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[3.12] gh-118314: Fix padding edge case in binascii.a2b_base64 strict mode (GH-118320) #118691

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Merged
merged 1 commit into from
May 7, 2024
Merged

[3.12] gh-118314: Fix padding edge case in binascii.a2b_base64 strict mode (GH-118320) #118691

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15 changes: 15 additions & 0 deletions Lib/test/test_binascii.py
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Original file line number Diff line number Diff line change
Expand Up @@ -132,13 +132,21 @@ def assertLeadingPadding(data, non_strict_mode_expected_result: bytes):
def assertDiscontinuousPadding(data, non_strict_mode_expected_result: bytes):
_assertRegexTemplate(r'(?i)Discontinuous padding', data, non_strict_mode_expected_result)

def assertExcessPadding(data, non_strict_mode_expected_result: bytes):
_assertRegexTemplate(r'(?i)Excess padding', data, non_strict_mode_expected_result)

# Test excess data exceptions
assertExcessData(b'ab==a', b'i')
assertExcessData(b'ab===', b'i')
assertExcessData(b'ab====', b'i')
assertExcessData(b'ab==:', b'i')
assertExcessData(b'abc=a', b'i\xb7')
assertExcessData(b'abc=:', b'i\xb7')
assertExcessData(b'ab==\n', b'i')
assertExcessData(b'abc==', b'i\xb7')
assertExcessData(b'abc===', b'i\xb7')
assertExcessData(b'abc====', b'i\xb7')
assertExcessData(b'abc=====', b'i\xb7')

# Test non-base64 data exceptions
assertNonBase64Data(b'\nab==', b'i')
Expand All @@ -150,8 +158,15 @@ def assertDiscontinuousPadding(data, non_strict_mode_expected_result: bytes):
assertLeadingPadding(b'=', b'')
assertLeadingPadding(b'==', b'')
assertLeadingPadding(b'===', b'')
assertLeadingPadding(b'====', b'')
assertLeadingPadding(b'=====', b'')
assertDiscontinuousPadding(b'ab=c=', b'i\xb7')
assertDiscontinuousPadding(b'ab=ab==', b'i\xb6\x9b')
assertExcessPadding(b'abcd=', b'i\xb7\x1d')
assertExcessPadding(b'abcd==', b'i\xb7\x1d')
assertExcessPadding(b'abcd===', b'i\xb7\x1d')
assertExcessPadding(b'abcd====', b'i\xb7\x1d')
assertExcessPadding(b'abcd=====', b'i\xb7\x1d')


def test_base64errors(self):
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1 change: 1 addition & 0 deletions Misc/NEWS.d/next/Library/2024-04-26-12-42-29.gh-issue-118314.Z7reGc.rst
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@@ -0,0 +1 @@
Fix an edge case in :func:`binascii.a2b_base64` strict mode, where excessive padding is not detected when no padding is necessary.
7 changes: 7 additions & 0 deletions Modules/binascii.c
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Expand Up @@ -424,6 +424,13 @@ binascii_a2b_base64_impl(PyObject *module, Py_buffer *data, int strict_mode)
if (this_ch == BASE64_PAD) {
padding_started = 1;

if (strict_mode && quad_pos == 0) {
state = get_binascii_state(module);
if (state) {
PyErr_SetString(state->Error, "Excess padding not allowed");
}
goto error_end;
}
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.
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