https://github.com/python/cpython/commit/9060b4abbe475591b6230b23c2afefeff26fcca5
commit: 9060b4abbe475591b6230b23c2afefeff26fcca5
branch: main
author: Serhiy Storchaka <[email protected]>
committer: serhiy-storchaka <[email protected]>
date: 2026-01-21T09:41:58+02:00
summary:
gh-125346: Deprecate accepting standard Base64 alphabet when alternative
alphabet is used (GH-141128)
Emit a warning in base64.urlsafe_b64decode() and base64.b64decode() when
the "+" or "/" characters occur in the Base64 data with alternative
alphabet if they are not the part of the alternative alphabet.
It is a DeprecationWarning in the strict mode (will be error) and
a FutureWarning in non-strict mode (will be ignored).
files:
A Misc/NEWS.d/next/Library/2025-11-06-12-03-29.gh-issue-125346.7Gfpgw.rst
M Doc/library/base64.rst
M Doc/whatsnew/3.15.rst
M Lib/base64.py
M Lib/test/test_base64.py
diff --git a/Doc/library/base64.rst b/Doc/library/base64.rst
index 4876117f6403b2..3e7884debd5948 100644
--- a/Doc/library/base64.rst
+++ b/Doc/library/base64.rst
@@ -84,15 +84,20 @@ POST request.
A :exc:`binascii.Error` exception is raised
if *s* is incorrectly padded.
- If *validate* is ``False`` (the default), characters that are neither
+ If *validate* is false (the default), characters that are neither
in the normal base-64 alphabet nor the alternative alphabet are
- discarded prior to the padding check. If *validate* is ``True``,
- these non-alphabet characters in the input result in a
- :exc:`binascii.Error`.
+ discarded prior to the padding check, but the ``+`` and ``/`` characters
+ keep their meaning if they are not in *altchars* (they will be discarded
+ in future Python versions).
+ If *validate* is true, these non-alphabet characters in the input
+ result in a :exc:`binascii.Error`.
For more information about the strict base64 check, see
:func:`binascii.a2b_base64`
- May assert or raise a :exc:`ValueError` if the length of *altchars* is not
2.
+ .. deprecated:: next
+ Accepting the ``+`` and ``/`` characters with an alternative alphabet
+ is now deprecated.
+
.. function:: standard_b64encode(s)
@@ -123,6 +128,9 @@ POST request.
``/`` in the standard Base64 alphabet, and return the decoded
:class:`bytes`.
+ .. deprecated:: next
+ Accepting the ``+`` and ``/`` characters is now deprecated.
+
.. function:: b32encode(s)
diff --git a/Doc/whatsnew/3.15.rst b/Doc/whatsnew/3.15.rst
index 1dd66065b0f93d..1d4961d7293631 100644
--- a/Doc/whatsnew/3.15.rst
+++ b/Doc/whatsnew/3.15.rst
@@ -1149,6 +1149,15 @@ Deprecated
New deprecations
----------------
+* :mod:`base64`:
+
+ * Accepting the ``+`` and ``/`` characters with an alternative alphabet in
+ :func:`~base64.b64decode` and :func:`~base64.urlsafe_b64decode` is now
+ deprecated.
+ In future Python versions they will be errors in the strict mode and
+ discarded in the non-strict mode.
+ (Contributed by Serhiy Storchaka in :gh:`125346`.)
+
* CLI:
* Deprecate :option:`-b` and :option:`!-bb` command-line options
diff --git a/Lib/base64.py b/Lib/base64.py
index e62ae6aff580fa..6e0da16b23ce99 100644
--- a/Lib/base64.py
+++ b/Lib/base64.py
@@ -72,20 +72,39 @@ def b64decode(s, altchars=None, validate=False):
The result is returned as a bytes object. A binascii.Error is raised if
s is incorrectly padded.
- If validate is False (the default), characters that are neither in the
+ If validate is false (the default), characters that are neither in the
normal base-64 alphabet nor the alternative alphabet are discarded prior
- to the padding check. If validate is True, these non-alphabet characters
+ to the padding check. If validate is true, these non-alphabet characters
in the input result in a binascii.Error.
For more information about the strict base64 check, see:
https://docs.python.org/3.11/library/binascii.html#binascii.a2b_base64
"""
s = _bytes_from_decode_data(s)
+ badchar = None
if altchars is not None:
altchars = _bytes_from_decode_data(altchars)
- assert len(altchars) == 2, repr(altchars)
+ if len(altchars) != 2:
+ raise ValueError(f'invalid altchars: {altchars!r}')
+ for b in b'+/':
+ if b not in altchars and b in s:
+ badchar = b
+ break
s = s.translate(bytes.maketrans(altchars, b'+/'))
- return binascii.a2b_base64(s, strict_mode=validate)
+ result = binascii.a2b_base64(s, strict_mode=validate)
+ if badchar is not None:
+ import warnings
+ if validate:
+ warnings.warn(f'invalid character {chr(badchar)!a} in Base64 data '
+ f'with altchars={altchars!r} and validate=True '
+ f'will be an error in future Python versions',
+ DeprecationWarning, stacklevel=2)
+ else:
+ warnings.warn(f'invalid character {chr(badchar)!a} in Base64 data '
+ f'with altchars={altchars!r} and validate=False '
+ f'will be discarded in future Python versions',
+ FutureWarning, stacklevel=2)
+ return result
def standard_b64encode(s):
@@ -130,8 +149,19 @@ def urlsafe_b64decode(s):
The alphabet uses '-' instead of '+' and '_' instead of '/'.
"""
s = _bytes_from_decode_data(s)
+ badchar = None
+ for b in b'+/':
+ if b in s:
+ badchar = b
+ break
s = s.translate(_urlsafe_decode_translation)
- return b64decode(s)
+ result = binascii.a2b_base64(s, strict_mode=False)
+ if badchar is not None:
+ import warnings
+ warnings.warn(f'invalid character {chr(badchar)!a} in URL-safe Base64
data '
+ f'will be discarded in future Python versions',
+ FutureWarning, stacklevel=2)
+ return result
diff --git a/Lib/test/test_base64.py b/Lib/test/test_base64.py
index 120c5824a42a40..d02992903f15a7 100644
--- a/Lib/test/test_base64.py
+++ b/Lib/test/test_base64.py
@@ -292,6 +292,11 @@ def test_b64decode_altchars(self):
eq(base64.b64decode(data, altchars=altchars_str), res)
eq(base64.b64decode(data_str, altchars=altchars_str), res)
+ self.assertRaises(ValueError, base64.b64decode, b'', altchars=b'+')
+ self.assertRaises(ValueError, base64.b64decode, b'', altchars=b'+/-')
+ self.assertRaises(ValueError, base64.b64decode, '', altchars='+')
+ self.assertRaises(ValueError, base64.b64decode, '', altchars='+/-')
+
def test_b64decode_padding_error(self):
self.assertRaises(binascii.Error, base64.b64decode, b'abc')
self.assertRaises(binascii.Error, base64.b64decode, 'abc')
@@ -323,13 +328,25 @@ def test_b64decode_invalid_chars(self):
with self.assertRaises(binascii.Error):
base64.b64decode(bstr.decode('ascii'), validate=True)
- # Normal alphabet characters not discarded when alternative given
- res = b'\xfb\xef\xff'
- self.assertEqual(base64.b64decode(b'++//', validate=True), res)
- self.assertEqual(base64.b64decode(b'++//', '-_', validate=True), res)
- self.assertEqual(base64.b64decode(b'--__', '-_', validate=True), res)
- self.assertEqual(base64.urlsafe_b64decode(b'++//'), res)
- self.assertEqual(base64.urlsafe_b64decode(b'--__'), res)
+ # Normal alphabet characters will be discarded when alternative given
+ with self.assertWarns(FutureWarning):
+ self.assertEqual(base64.b64decode(b'++++', altchars=b'-_'),
+ b'\xfb\xef\xbe')
+ with self.assertWarns(FutureWarning):
+ self.assertEqual(base64.b64decode(b'////', altchars=b'-_'),
+ b'\xff\xff\xff')
+ with self.assertWarns(DeprecationWarning):
+ self.assertEqual(base64.b64decode(b'++++', altchars=b'-_',
validate=True),
+ b'\xfb\xef\xbe')
+ with self.assertWarns(DeprecationWarning):
+ self.assertEqual(base64.b64decode(b'////', altchars=b'-_',
validate=True),
+ b'\xff\xff\xff')
+ with self.assertWarns(FutureWarning):
+ self.assertEqual(base64.urlsafe_b64decode(b'++++'),
b'\xfb\xef\xbe')
+ with self.assertWarns(FutureWarning):
+ self.assertEqual(base64.urlsafe_b64decode(b'////'),
b'\xff\xff\xff')
+ with self.assertRaises(binascii.Error):
+ base64.b64decode(b'+/!', altchars=b'-_')
def _altchars_strategy():
"""Generate 'altchars' for base64 encoding."""
diff --git
a/Misc/NEWS.d/next/Library/2025-11-06-12-03-29.gh-issue-125346.7Gfpgw.rst
b/Misc/NEWS.d/next/Library/2025-11-06-12-03-29.gh-issue-125346.7Gfpgw.rst
new file mode 100644
index 00000000000000..187a6ebbe79b26
--- /dev/null
+++ b/Misc/NEWS.d/next/Library/2025-11-06-12-03-29.gh-issue-125346.7Gfpgw.rst
@@ -0,0 +1,5 @@
+Accepting ``+`` and ``/`` characters with an alternative alphabet in
+:func:`base64.b64decode` and :func:`base64.urlsafe_b64decode` is now
+deprecated.
+In future Python versions they will be errors in the strict mode and
+discarded in the non-strict mode.
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