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gh-146311: Reject non-zero padding bits in base64/base32 decoding #146312

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1a71757
Add tests for rejecting non-zero padding bits in base64/base32
gpshead Mar 22, 2026
0ca2563
Reject non-zero padding bits in base64/base32 decoding
gpshead Mar 22, 2026
615b227
Fix test_base64 test data with non-zero padding bits
gpshead Mar 22, 2026
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10 changes: 5 additions & 5 deletions Lib/test/test_base64.py
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Original file line number Diff line number Diff line change
Expand Up @@ -311,12 +311,12 @@ def test_b64decode_padding_error(self):

def test_b64decode_invalid_chars(self):
# issue 1466065: Test some invalid characters.
tests = ((b'%3d==', b'\xdd', b'%$'),
(b'$3d==', b'\xdd', b'%$'),
tests = ((b'%3Q==', b'\xdd', b'%$'),
(b'$3Q==', b'\xdd', b'%$'),
(b'[==', b'', b'[='),
(b'YW]3=', b'am', b']'),
(b'3{d==', b'\xdd', b'{}'),
(b'3d}==', b'\xdd', b'{}'),
(b'YW]0=', b'am', b']'),
(b'3{Q==', b'\xdd', b'{}'),
(b'3Q}==', b'\xdd', b'{}'),
(b'@@', b'', b'@!'),
(b'!', b'', b'@!'),
(b"YWJj\n", b"abc", b'\n'),
Expand Down
128 changes: 99 additions & 29 deletions Lib/test/test_binascii.py
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Original file line number Diff line number Diff line change
Expand Up @@ -183,8 +183,8 @@ def assertExcessPadding(*args):
def assertInvalidLength(*args):
_assertRegexTemplate(r'(?i)Invalid.+number of data characters', *args)

assertExcessPadding(b'ab===', b'i')
assertExcessPadding(b'ab====', b'i')
assertExcessPadding(b'aQ===', b'i')
assertExcessPadding(b'aQ====', b'i')
assertExcessPadding(b'abc==', b'i\xb7')
assertExcessPadding(b'abc===', b'i\xb7')
assertExcessPadding(b'abc====', b'i\xb7')
Expand All @@ -201,7 +201,7 @@ def assertInvalidLength(*args):
assertLeadingPadding(b'====abcd', b'i\xb7\x1d')
assertLeadingPadding(b'=====abcd', b'i\xb7\x1d')

assertInvalidLength(b'a=b==', b'i')
assertInvalidLength(b'a=Q==', b'i')
assertInvalidLength(b'a=bc=', b'i\xb7')
assertInvalidLength(b'a=bc==', b'i\xb7')
assertInvalidLength(b'a=bcd', b'i\xb7\x1d')
Expand Down Expand Up @@ -241,17 +241,17 @@ def assertNonBase64Data(data, expected, ignorechars):
self.assertEqual(binascii.a2b_base64(data, strict_mode=False, ignorechars=b''),
expected)

assertNonBase64Data(b'\nab==', b'i', ignorechars=b'\n')
assertNonBase64Data(b'ab:(){:|:&};:==', b'i', ignorechars=b':;(){}|&')
assertNonBase64Data(b'a\nb==', b'i', ignorechars=b'\n')
assertNonBase64Data(b'a\x00b==', b'i', ignorechars=b'\x00')
assertNonBase64Data(b'ab:==', b'i', ignorechars=b':')
assertNonBase64Data(b'ab=:=', b'i', ignorechars=b':')
assertNonBase64Data(b'ab==:', b'i', ignorechars=b':')
assertNonBase64Data(b'\naQ==', b'i', ignorechars=b'\n')
assertNonBase64Data(b'aQ:(){:|:&};:==', b'i', ignorechars=b':;(){}|&')
assertNonBase64Data(b'a\nQ==', b'i', ignorechars=b'\n')
assertNonBase64Data(b'a\x00Q==', b'i', ignorechars=b'\x00')
assertNonBase64Data(b'aQ:==', b'i', ignorechars=b':')
assertNonBase64Data(b'aQ=:=', b'i', ignorechars=b':')
assertNonBase64Data(b'aQ==:', b'i', ignorechars=b':')
assertNonBase64Data(b'abc=:', b'i\xb7', ignorechars=b':')
assertNonBase64Data(b'ab==\n', b'i', ignorechars=b'\n')
assertNonBase64Data(b'a\nb==', b'i', ignorechars=bytearray(b'\n'))
assertNonBase64Data(b'a\nb==', b'i', ignorechars=memoryview(b'\n'))
assertNonBase64Data(b'aQ==\n', b'i', ignorechars=b'\n')
assertNonBase64Data(b'a\nQ==', b'i', ignorechars=bytearray(b'\n'))
assertNonBase64Data(b'a\nQ==', b'i', ignorechars=memoryview(b'\n'))

# Same cell in the cache: '\r' >> 3 == '\n' >> 3.
data = self.type2test(b'\r\n')
Expand Down Expand Up @@ -334,6 +334,34 @@ def assertInvalidLength(data, strict_mode=True):
assertInvalidLength(b'A\tB\nC ??DE', # only 5 valid characters
strict_mode=False)

def test_base64_nonzero_padding_bits(self):
# https://datatracker.ietf.org/doc/html/rfc4648.html#section-3.5
# Decoders MAY reject encoded data if the pad bits are not zero.

# 2 data chars + "==": last char has 4 padding bits
# 'A' = 0, 'B' = 1 ->000000 000001 ->byte 0x00, leftover 0001 (non-zero)
with self.assertRaises(binascii.Error):
binascii.a2b_base64(self.type2test(b'AB=='), strict_mode=True)
# 'A' = 0, 'P' = 15 ->000000 001111 ->byte 0x00, leftover 1111 (non-zero)
with self.assertRaises(binascii.Error):
binascii.a2b_base64(self.type2test(b'AP=='), strict_mode=True)

# 3 data chars + "=": last char has 2 padding bits
# 'A' = 0, 'A' = 0, 'B' = 1 ->000000 000000 000001 ->bytes 0x00 0x00,
# leftover 01 (non-zero)
with self.assertRaises(binascii.Error):
binascii.a2b_base64(self.type2test(b'AAB='), strict_mode=True)
# 'A' = 0, 'A' = 0, 'D' = 3 ->leftover 11 (non-zero)
with self.assertRaises(binascii.Error):
binascii.a2b_base64(self.type2test(b'AAD='), strict_mode=True)

# Verify that zero padding bits are accepted
binascii.a2b_base64(self.type2test(b'AA=='), strict_mode=True)
binascii.a2b_base64(self.type2test(b'AAA='), strict_mode=True)

# Full quads with no padding have no leftover bits --always valid
binascii.a2b_base64(self.type2test(b'AAAA'), strict_mode=True)

def test_base64_alphabet(self):
alphabet = (b'!"#$%&\'()*+,-012345689@'
b'ABCDEFGHIJKLMNPQRSTUVXYZ[`abcdefhijklmpqr')
Expand Down Expand Up @@ -738,19 +766,19 @@ def assertInvalidLength(*args):
assertExcessData(b"ABCDEFG=H")
assertExcessData(b"432Z====55555555")

assertExcessData(b"BE======EF", b"\t\x08")
assertExcessData(b"BE======EA", b"\t\x08")
assertExcessData(b"BEEF====C", b"\t\x08Q")
assertExcessData(b"BEEFC===AK", b"\t\x08Q\x01")
assertExcessData(b"BEEFC===AI", b"\t\x08Q\x01")
assertExcessData(b"BEEFCAK=E", b"\t\x08Q\x01D")

assertExcessPadding(b"BE=======", b"\t")
assertExcessPadding(b"BE========", b"\t")
assertExcessPadding(b"BEEF=====", b"\t\x08")
assertExcessPadding(b"BEEF======", b"\t\x08")
assertExcessPadding(b"BEEA=====", b"\t\x08")
assertExcessPadding(b"BEEA======", b"\t\x08")
assertExcessPadding(b"BEEFC====", b"\t\x08Q")
assertExcessPadding(b"BEEFC=====", b"\t\x08Q")
assertExcessPadding(b"BEEFCAK==", b"\t\x08Q\x01")
assertExcessPadding(b"BEEFCAK===", b"\t\x08Q\x01")
assertExcessPadding(b"BEEFCAI==", b"\t\x08Q\x01")
assertExcessPadding(b"BEEFCAI===", b"\t\x08Q\x01")
assertExcessPadding(b"BEEFCAKE=", b"\t\x08Q\x01D")
assertExcessPadding(b"BEEFCAKE==", b"\t\x08Q\x01D")
assertExcessPadding(b"BEEFCAKE===", b"\t\x08Q\x01D")
Expand Down Expand Up @@ -790,16 +818,16 @@ def assertInvalidLength(*args):
assertIncorrectPadding(b"BE===", b"\t")
assertIncorrectPadding(b"BE====", b"\t")
assertIncorrectPadding(b"BE=====", b"\t")
assertIncorrectPadding(b"BEEF=", b"\t\x08")
assertIncorrectPadding(b"BEEF==", b"\t\x08")
assertIncorrectPadding(b"BEEF===", b"\t\x08")
assertIncorrectPadding(b"BEEA=", b"\t\x08")
assertIncorrectPadding(b"BEEA==", b"\t\x08")
assertIncorrectPadding(b"BEEA===", b"\t\x08")
assertIncorrectPadding(b"BEEFC=", b"\t\x08Q")
assertIncorrectPadding(b"BEEFC==", b"\t\x08Q")

assertDiscontinuousPadding(b"BE=EF===", b"\t\x08")
assertDiscontinuousPadding(b"BE==EF==", b"\t\x08")
assertDiscontinuousPadding(b"BE=EA===", b"\t\x08")
assertDiscontinuousPadding(b"BE==EA==", b"\t\x08")
assertDiscontinuousPadding(b"BEEF=C==", b"\t\x08Q")
assertDiscontinuousPadding(b"BEEFC=AK", b"\t\x08Q\x01")
assertDiscontinuousPadding(b"BEEFC=AI", b"\t\x08Q\x01")

assertInvalidLength(b"A")
assertInvalidLength(b"ABC")
Expand All @@ -819,10 +847,52 @@ def assertInvalidLength(*args):

assertInvalidLength(b"B=E=====", b"\t")
assertInvalidLength(b"B==E====", b"\t")
assertInvalidLength(b"BEE=F===", b"\t\x08")
assertInvalidLength(b"BEE==F==", b"\t\x08")
assertInvalidLength(b"BEEFCA=K", b"\t\x08Q\x01")
assertInvalidLength(b"BEEFCA=====K", b"\t\x08Q\x01")
assertInvalidLength(b"BEE=A===", b"\t\x08")
assertInvalidLength(b"BEE==A==", b"\t\x08")
assertInvalidLength(b"BEEFCA=I", b"\t\x08Q\x01")
assertInvalidLength(b"BEEFCA=====I", b"\t\x08Q\x01")

def test_base32_nonzero_padding_bits(self):
# https://datatracker.ietf.org/doc/html/rfc4648.html#section-3.5
# Decoders MAY reject encoded data if the pad bits are not zero.

# 2 data chars + "======": last char has 2 padding bits
# 'AB' ->00000 00001 ->byte 0x00, leftover 01 (non-zero)
with self.assertRaises(binascii.Error):
binascii.a2b_base32(self.type2test(b'AB======'))
# 'AD' ->00000 00011 ->byte 0x00, leftover 11 (non-zero)
with self.assertRaises(binascii.Error):
binascii.a2b_base32(self.type2test(b'AD======'))

# 4 data chars + "====": last char has 4 padding bits
# 'AAAB' ->00000 00000 00000 00001 ->bytes 0x00 0x00, leftover 0001
with self.assertRaises(binascii.Error):
binascii.a2b_base32(self.type2test(b'AAAB===='))
# 'AAAP' ->leftover 1111
with self.assertRaises(binascii.Error):
binascii.a2b_base32(self.type2test(b'AAAP===='))

# 5 data chars + "===": last char has 1 padding bit
# 'AAAAB' ->4*00000 + 00001 ->bytes 0x00*3, leftover 1 (non-zero)
with self.assertRaises(binascii.Error):
binascii.a2b_base32(self.type2test(b'AAAAB==='))

# 7 data chars + "=": last char has 3 padding bits
# 'AAAAAAB' ->6*00000 + 00001 ->bytes 0x00*4, leftover 001
with self.assertRaises(binascii.Error):
binascii.a2b_base32(self.type2test(b'AAAAAAB='))
# 'AAAAAAH' ->leftover 111
with self.assertRaises(binascii.Error):
binascii.a2b_base32(self.type2test(b'AAAAAAH='))

# Verify that zero padding bits are accepted
binascii.a2b_base32(self.type2test(b'AA======'))
binascii.a2b_base32(self.type2test(b'AAAA===='))
binascii.a2b_base32(self.type2test(b'AAAAA==='))
binascii.a2b_base32(self.type2test(b'AAAAAAA='))

# Full octet with no padding --always valid
binascii.a2b_base32(self.type2test(b'AAAAAAAA'))

def test_base32_alphabet(self):
alphabet = b'0Aa1Bb2Cc3Dd4Ee5Ff6Gg7Hh8Ii9JjKk'
Expand Down
20 changes: 20 additions & 0 deletions Modules/binascii.c
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Original file line number Diff line number Diff line change
Expand Up @@ -902,6 +902,16 @@ binascii_a2b_base64_impl(PyObject *module, Py_buffer *data, int strict_mode,
goto error_end;
}

/* https://datatracker.ietf.org/doc/html/rfc4648.html#section-3.5
* Decoders MAY reject non-zero padding bits. */
if (strict_mode && leftchar != 0) {
state = get_binascii_state(module);
if (state) {
PyErr_SetString(state->Error, "Non-zero padding bits");
}
goto error_end;
}

Py_XDECREF(table_obj);
return PyBytesWriter_FinishWithPointer(writer, bin_data);

Expand Down Expand Up @@ -1652,6 +1662,16 @@ binascii_a2b_base32_impl(PyObject *module, Py_buffer *data,
goto error;
}

/* https://datatracker.ietf.org/doc/html/rfc4648.html#section-3.5
* Decoders MAY reject non-zero padding bits. */
if (leftchar != 0) {
state = get_binascii_state(module);
if (state) {
PyErr_SetString(state->Error, "Non-zero padding bits");
}
goto error;
}

Py_XDECREF(table_obj);
return PyBytesWriter_FinishWithPointer(writer, bin_data);

Expand Down
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