bpo-22602: Raise an exception in the UTF-7 decoder for ill-formed sequences starting with "+"

The UTF-7 decoder now raises UnicodeDecodeError for ill-formed
sequences starting with "+" (as specified in RFC 2152).

Author: ZackerySpytz

Lib/test/test_unicode.py

@@ -1630,6 +1630,10 @@ def test_codecs_utf7(self):
         for c in set_o:
             self.assertEqual(c.encode('ascii').decode('utf7'), c)
 
+        with self.assertRaisesRegex(UnicodeDecodeError,
+                                    'ill-formed sequence'):
+            b'+@'.decode('utf-7')
+
     def test_codecs_utf8(self):
         self.assertEqual(''.encode('utf-8'), b'')
         self.assertEqual('\u20ac'.encode('utf-8'), b'\xe2\x82\xac')

Misc/NEWS.d/next/Library/2018-08-12-00-14-54.bpo-22602.ybG9K8.rst

@@ -0,0 +1,3 @@
+The UTF-7 decoder now raises :exc:`UnicodeDecodeError` for ill-formed
+sequences starting with "+" (as specified in RFC 2152).  Patch by Zackery
+Spytz.

Objects/unicodeobject.c

@@ -4479,6 +4479,11 @@ PyUnicode_DecodeUTF7Stateful(const char *s,
                 if (_PyUnicodeWriter_WriteCharInline(&writer, '+') < 0)
                     goto onError;
             }
+            else if (s < e && !IS_BASE64(*s)) {
+                s++;
+                errmsg = "ill-formed sequence";
+                goto utf7Error;
+            }
             else { /* begin base64-encoded section */
                 inShift = 1;
                 surrogate = 0;