bpo-30835: email: Fix AttributeError when parsing invalid CTE (GH-13598)

maxking · warsaw · commit aa79707262f8 · 2019-06-04T11:00:47.000-07:00
* bpo-30835: email: Fix AttributeError when parsing invalid Content-Transfer-Encoding Parsing an email containing a multipart Content-Type, along with a Content-Transfer-Encoding containing an invalid (non-ASCII-decodable) byte will fail. email.feedparser.FeedParser._parsegen() gets the header and attempts to convert it to lowercase before comparing it with the accepted encodings, but as the header contains an invalid byte, it's returned as a Header object rather than a str. Cast the Content-Transfer-Encoding header to a str to avoid this. Found using the AFL fuzzer. Reported-by: Daniel Axtens <dja@axtens.net> Signed-off-by: Andrew Donnellan <andrew@donnellan.id.au> * Add email and NEWS entry for the bugfix.
diff --git a/Lib/email/feedparser.py b/Lib/email/feedparser.py
@@ -320,7 +320,7 @@ def _parsegen(self):
                 self._cur.set_payload(EMPTYSTRING.join(lines))
                 return
             # Make sure a valid content type was specified per RFC 2045:6.4.
-            if (self._cur.get('content-transfer-encoding', '8bit').lower()
+            if (str(self._cur.get('content-transfer-encoding', '8bit')).lower()
                     not in ('7bit', '8bit', 'binary')):
                 defect = errors.InvalidMultipartContentTransferEncodingDefect()
                 self.policy.handle_defect(self._cur, defect)
diff --git a/Lib/test/test_email/test_email.py b/Lib/test/test_email/test_email.py
@@ -1466,6 +1466,15 @@ def test_mangled_from_with_bad_bytes(self):
         g.flatten(msg)
         self.assertEqual(b.getvalue(), source + b'>From R\xc3\xb6lli\n')
 
+    def test_mutltipart_with_bad_bytes_in_cte(self):
+        # bpo30835
+        source = textwrap.dedent("""\
+            From: aperson@example.com
+            Content-Type: multipart/mixed; boundary="1"
+            Content-Transfer-Encoding: \xc8
+        """).encode('utf-8')
+        msg = email.message_from_bytes(source)
+
 
 # Test the basic MIMEAudio class
 class TestMIMEAudio(unittest.TestCase):
diff --git a/Misc/NEWS.d/next/Library/2019-05-27-15-29-46.bpo-30835.3FoaWH.rst b/Misc/NEWS.d/next/Library/2019-05-27-15-29-46.bpo-30835.3FoaWH.rst
@@ -0,0 +1,3 @@
+Fixed a bug in email parsing where a message with invalid bytes in
+content-transfer-encoding of a multipart message can cause an AttributeError.
+Patch by Andrew Donnellan.

Original file line number	Diff line number	Diff line change
`@@ -0,0 +1,3 @@`
	`1`	`+Fixed a bug in email parsing where a message with invalid bytes in`
	`2`	`+content-transfer-encoding of a multipart message can cause an AttributeError.`
	`3`	`+Patch by Andrew Donnellan.`