diff --git a/Lib/test/test_codecs.py b/Lib/test/test_codecs.py
index a59a5e21358e7b..86d0dde1705763 100644
--- a/Lib/test/test_codecs.py
+++ b/Lib/test/test_codecs.py
@@ -1020,6 +1020,7 @@ def test_errors(self):
             (b'a+////,+IKw-b', 'a\uffff\ufffd\u20acb'),
             (b'a+IKw-b\xff', 'a\u20acb\ufffd'),
             (b'a+IKw\xffb', 'a\u20ac\ufffdb'),
+            (b'a+@b', 'a\ufffdb'),
         ]
         for raw, expected in tests:
             with self.subTest(raw=raw):
diff --git a/Lib/test/test_unicode.py b/Lib/test/test_unicode.py
index 3cc018c0cc2caa..fb7bb2d523fe6e 100644
--- a/Lib/test/test_unicode.py
+++ b/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')
diff --git a/Misc/NEWS.d/next/Library/2018-08-12-00-14-54.bpo-22602.ybG9K8.rst b/Misc/NEWS.d/next/Library/2018-08-12-00-14-54.bpo-22602.ybG9K8.rst
new file mode 100644
index 00000000000000..5b113e3204c104
--- /dev/null
+++ b/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.
diff --git a/Objects/unicodeobject.c b/Objects/unicodeobject.c
index 04fd6d03b464e8..0460d184932ee0 100644
--- a/Objects/unicodeobject.c
+++ b/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;