gh-121284: Fix email address header folding with parsed encoded-word

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.
python · encukou · Mar 18, 2025 · Aug 6, 2024 · Oct 21, 2024 · Jan 18, 2025
commit c540c9f6a5e1d7ebe14e2a4b737e8b1e76a7bb1b
@@ -2829,6 +2829,13 @@ def _refold_parse_tree(parse_tree, *, policy):
             _fold_mime_parameters(part, lines, maxlen, encoding)
             continue
 
+        allow_refolding_subparts = True
+        if part.token_type == 'encoded-word':
+            # A parsed encoded-word containing specials must remain encoded,
+            # to keep specials from sneaking into a structured header unquoted.
+            # (The encoded-word can be split for folding.)
+            allow_refolding_subparts = SPECIALSNL.isdisjoint(tstr)
+
         if want_encoding and not wrap_as_ew_blocked:
             if not part.as_ew_allowed:
                 want_encoding = False
@@ -2848,7 +2855,7 @@ def _refold_parse_tree(parse_tree, *, policy):
                 # want it on a line by itself even if it fits, or it
                 # doesn't fit on a line by itself.  Either way, fall through
                 # to unpacking the subparts and wrapping them.
-            if not hasattr(part, 'encode'):
+            if allow_refolding_subparts and not hasattr(part, 'encode'):
                 # It's not a Terminal, do each piece individually.
                 parts = list(part) + parts
                 want_encoding = False
@@ -2902,7 +2909,7 @@ def _refold_parse_tree(parse_tree, *, policy):
                 leading_whitespace = ''.join(whitespace_accumulator)
                 last_ew = None
                 continue
-        if not hasattr(part, 'encode'):
+        if allow_refolding_subparts and not hasattr(part, 'encode'):
             # It's not a terminal, try folding the subparts.
             newparts = list(part)
             if not part.as_ew_allowed:

@@ -3076,6 +3076,31 @@ def test_address_list_with_unicode_names_in_quotes(self):
             '=?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>',
+             '=?utf-8?q?A_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>'

diff --git a/Misc/NEWS.d/next/Security/2024-08-06-12-27-34.gh-issue-121284.8rwPxe.rst b/Misc/NEWS.d/next/Security/2024-08-06-12-27-34.gh-issue-121284.8rwPxe.rst
@@ -0,0 +1,4 @@
+Fix a problem where email.policy.default header refolding could incorrectly
+convert an RFC 2047 encoded-word containing commas or other special
+characters to unencoded, unquoted text, enabling sender or recipient
+spoofing via a carefully crafted display-name.