#16983: Apply postel's law to encoded words inside quoted strings.

This applies only to the new parser.  The old parser decodes encoded words
inside quoted strings already, although it gets the whitespace wrong
when it does so.

This version of the patch only handles the most common case (a single encoded
word surrounded by quotes), but I haven't seen any other variations of this in
the wild yet, so its good enough for now.

Author: bitdancer

Lib/email/_header_value_parser.py

@@ -1559,6 +1559,13 @@ def get_bare_quoted_string(value):
     while value and value[0] != '"':
         if value[0] in WSP:
             token, value = get_fws(value)
+        elif value[:2] == '=?':
+            try:
+                token, value = get_encoded_word(value)
+                bare_quoted_string.defects.append(errors.InvalidHeaderDefect(
+                    "encoded word inside quoted string"))
+            except errors.HeaderParseError:
+                token, value = get_qcontent(value)
         else:
             token, value = get_qcontent(value)
         bare_quoted_string.append(token)

Lib/test/test_email/test__header_value_parser.py

@@ -540,6 +540,15 @@ def test_get_bare_quoted_string_empty_quotes(self):
         self._test_get_x(parser.get_bare_quoted_string,
             '""', '""', '', [], '')
 
+    # Issue 16983: apply postel's law to some bad encoding.
+    def test_encoded_word_inside_quotes(self):
+        self._test_get_x(parser.get_bare_quoted_string,
+            '"=?utf-8?Q?not_really_valid?="',
+            '"not really valid"',
+            'not really valid',
+            [errors.InvalidHeaderDefect],
+            '')
+
     # get_comment
 
     def test_get_comment_only(self):

Lib/test/test_email/test_headerregistry.py

@@ -1143,6 +1143,16 @@ class TestAddressHeader(TestHeaderBase):
             'example.com',
             None),
 
+        'rfc2047_atom_in_quoted_string_is_decoded':
+            ('"=?utf-8?q?=C3=89ric?=" <foo@example.com>',
+            [errors.InvalidHeaderDefect],
+            'Éric <foo@example.com>',
+            'Éric',
+            'foo@example.com',
+            'foo',
+            'example.com',
+            None),
+
         }
 
         # XXX: Need many more examples, and in particular some with names in

Misc/NEWS

@@ -48,6 +48,9 @@ Core and Builtins
 Library
 -------
 
+- Issue #16983: the new email header parsing code will now decode encoded words
+  that are (incorrectly) surrounded by quotes, and register a defect.
+
 - Issue #19772: email.generator no longer mutates the message object when
   doing a down-transform from 8bit to 7bit CTEs.