From b19c0d77e6f25ea831ab608c71f15d0d9266c8c4 Mon Sep 17 00:00:00 2001
From: Serhiy Storchaka <storchaka@gmail.com>
Date: Sun, 5 Jan 2020 14:15:50 +0200
Subject: [PATCH] bpo-39055: Reject a trailing \n in base64.b64decode() with
 validate=True. (GH-17616)

---
 Lib/base64.py                                                   | 2 +-
 Lib/test/test_base64.py                                         | 1 +
 .../next/Library/2019-12-15-19-23-23.bpo-39055.FmN3un.rst       | 2 ++
 3 files changed, 4 insertions(+), 1 deletion(-)
 create mode 100644 Misc/NEWS.d/next/Library/2019-12-15-19-23-23.bpo-39055.FmN3un.rst

diff --git a/Lib/base64.py b/Lib/base64.py
index 2be9c395a96..2e70223dfe7 100755
--- a/Lib/base64.py
+++ b/Lib/base64.py
@@ -82,7 +82,7 @@ def b64decode(s, altchars=None, validate=False):
         altchars = _bytes_from_decode_data(altchars)
         assert len(altchars) == 2, repr(altchars)
         s = s.translate(bytes.maketrans(altchars, b'+/'))
-    if validate and not re.match(b'^[A-Za-z0-9+/]*={0,2}$', s):
+    if validate and not re.fullmatch(b'[A-Za-z0-9+/]*={0,2}', s):
         raise binascii.Error('Non-base64 digit found')
     return binascii.a2b_base64(s)
 
diff --git a/Lib/test/test_base64.py b/Lib/test/test_base64.py
index 2a4cc2acad2..7dba6635d4e 100644
--- a/Lib/test/test_base64.py
+++ b/Lib/test/test_base64.py
@@ -250,6 +250,7 @@ class BaseXYTestCase(unittest.TestCase):
                  (b'3d}==', b'\xdd'),
                  (b'@@', b''),
                  (b'!', b''),
+                 (b"YWJj\n", b"abc"),
                  (b'YWJj\nYWI=', b'abcab'))
         funcs = (
             base64.b64decode,
diff --git a/Misc/NEWS.d/next/Library/2019-12-15-19-23-23.bpo-39055.FmN3un.rst b/Misc/NEWS.d/next/Library/2019-12-15-19-23-23.bpo-39055.FmN3un.rst
new file mode 100644
index 00000000000..83b1431e92f
--- /dev/null
+++ b/Misc/NEWS.d/next/Library/2019-12-15-19-23-23.bpo-39055.FmN3un.rst
@@ -0,0 +1,2 @@
+:func:`base64.b64decode` with ``validate=True`` raises now a binascii.Error
+if the input ends with a single ``\n``.