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gh-145264: Do not ignore excess Base64 data after the first padded quad #145267

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Merged
serhiy-storchaka merged 4 commits into from
Mar 22, 2026
+36 −41
Merged

gh-145264: Do not ignore excess Base64 data after the first padded quad #145267

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0229b06
gh-145264: Do not ignore excess Base64 data after the first padded quad
serhiy-storchaka Feb 26, 2026
1307572
Merge branch 'main' into base64-excess-data
serhiy-storchaka Feb 26, 2026
2676401
Address commit review.
serhiy-storchaka Feb 28, 2026
94cb78e
Merge branch 'main' into base64-excess-data
serhiy-storchaka Mar 22, 2026
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Address commit review.
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@serhiy-storchaka
serhiy-storchaka committed Feb 28, 2026
commit 2676401771de27d370b81fc249cbd1f472f8c51c
8 changes: 4 additions & 4 deletions Lib/test/test_binascii.py
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Original file line number Diff line number Diff line change
Expand Up @@ -240,17 +240,17 @@ def assertNonBase64Data(data, expected, ignorechars):

def test_base64_excess_data(self):
# Test excess data exceptions
def assertExcessData(data, non_strict_expected):
def assertExcessData(data, expected):
assert_regex = r'(?i)Excess data'
data = self.type2test(data)
with self.assertRaisesRegex(binascii.Error, assert_regex):
binascii.a2b_base64(data, strict_mode=True)
self.assertEqual(binascii.a2b_base64(data, strict_mode=False),
non_strict_expected)
expected)
self.assertEqual(binascii.a2b_base64(data, strict_mode=True,
ignorechars=b'='),
non_strict_expected)
self.assertEqual(binascii.a2b_base64(data), non_strict_expected)
expected)
self.assertEqual(binascii.a2b_base64(data), expected)

assertExcessData(b'ab==c=', b'i\xb7')
assertExcessData(b'ab==cd', b'i\xb7\x1d')
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2 changes: 1 addition & 1 deletion Misc/NEWS.d/next/Library/2026-02-26-20-13-16.gh-issue-145264.4pggX_.rst
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@@ -1,4 +1,4 @@
Base64 decoder (see :func:`binascii.a2b_base64`, :func:`base64.b64decode`, etc) no
longer ignores excess data after the first padded quad in non-strict
(default) mode. Instead, in conformance with :rfc:`4648`, it ignores
(default) mode. Instead, in conformance with :rfc:`4648`, section 3.3, it now ignores
the pad character, "=", if it is present before the end of the encoded data.
8 changes: 7 additions & 1 deletion Modules/binascii.c
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Expand Up @@ -643,6 +643,10 @@ binascii_a2b_base64_impl(PyObject *module, Py_buffer *data, int strict_mode,
if (quad_pos >= 2 && quad_pos + pads <= 4) {
continue;
}
// See RFC 4648, section-3.3: "specifications MAY ignore the
// pad character, "=", treating it as non-alphabet data, if
// it is present before the end of the encoded data" and
// "the excess pad characters MAY also be ignored."
if (!strict_mode || ignorechar(BASE64_PAD, ignorechars, ignorecache)) {
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continue;
}
Expand All @@ -662,6 +666,7 @@ binascii_a2b_base64_impl(PyObject *module, Py_buffer *data, int strict_mode,

unsigned char v = table_a2b_base64[this_ch];
if (v >= 64) {
// See RFC 4648, section-3.3.
if (strict_mode && !ignorechar(this_ch, ignorechars, ignorecache)) {
state = get_binascii_state(module);
if (state) {
Expand All @@ -672,7 +677,8 @@ binascii_a2b_base64_impl(PyObject *module, Py_buffer *data, int strict_mode,
continue;
}

// Characters that are not '=', in the middle of the padding, are not allowed
// Characters that are not '=', in the middle of the padding, are
// not allowed (except when they are). See RFC 4648, section-3.3.
if (pads && strict_mode &&
!ignorechar(BASE64_PAD, ignorechars, ignorecache))
{
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