bpo-21315: Fix parsing of encoded words with missing leading ws. (#13425)

* bpo-21315: Fix parsing of encoded words with missing leading ws.

Because of missing leading whitespace, encoded word would get parsed as
unstructured token. This patch fixes that by looking for encoded words when
splitting tokens with whitespace.

Missing trailing whitespace around encoded word now register a defect
instead.

Original patch suggestion by David R. Murray on bpo-21315.

1 files changed, 21 insertions, 0 deletions

diff --git a/Lib/email/_header_value_parser.py b/Lib/email/_header_value_parser.py
index 34969ab5915..35d746aa508 100644
--- a/Lib/email/_header_value_parser.py
+++ b/Lib/email/_header_value_parser.py
@@ -96,6 +96,18 @@ EXTENDED_ATTRIBUTE_ENDS = ATTRIBUTE_ENDS - set('%')
 def quote_string(value):
     return '"'+str(value).replace('\\', '\\\\').replace('"', r'\"')+'"'
 
+# Match a RFC 2047 word, looks like =?utf-8?q?someword?=
+rfc2047_matcher = re.compile(r'''
+   =\?            # literal =?
+   [^?]*          # charset
+   \?             # literal ?
+   [qQbB]         # literal 'q' or 'b', case insensitive
+   \?             # literal ?
+  .*?             # encoded word
+  \?=             # literal ?=
+''', re.VERBOSE | re.MULTILINE)
+
+
 #
 # TokenList and its subclasses
 #
@@ -1052,6 +1064,10 @@ def get_encoded_word(value):
         _validate_xtext(vtext)
         ew.append(vtext)
         text = ''.join(remainder)
+    # Encoded words should be followed by a WS
+    if value and value[0] not in WSP:
+        ew.defects.append(errors.InvalidHeaderDefect(
+            "missing trailing whitespace after encoded-word"))
     return ew, value
 
 def get_unstructured(value):
@@ -1104,6 +1120,11 @@ def get_unstructured(value):
                 unstructured.append(token)
                 continue
         tok, *remainder = _wsp_splitter(value, 1)
+        # Split in the middle of an atom if there is a rfc2047 encoded word
+        # which does not have WSP on both sides. The defect will be registered
+        # the next time through the loop.
+        if rfc2047_matcher.search(tok):
+            tok, *remainder = value.partition('=?')
         vtext = ValueTerminal(tok, 'vtext')
         _validate_xtext(vtext)
         unstructured.append(vtext)