
# the parser to go in an infinite loop.
if valid_ew and rfc2047_matcher.search(tok):
tok, *remainder = value.partition('=?')
vtext = ValueTerminal(tok, 'vtext')
vtext = ValueTerminal(tok, 'utext')
_validate_xtext(vtext)
unstructured.append(vtext)
value = ''.join(remainder)
@ -2813,7 +2813,7 @@ def _refold_parse_tree(parse_tree, *, policy):
continue
tstr = str(part)
if not want_encoding:
if part.token_type == 'ptext':
if part.token_type in ('ptext', 'vtext'):
# Encode if tstr contains special characters.
want_encoding = not SPECIALSNL.isdisjoint(tstr)
else:

25
Lib/test/test_email/test__header_value_parser.py
View file

@ -3076,6 +3076,31 @@ class TestFolding(TestEmailBase):
'=?utf-8?q?H=C3=BCbsch?= Kaktus <beautiful@example.com>,\n'
' =?utf-8?q?bei=C3=9Ft_bei=C3=9Ft?= <biter@example.com>\n')
def test_address_list_with_specials_in_encoded_word(self):
# An encoded-word parsed from a structured header must remain
# encoded when it contains specials. Regression for gh-121284.
policy = self.policy.clone(max_line_length=40)
cases = [
# (to, folded)
('=?utf-8?q?A_v=C3=A9ry_long_name_with=2C_comma?= <to@example.com>',
'A =?utf-8?q?v=C3=A9ry_long_name_with?=\n'
' =?utf-8?q?=2C?= comma <to@example.com>\n'),
('=?utf-8?q?This_long_name_does_not_need_encoded=2Dword?= <to@example.com>',
'This long name does not need\n'
' encoded-word <to@example.com>\n'),
('"A véry long name with, comma" <to@example.com>',
# (This isn't the best fold point, but it's not invalid.)
'A =?utf-8?q?v=C3=A9ry_long_name_with?=\n'
' =?utf-8?q?=2C?= comma <to@example.com>\n'),
('"A véry long name containing a, comma" <to@example.com>',
'A =?utf-8?q?v=C3=A9ry?= long name\n'
' containing =?utf-8?q?a=2C?= comma\n'
' <to@example.com>\n'),
]
for (to, folded) in cases:
with self.subTest(to=to):
self._test(parser.get_address_list(to)[0], folded, policy=policy)
def test_address_list_with_list_separator_after_fold(self):
a = 'x' * 66 + '@example.com'
to = f'{a}, "Hübsch Kaktus" <beautiful@example.com>'

7
Misc/NEWS.d/next/Security/2024-08-06-12-27-34.gh-issue-121284.8rwPxe.rst Normal file
View file

@ -0,0 +1,7 @@
Fix bug in the folding of rfc2047 encoded-words when flattening an email message
using a modern email policy. Previously when an encoded-word was too long
for a line, it would be decoded, split across lines, and re-encoded. But commas
and other special characters in the original text could be left unencoded and
unquoted. This could theoretically be used to spoof header lines using
a carefully constructed encoded-word if the resulting rendered email was
transmitted or re-parsed.