From 366fcbac18e3adc41e3901580dbedb6a91e41a10 Mon Sep 17 00:00:00 2001
From: Idan Moral <idan22moral@gmail.com>
Date: Tue, 20 Jul 2021 01:42:19 +0300
Subject: [PATCH] bpo-44678: Separate error message for discontinuous padding
 in binascii.a2b_base64 strict mode (GH-27249)

* Renamed assertLeadingPadding function to match logic
* Added a separate error message for discontinuous padding
* Updated the tests for discontinuous padding
---
 Lib/test/test_binascii.py                         | 15 +++++++++------
 .../2021-07-19-18-45-00.bpo-44678.YMEAu0.rst      |  1 +
 Modules/binascii.c                                |  7 +++++--
 3 files changed, 15 insertions(+), 8 deletions(-)
 create mode 100644 Misc/NEWS.d/next/Library/2021-07-19-18-45-00.bpo-44678.YMEAu0.rst

diff --git a/Lib/test/test_binascii.py b/Lib/test/test_binascii.py
index 74438d837852c8..da8f9d1f213f7d 100644
--- a/Lib/test/test_binascii.py
+++ b/Lib/test/test_binascii.py
@@ -130,9 +130,12 @@ def assertExcessData(data, non_strict_mode_expected_result: bytes):
         def assertNonBase64Data(data, non_strict_mode_expected_result: bytes):
             _assertRegexTemplate(r'(?i)Only base64 data', data, non_strict_mode_expected_result)
 
-        def assertMalformedPadding(data, non_strict_mode_expected_result: bytes):
+        def assertLeadingPadding(data, non_strict_mode_expected_result: bytes):
             _assertRegexTemplate(r'(?i)Leading padding', data, non_strict_mode_expected_result)
 
+        def assertDiscontinuousPadding(data, non_strict_mode_expected_result: bytes):
+            _assertRegexTemplate(r'(?i)Discontinuous padding', data, non_strict_mode_expected_result)
+
         # Test excess data exceptions
         assertExcessData(b'ab==a', b'i')
         assertExcessData(b'ab===', b'i')
@@ -148,11 +151,11 @@ def assertMalformedPadding(data, non_strict_mode_expected_result: bytes):
         assertNonBase64Data(b'a\x00b==', b'i')
 
         # Test malformed padding
-        assertMalformedPadding(b'=', b'')
-        assertMalformedPadding(b'==', b'')
-        assertMalformedPadding(b'===', b'')
-        assertMalformedPadding(b'ab=c=', b'i\xb7')
-        assertMalformedPadding(b'ab=ab==', b'i\xb6\x9b')
+        assertLeadingPadding(b'=', b'')
+        assertLeadingPadding(b'==', b'')
+        assertLeadingPadding(b'===', b'')
+        assertDiscontinuousPadding(b'ab=c=', b'i\xb7')
+        assertDiscontinuousPadding(b'ab=ab==', b'i\xb6\x9b')
 
 
     def test_base64errors(self):
diff --git a/Misc/NEWS.d/next/Library/2021-07-19-18-45-00.bpo-44678.YMEAu0.rst b/Misc/NEWS.d/next/Library/2021-07-19-18-45-00.bpo-44678.YMEAu0.rst
new file mode 100644
index 00000000000000..991b1579b2e2b6
--- /dev/null
+++ b/Misc/NEWS.d/next/Library/2021-07-19-18-45-00.bpo-44678.YMEAu0.rst
@@ -0,0 +1 @@
+Added a separate error message for discontinuous padding in *binascii.a2b_base64* strict mode.
diff --git a/Modules/binascii.c b/Modules/binascii.c
index 50f25b406a924e..e80eb2af49cf7e 100644
--- a/Modules/binascii.c
+++ b/Modules/binascii.c
@@ -464,7 +464,6 @@ binascii_a2b_base64_impl(PyObject *module, Py_buffer *data, int strict_mode)
     unsigned char *bin_data_start = bin_data;
 
     if (strict_mode && ascii_len > 0 && ascii_data[0] == '=') {
-        malformed_padding:
         state = get_binascii_state(module);
         if (state) {
             PyErr_SetString(state->Error, "Leading padding not allowed");
@@ -516,7 +515,11 @@ binascii_a2b_base64_impl(PyObject *module, Py_buffer *data, int strict_mode)
 
         // Characters that are not '=', in the middle of the padding, are not allowed
         if (strict_mode && padding_started) {
-            goto malformed_padding;
+            state = get_binascii_state(module);
+            if (state) {
+                PyErr_SetString(state->Error, "Discontinuous padding not allowed");
+            }
+            goto error_end;
         }
         pads = 0;