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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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16 changes: 7 additions & 9 deletions Lib/test/test_binascii.py
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Original file line number Diff line number Diff line change
Expand Up @@ -240,23 +240,21 @@ def assertNonBase64Data(data, expected, ignorechars):

def test_base64_excess_data(self):
# Test excess data exceptions
def assertExcessData(data, non_strict_expected,
ignore_padchar_expected=None):
def assertExcessData(data, non_strict_expected):
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@gpshead gpshead Feb 28, 2026

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rename this from non_strict_expected to just expected.

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@serhiy-storchaka serhiy-storchaka Feb 28, 2026

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In strict mode you get an error. You get that value only in non-strict mode, either when strict_mode=False, or when ignorechars contains "=".

But I agree that expected is shorter. The old name was even longer: non_strict_mode_expected_result.

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)
if ignore_padchar_expected is not None:
self.assertEqual(binascii.a2b_base64(data, strict_mode=True,
ignorechars=b'='),
ignore_padchar_expected)
self.assertEqual(binascii.a2b_base64(data, strict_mode=True,
ignorechars=b'='),
non_strict_expected)
self.assertEqual(binascii.a2b_base64(data), non_strict_expected)

assertExcessData(b'ab==c', b'i')
assertExcessData(b'ab==cd', b'i', b'i\xb7\x1d')
assertExcessData(b'abc=d', b'i\xb7', b'i\xb7\x1d')
assertExcessData(b'ab==c=', b'i\xb7')
assertExcessData(b'ab==cd', b'i\xb7\x1d')
assertExcessData(b'abc=d', b'i\xb7\x1d')

def test_base64errors(self):
# Test base64 with invalid padding
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4 changes: 4 additions & 0 deletions Misc/NEWS.d/next/Library/2026-02-26-20-13-16.gh-issue-145264.4pggX_.rst
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@@ -0,0 +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
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@gpshead gpshead Feb 28, 2026

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I guess this is in accordance with the MAY in https://datatracker.ietf.org/doc/html/rfc4648#section-3.3 about ignoring PADs as non-alphabet data? it'd be good to cite the specific section.

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@serhiy-storchaka serhiy-storchaka Feb 28, 2026

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Done.

the pad character, "=", if it is present before the end of the encoded data.
43 changes: 15 additions & 28 deletions Modules/binascii.c
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Original file line number Diff line number Diff line change
Expand Up @@ -640,36 +640,24 @@ binascii_a2b_base64_impl(PyObject *module, Py_buffer *data, int strict_mode,
*/
if (this_ch == BASE64_PAD) {
pads++;

if (strict_mode) {
if (quad_pos >= 2 && quad_pos + pads <= 4) {
continue;
}
if (ignorechar(BASE64_PAD, ignorechars, ignorecache)) {
continue;
}
if (quad_pos == 1) {
/* Set an error below. */
break;
}
state = get_binascii_state(module);
if (state) {
PyErr_SetString(state->Error,
(quad_pos == 0 && ascii_data == data->buf)
? "Leading padding not allowed"
: "Excess padding not allowed");
}
goto error_end;
if (quad_pos >= 2 && quad_pos + pads <= 4) {
continue;
}
else {
if (quad_pos >= 2 && quad_pos + pads >= 4) {
/* A pad sequence means we should not parse more input.
** We've already interpreted the data from the quad at this point.
*/
goto done;
}
if (!strict_mode || ignorechar(BASE64_PAD, ignorechars, ignorecache)) {
continue;
}
if (quad_pos == 1) {
/* Set an error below. */
break;
}
state = get_binascii_state(module);
if (state) {
PyErr_SetString(state->Error,
(quad_pos == 0 && ascii_data == data->buf)
? "Leading padding not allowed"
: "Excess padding not allowed");
}
goto error_end;
}

unsigned char v = table_a2b_base64[this_ch];
Expand Down Expand Up @@ -748,7 +736,6 @@ binascii_a2b_base64_impl(PyObject *module, Py_buffer *data, int strict_mode,
goto error_end;
}

done:
return PyBytesWriter_FinishWithPointer(writer, bin_data);

error_end:
Expand Down
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