8000 gh-118314: Fix padding edge case in binascii.a2b_base64 strict mode (… · python/cpython@fe47d9b · GitHub
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gh-118314: Fix padding edge case in binascii.a2b_base64 strict mode (GH-118320)
Fix an edge case in `binascii.a2b_base64` strict mode, where excessive padding was not detected when no padding is necessary. Co-authored-by: Terry Jan Reedy <tjreedy@udel.edu> Co-authored-by: Pieter Eendebak <pieter.eendebak@gmail.com>
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‎Lib/test/test_binascii.py

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@@ -139,13 +139,21 @@ def assertLeadingPadding(data, non_strict_mode_expected_result: bytes):
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def assertDiscontinuousPadding(data, non_strict_mode_expected_result: bytes):
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_assertRegexTemplate(r'(?i)Discontinuous padding', data, non_strict_mode_expected_result)
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def assertExcessPadding(data, non_strict_mode_expected_result: bytes):
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_assertRegexTemplate(r'(?i)Excess padding', data, non_strict_mode_expected_result)
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# Test excess data exceptions
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assertExcessData(b'ab==a', b'i')
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assertExcessData(b'ab===', b'i')
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assertExcessData(b'ab====', b'i')
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assertExcessData(b'ab==:', b'i')
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assertExcessData(b'abc=a', b'i\xb7')
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assertExcessData(b'abc=:', b'i\xb7')
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assertExcessData(b'ab==\n', b'i')
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assertExcessData(b'abc==', b'i\xb7')
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assertExcessData(b'abc===', b'i\xb7')
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assertExcessData(b'abc====', b'i\xb7')
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assertExcessData(b'abc=====', b'i\xb7')
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# Test non-base64 data exceptions
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assertNonBase64Data(b'\nab==', b'i')
@@ -157,8 +165,15 @@ def assertDiscontinuousPadding(data, non_strict_mode_expected_result: bytes):
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assertLeadingPadding(b'=', b'')
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assertLeadingPadding(b'==', b'')
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assertLeadingPadding(b'===', b'')
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assertLeadingPadding(b'====', b'')
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assertLeadingPadding(b'=====', b'')
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assertDiscontinuousPadding(b'ab=c=', b'i\xb7')
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assertDiscontinuousPadding(b'ab=ab==', b'i\xb6\x9b')
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assertExcessPadding(b'abcd=', b'i\xb7\x1d')
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assertExcessPadding(b'abcd==', b'i\xb7\x1d')
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assertExcessPadding(b'abcd===', b'i\xb7\x1d')
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assertExcessPadding(b'abcd====', b'i\xb7\x1d')
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assertExcessPadding(b'abcd=====', b'i\xb7\x1d')
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def test_base64errors(self):

‎Misc/NEWS.d/next/Library/2024-04-26-12-42-29.gh-issue-118314.Z7reGc.rst

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Fix an edge case in :func:`binascii.a2b_base64` strict mode, where excessive padding is not detected when no padding is necessary.

‎Modules/binascii.c

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@@ -414,6 +414,13 @@ binascii_a2b_base64_impl(PyObject *module, Py_buffer *data, int strict_mode)
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if (this_ch == BASE64_PAD) {
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padding_started = 1;
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if (strict_mode && quad_pos == 0) {
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state = get_binascii_state(module);
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if (state) {
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PyErr_SetString(state->Error, "Excess padding not allowed");
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}
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goto error_end;
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}
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if (quad_pos >= 2 && quad_pos + ++pads >= 4) {
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/* A pad sequence means we should not parse more input.
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** We've already interpreted the data from the quad at this point.

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