[3.7] gh-95778: CVE-2020-10735: Prevent DoS by very large int() (GH-96504)

Converting between `int` and `str` in bases other than 2
(binary), 4, 8 (octal), 16 (hexadecimal), or 32 such as base 10 (decimal) now
raises a `ValueError` if the number of digits in string form is above a
limit to avoid potential denial of service attacks due to the algorithmic
complexity. This is a mitigation for CVE-2020-10735
(https://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2020-10735).

This new limit can be configured or disabled by environment variable, command
line flag, or :mod:`sys` APIs. See the `Integer String Conversion Length
Limitation` documentation.  The default limit is 4300
digits in string form.

Patch by Gregory P. Smith [Google] and Christian Heimes [Red Hat] with feedback
from Victor Stinner, Thomas Wouters, Steve Dower, Ned Deily, and Mark Dickinson.

26 files changed, 882 insertions, 22 deletions

diff --git a/Doc/data/python3.7m.abi b/Doc/data/python3.7m.abi
index c42f67ca89..d5c17dc437 100644
--- a/Doc/data/python3.7m.abi
+++ b/Doc/data/python3.7m.abi
@@ -1860,6 +1860,7 @@
     <elf-symbol name='_Py_endswith__doc__' size='264' type='object-type' binding='global-binding' visibility='default-visibility' is-defined='yes'/>
     <elf-symbol name='_Py_expandtabs__doc__' size='173' type='object-type' binding='global-binding' visibility='default-visibility' is-defined='yes'/>
     <elf-symbol name='_Py_find__doc__' size='239' type='object-type' binding='global-binding' visibility='default-visibility' is-defined='yes'/>
+    <elf-symbol name='_Py_global_config_int_max_str_digits' size='4' type='object-type' binding='global-binding' visibility='default-visibility' is-defined='yes'/>
     <elf-symbol name='_Py_index__doc__' size='270' type='object-type' binding='global-binding' visibility='default-visibility' is-defined='yes'/>
     <elf-symbol name='_Py_isalnum__doc__' size='132' type='object-type' binding='global-binding' visibility='default-visibility' is-defined='yes'/>
     <elf-symbol name='_Py_isalpha__doc__' size='130' type='object-type' binding='global-binding' visibility='default-visibility' is-defined='yes'/>
@@ -11958,6 +11959,9 @@
       <data-member access='public' layout-offset-in-bits='11776'>
         <var-decl name='gilstate' type-id='type-id-600' visibility='default' filepath='../gpshead/Include/internal/pystate.h' line='105' column='1'/>
       </data-member>
+      <data-member access='public' layout-offset-in-bits='12096'>
+        <var-decl name='int_max_str_digits' type-id='type-id-7' visibility='default' filepath='../gpshead/Include/internal/pystate.h' line='109' column='1'/>
+      </data-member>
     </class-decl>
     <class-decl name='pyinterpreters' size-in-bits='256' is-struct='yes' visibility='default' filepath='../gpshead/Include/internal/pystate.h' line='83' column='1' id='type-id-596'>
       <data-member access='public' layout-offset-in-bits='0'>
diff --git a/Doc/library/functions.rst b/Doc/library/functions.rst
index d4553e045d..c0a22d5f16 100644
--- a/Doc/library/functions.rst
+++ b/Doc/library/functions.rst
@@ -774,6 +774,14 @@ are always available.  They are listed here in alphabetical order.
    .. versionchanged:: 3.7
       *x* is now a positional-only parameter.
 
+   .. versionchanged:: 3.7.14
+      :class:`int` string inputs and string representations can be limited to
+      help avoid denial of service attacks. A :exc:`ValueError` is raised when
+      the limit is exceeded while converting a string *x* to an :class:`int` or
+      when converting an :class:`int` into a string would exceed the limit.
+      See the :ref:`integer string conversion length limitation
+      <int_max_str_digits>` documentation.
+
 
 .. function:: isinstance(object, classinfo)
 
diff --git a/Doc/library/json.rst b/Doc/library/json.rst
index 510e30733f..feec0d0725 100644
--- a/Doc/library/json.rst
+++ b/Doc/library/json.rst
@@ -18,6 +18,11 @@ is a lightweight data interchange format inspired by
 `JavaScript <https://en.wikipedia.org/wiki/JavaScript>`_ object literal syntax
 (although it is not a strict subset of JavaScript [#rfc-errata]_ ).
 
+.. warning::
+   Be cautious when parsing JSON data from untrusted sources. A malicious
+   JSON string may cause the decoder to consume considerable CPU and memory
+   resources. Limiting the size of data to be parsed is recommended.
+
 :mod:`json` exposes an API familiar to users of the standard library
 :mod:`marshal` and :mod:`pickle` modules.
 
@@ -243,6 +248,12 @@ Basic Usage
    be used to use another datatype or parser for JSON integers
    (e.g. :class:`float`).
 
+   .. versionchanged:: 3.7.14
+      The default *parse_int* of :func:`int` now limits the maximum length of
+      the integer string via the interpreter's :ref:`integer string
+      conversion length limitation <int_max_str_digits>` to help avoid denial
+      of service attacks.
+
    *parse_constant*, if specified, will be called with one of the following
    strings: ``'-Infinity'``, ``'Infinity'``, ``'NaN'``.
    This can be used to raise an exception if invalid JSON numbers
diff --git a/Doc/library/stdtypes.rst b/Doc/library/stdtypes.rst
index c35cb2e11d..36e0bed90d 100644
--- a/Doc/library/stdtypes.rst
+++ b/Doc/library/stdtypes.rst
@@ -4741,6 +4741,165 @@ types, where they are relevant.  Some of these are not reported by the
       [<class 'bool'>]
 
 
+.. _int_max_str_digits:
+
+Integer string conversion length limitation
+===========================================
+
+CPython has a global limit for converting between :class:`int` and :class:`str`
+to mitigate denial of service attacks. This limit *only* applies to decimal or
+other non-power-of-two number bases. Hexadecimal, octal, and binary conversions
+are unlimited. The limit can be configured.
+
+The :class:`int` type in CPython is an abitrary length number stored in binary
+form (commonly known as a "bignum"). There exists no algorithm that can convert
+a string to a binary integer or a binary integer to a string in linear time,
+*unless* the base is a power of 2. Even the best known algorithms for base 10
+have sub-quadratic complexity. Converting a large value such as ``int('1' *
+500_000)`` can take over a second on a fast CPU.
+
+Limiting conversion size offers a practical way to avoid `CVE-2020-10735
+<https://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2020-10735>`_.
+
+The limit is applied to the number of digit characters in the input or output
+string when a non-linear conversion algorithm would be involved.  Underscores
+and the sign are not counted towards the limit.
+
+When an operation would exceed the limit, a :exc:`ValueError` is raised:
+
+.. doctest::
+
+   >>> import sys
+   >>> sys.set_int_max_str_digits(4300)  # Illustrative, this is the default.
+   >>> _ = int('2' * 5432)
+   Traceback (most recent call last):
+   ...
+   ValueError: Exceeds the limit (4300) for integer string conversion: value has 5432 digits.
+   >>> i = int('2' * 4300)
+   >>> len(str(i))
+   4300
+   >>> i_squared = i*i
+   >>> len(str(i_squared))
+   Traceback (most recent call last):
+   ...
+   ValueError: Exceeds the limit (4300) for integer string conversion: value has 8599 digits.
+   >>> len(hex(i_squared))
+   7144
+   >>> assert int(hex(i_squared), base=16) == i*i  # Hexadecimal is unlimited.
+
+The default limit is 4300 digits as provided in
+:data:`sys.int_info.default_max_str_digits <sys.int_info>`.
+The lowest limit that can be configured is 640 digits as provided in
+:data:`sys.int_info.str_digits_check_threshold <sys.int_info>`.
+
+Verification:
+
+.. doctest::
+
+   >>> import sys
+   >>> assert sys.int_info.default_max_str_digits == 4300, sys.int_info
+   >>> assert sys.int_info.str_digits_check_threshold == 640, sys.int_info
+   >>> msg = int('578966293710682886880994035146873798396722250538762761564'
+   ...           '9252925514383915483333812743580549779436104706260696366600'
+   ...           '571186405732').to_bytes(53, 'big')
+   ...
+
+.. versionadded:: 3.7.14
+
+Affected APIs
+-------------
+
+The limitation only applies to potentially slow conversions between :class:`int`
+and :class:`str` or :class:`bytes`:
+
+* ``int(string)`` with default base 10.
+* ``int(string, base)`` for all bases that are not a power of 2.
+* ``str(integer)``.
+* ``repr(integer)``
+* any other string conversion to base 10, for example ``f"{integer}"``,
+  ``"{}".format(integer)``, or ``b"%d" % integer``.
+
+The limitations do not apply to functions with a linear algorithm:
+
+* ``int(string, base)`` with base 2, 4, 8, 16, or 32.
+* :func:`int.from_bytes` and :func:`int.to_bytes`.
+* :func:`hex`, :func:`oct`, :func:`bin`.
+* :ref:`formatspec` for hex, octal, and binary numbers.
+* :class:`str` to :class:`float`.
+* :class:`str` to :class:`decimal.Decimal`.
+
+Configuring the limit
+---------------------
+
+Before Python starts up you can use an environment variable or an interpreter
+command line flag to configure the limit:
+
+* :envvar:`PYTHONINTMAXSTRDIGITS`, e.g.
+  ``PYTHONINTMAXSTRDIGITS=640 python3`` to set the limit to 640 or
+  ``PYTHONINTMAXSTRDIGITS=0 python3`` to disable the limitation.
+* :option:`-X int_max_str_digits <-X>`, e.g.
+  ``python3 -X int_max_str_digits=640``
+* :data:`sys.flags.int_max_str_digits` contains the value of
+  :envvar:`PYTHONINTMAXSTRDIGITS` or :option:`-X int_max_str_digits <-X>`.
+  If both the env var and the ``-X`` option are set, the ``-X`` option takes
+  precedence. A value of *-1* indicates that both were unset, thus a value of
+  :data:`sys.int_info.default_max_str_digits` was used during initilization.
+
+From code, you can inspect the current limit and set a new one using these
+:mod:`sys` APIs:
+
+* :func:`sys.get_int_max_str_digits` and :func:`sys.set_int_max_str_digits` are
+  a getter and setter for the interpreter-wide limit. Subinterpreters have
+  their own limit.
+
+Information about the default and minimum can be found in :attr:`sys.int_info`:
+
+* :data:`sys.int_info.default_max_str_digits <sys.int_info>` is the compiled-in
+  default limit.
+* :data:`sys.int_info.str_digits_check_threshold <sys.int_info>` is the lowest
+  accepted value for the limit (other than 0 which disables it).
+
+.. versionadded:: 3.7.14
+
+.. caution::
+
+   Setting a low limit *can* lead to problems. While rare, code exists that
+   contains integer constants in decimal in their source that exceed the
+   minimum threshold. A consequence of setting the limit is that Python source
+   code containing decimal integer literals longer than the limit will
+   encounter an error during parsing, usually at startup time or import time or
+   even at installation time - anytime an up to date ``.pyc`` does not already
+   exist for the code. A workaround for source that contains such large
+   constants is to convert them to ``0x`` hexadecimal form as it has no limit.
+
+   Test your application thoroughly if you use a low limit. Ensure your tests
+   run with the limit set early via the environment or flag so that it applies
+   during startup and even during any installation step that may invoke Python
+   to precompile ``.py`` sources to ``.pyc`` files.
+
+Recommended configuration
+-------------------------
+
+The default :data:`sys.int_info.default_max_str_digits` is expected to be
+reasonable for most applications. If your application requires a different
+limit, set it from your main entry point using Python version agnostic code as
+these APIs were added in security patch releases in versions before 3.11.
+
+Example::
+
+   >>> import sys
+   >>> if hasattr(sys, "set_int_max_str_digits"):
+   ...     upper_bound = 68000
+   ...     lower_bound = 4004
+   ...     current_limit = sys.get_int_max_str_digits()
+   ...     if current_limit == 0 or current_limit > upper_bound:
+   ...         sys.set_int_max_str_digits(upper_bound)
+   ...     elif current_limit < lower_bound:
+   ...         sys.set_int_max_str_digits(lower_bound)
+
+If you need to disable it entirely, set it to ``0``.
+
+
 .. rubric:: Footnotes
 
 .. [1] Additional information on these special methods may be found in the Python
diff --git a/Doc/library/sys.rst b/Doc/library/sys.rst
index 1760ae3cd5..c636b73925 100644
--- a/Doc/library/sys.rst
+++ b/Doc/library/sys.rst
@@ -327,9 +327,9 @@ always available.
    The :term:`named tuple` *flags* exposes the status of command line
    flags. The attributes are read only.
 
-   ============================= =============================
+   ============================= ==============================================================================================================
    attribute                     flag
-   ============================= =============================
+   ============================= ==============================================================================================================
    :const:`debug`                :option:`-d`
    :const:`inspect`              :option:`-i`
    :const:`interactive`          :option:`-i`
@@ -345,7 +345,8 @@ always available.
    :const:`hash_randomization`   :option:`-R`
    :const:`dev_mode`             :option:`-X` ``dev``
    :const:`utf8_mode`            :option:`-X` ``utf8``
-   ============================= =============================
+   :const:`int_max_str_digits`   :option:`-X int_max_str_digits <-X>` (:ref:`integer string conversion length limitation <int_max_str_digits>`)
+   ============================= ==============================================================================================================
 
    .. versionchanged:: 3.2
       Added ``quiet`` attribute for the new :option:`-q` flag.
@@ -363,6 +364,9 @@ always available.
       Added ``dev_mode`` attribute for the new :option:`-X` ``dev`` flag
       and ``utf8_mode`` attribute for the new  :option:`-X` ``utf8`` flag.
 
+   .. versionchanged:: 3.7.14
+      Added the ``int_max_str_digits`` attribute.
+
 
 .. data:: float_info
 
@@ -539,6 +543,15 @@ always available.
 
    .. versionadded:: 3.6
 
+
+.. function:: get_int_max_str_digits()
+
+   Returns the current value for the :ref:`integer string conversion length
+   limitation <int_max_str_digits>`. See also :func:`set_int_max_str_digits`.
+
+   .. versionadded:: 3.7.14
+
+
 .. function:: getrefcount(object)
 
    Return the reference count of the *object*.  The count returned is generally one
@@ -821,19 +834,31 @@ always available.
 
    .. tabularcolumns:: |l|L|
 
-   +-------------------------+----------------------------------------------+
-   | Attribute               | Explanation                                  |
-   +=========================+==============================================+
-   | :const:`bits_per_digit` | number of bits held in each digit.  Python   |
-   |                         | integers are stored internally in base       |
-   |                         | ``2**int_info.bits_per_digit``               |
-   +-------------------------+----------------------------------------------+
-   | :const:`sizeof_digit`   | size in bytes of the C type used to          |
-   |                         | represent a digit                            |
-   +-------------------------+----------------------------------------------+
+   +----------------------------------------+-----------------------------------------------+
+   | Attribute                              | Explanation                                   |
+   +========================================+===============================================+
+   | :const:`bits_per_digit`                | number of bits held in each digit.  Python    |
+   |                                        | integers are stored internally in base        |
+   |                                        | ``2**int_info.bits_per_digit``                |
+   +----------------------------------------+-----------------------------------------------+
+   | :const:`sizeof_digit`                  | size in bytes of the C type used to           |
+   |                                        | represent a digit                             |
+   +----------------------------------------+-----------------------------------------------+
+   | :const:`default_max_str_digits`        | default value for                             |
+   |                                        | :func:`sys.get_int_max_str_digits` when it    |
+   |                                        | is not otherwise explicitly configured.       |
+   +----------------------------------------+-----------------------------------------------+
+   | :const:`str_digits_check_threshold`    | minimum non-zero value for                    |
+   |                                        | :func:`sys.set_int_max_str_digits`,           |
+   |                                        | :envvar:`PYTHONINTMAXSTRDIGITS`, or           |
+   |                                        | :option:`-X int_max_str_digits <-X>`.         |
+   +----------------------------------------+-----------------------------------------------+
 
    .. versionadded:: 3.1
 
+   .. versionchanged:: 3.7.14
+      Added ``default_max_str_digits`` and ``str_digits_check_threshold``.
+
 
 .. data:: __interactivehook__
 
@@ -1092,6 +1117,14 @@ always available.
 
    .. availability:: Unix.
 
+.. function:: set_int_max_str_digits(n)
+
+   Set the :ref:`integer string conversion length limitation
+   <int_max_str_digits>` used by this interpreter. See also
+   :func:`get_int_max_str_digits`.
+
+   .. versionadded:: 3.7.14
+
 .. function:: setprofile(profilefunc)
 
    .. index::
diff --git a/Doc/library/test.rst b/Doc/library/test.rst
index e93ef450f0..d59cd405fa 100644
--- a/Doc/library/test.rst
+++ b/Doc/library/test.rst
@@ -1207,6 +1207,16 @@ The :mod:`test.support` module defines the following functions:
    .. versionadded:: 3.6
 
 
+.. function:: adjust_int_max_str_digits(max_digits)
+
+   This function returns a context manager that will change the global
+   :func:`sys.set_int_max_str_digits` setting for the duration of the
+   context to allow execution of test code that needs a different limit
+   on the number of digits when converting between an integer and string.
+
+   .. versionadded:: 3.7.14
+
+
 The :mod:`test.support` module defines the following classes:
 
 .. class:: TransientResource(exc, **kwargs)
diff --git a/Doc/using/cmdline.rst b/Doc/using/cmdline.rst
index be92642e2f..000fa6592e 100644
--- a/Doc/using/cmdline.rst
+++ b/Doc/using/cmdline.rst
@@ -432,6 +432,9 @@ Miscellaneous options
    * ``-X showalloccount`` to output the total count of allocated objects for
      each type when the program finishes. This only works when Python was built with
      ``COUNT_ALLOCS`` defined.
+   * ``-X int_max_str_digits`` configures the :ref:`integer string conversion
+     length limitation <int_max_str_digits>`.  See also
+     :envvar:`PYTHONINTMAXSTRDIGITS`.
    * ``-X importtime`` to show how long each import takes. It shows module
      name, cumulative time (including nested imports) and self time (excluding
      nested imports).  Note that its output may be broken in multi-threaded
@@ -474,6 +477,9 @@ Miscellaneous options
    .. versionadded:: 3.7
       The ``-X importtime``, ``-X dev`` and ``-X utf8`` options.
 
+   .. versionadded:: 3.7.14
+      The ``-X int_max_str_digits`` option.
+
 
 Options you shouldn't use
 ~~~~~~~~~~~~~~~~~~~~~~~~~
@@ -618,6 +624,13 @@ conflict.
 
    .. versionadded:: 3.2.3
 
+.. envvar:: PYTHONINTMAXSTRDIGITS
+
+   If this variable is set to an integer, it is used to configure the
+   interpreter's global :ref:`integer string conversion length limitation
+   <int_max_str_digits>`.
+
+   .. versionadded:: 3.7.14
 
 .. envvar:: PYTHONIOENCODING
 
diff --git a/Doc/whatsnew/3.7.rst b/Doc/whatsnew/3.7.rst
index 2cc380bf5a..21f96228cf 100644
--- a/Doc/whatsnew/3.7.rst
+++ b/Doc/whatsnew/3.7.rst
@@ -539,7 +539,6 @@ Other Language Changes
   the timing of each module import.
   (Contributed by Victor Stinner in :issue:`31415`.)
 
-
 New Modules
 ===========
 
@@ -2603,4 +2602,16 @@ URL by the parser :func:`urllib.parse` preventing such attacks. The removal
 characters are controlled by a new module level variable
 ``urllib.parse._UNSAFE_URL_BYTES_TO_REMOVE``. (See :issue:`43882`)
 
-
+Notable security feature in 3.7.14
+==================================
+
+Converting between :class:`int` and :class:`str` in bases other than 2
+(binary), 4, 8 (octal), 16 (hexadecimal), or 32 such as base 10 (decimal)
+now raises a :exc:`ValueError` if the number of digits in string form is
+above a limit to avoid potential denial of service attacks due to the
+algorithmic complexity. This is a mitigation for `CVE-2020-10735
+<https://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2020-10735>`_.
+This limit can be configured or disabled by environment variable, command
+line flag, or :mod:`sys` APIs. See the :ref:`integer string conversion
+length limitation <int_max_str_digits>` documentation.  The default limit
+is 4300 digits in string form.
diff --git a/Include/internal/pycore_long.h b/Include/internal/pycore_long.h
new file mode 100644
index 0000000000..ae04332a7a
--- /dev/null
+++ b/Include/internal/pycore_long.h
@@ -0,0 +1,49 @@
+#ifndef Py_INTERNAL_LONG_H
+#define Py_INTERNAL_LONG_H
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+#ifndef Py_BUILD_CORE
+#  error "this header requires Py_BUILD_CORE define"
+#endif
+
+/*
+ * Default int base conversion size limitation: Denial of Service prevention.
+ *
+ * Chosen such that this isn't wildly slow on modern hardware and so that
+ * everyone's existing deployed numpy test suite passes before
+ * https://github.com/numpy/numpy/issues/22098 is widely available.
+ *
+ * $ python -m timeit -s 's = "1"*4300' 'int(s)'
+ * 2000 loops, best of 5: 125 usec per loop
+ * $ python -m timeit -s 's = "1"*4300; v = int(s)' 'str(v)'
+ * 1000 loops, best of 5: 311 usec per loop
+ * (zen2 cloud VM)
+ *
+ * 4300 decimal digits fits a ~14284 bit number.
+ */
+#define _PY_LONG_DEFAULT_MAX_STR_DIGITS 4300
+/*
+ * Threshold for max digits check.  For performance reasons int() and
+ * int.__str__() don't checks values that are smaller than this
+ * threshold.  Acts as a guaranteed minimum size limit for bignums that
+ * applications can expect from CPython.
+ *
+ * % python -m timeit -s 's = "1"*640; v = int(s)' 'str(int(s))'
+ * 20000 loops, best of 5: 12 usec per loop
+ *
+ * "640 digits should be enough for anyone." - gps
+ * fits a ~2126 bit decimal number.
+ */
+#define _PY_LONG_MAX_STR_DIGITS_THRESHOLD 640
+
+#if ((_PY_LONG_DEFAULT_MAX_STR_DIGITS != 0) && \
+   (_PY_LONG_DEFAULT_MAX_STR_DIGITS < _PY_LONG_MAX_STR_DIGITS_THRESHOLD))
+# error "_PY_LONG_DEFAULT_MAX_STR_DIGITS smaller than threshold."
+#endif
+
+#ifdef __cplusplus
+}
+#endif
+#endif /* !Py_INTERNAL_LONG_H */
diff --git a/Include/internal/pystate.h b/Include/internal/pystate.h
index 5891339b54..f282708a86 100644
--- a/Include/internal/pystate.h
+++ b/Include/internal/pystate.h
@@ -105,6 +105,8 @@ typedef struct pyruntimestate {
     struct _gilstate_runtime_state gilstate;
 
     // XXX Consolidate globals found via the check-c-globals script.
+
+    int int_max_str_digits;
 } _PyRuntimeState;
 
 #define _PyRuntimeState_INIT {.initialized = 0, .core_initialized = 0}
@@ -120,6 +122,10 @@ PyAPI_FUNC(_PyInitError) _PyRuntime_Initialize(void);
 
 PyAPI_FUNC(void) _PyRuntime_Finalize(void);
 
+/* Excluded from public struct _PyCoreConfig for backporting reasons. */
+/* Modules/main.c config_init_int_max_str_digits() configures it. */
+/* Storage declared in pylifecycle.c */
+extern int _Py_global_config_int_max_str_digits;
 
 #define _Py_CURRENTLY_FINALIZING(tstate) \
     (_PyRuntime.finalizing == tstate)
diff --git a/Lib/test/support/__init__.py b/Lib/test/support/__init__.py
index b78451b9e6..8de486fa87 100644
--- a/Lib/test/support/__init__.py
+++ b/Lib/test/support/__init__.py
@@ -2985,3 +2985,13 @@ class _SMALLEST:
         return False
 
 SMALLEST = _SMALLEST()
+
+@contextlib.contextmanager
+def adjust_int_max_str_digits(max_digits):
+    """Temporarily change the integer string conversion length limit."""
+    current = sys.get_int_max_str_digits()
+    try:
+        sys.set_int_max_str_digits(max_digits)
+        yield
+    finally:
+        sys.set_int_max_str_digits(current)
diff --git a/Lib/test/test_ast.py b/Lib/test/test_ast.py
index 830fb58a02..63c4207d89 100644
--- a/Lib/test/test_ast.py
+++ b/Lib/test/test_ast.py
@@ -632,6 +632,14 @@ class ASTHelpers_Test(unittest.TestCase):
         self.assertRaises(ValueError, ast.literal_eval, '+True')
         self.assertRaises(ValueError, ast.literal_eval, '2+3')
 
+    def test_literal_eval_str_int_limit(self):
+        with support.adjust_int_max_str_digits(4000):
+            ast.literal_eval('3'*4000)  # no error
+            with self.assertRaises(SyntaxError) as err_ctx:
+                ast.literal_eval('3'*4001)
+            self.assertIn('Exceeds the limit ', str(err_ctx.exception))
+            self.assertIn(' Consider hexadecimal ', str(err_ctx.exception))
+
     def test_literal_eval_complex(self):
         # Issue #4907
         self.assertEqual(ast.literal_eval('6j'), 6j)
diff --git a/Lib/test/test_cmd_line.py b/Lib/test/test_cmd_line.py
index 95cdc8db7e..0ca90fd4ab 100644
--- a/Lib/test/test_cmd_line.py
+++ b/Lib/test/test_cmd_line.py
@@ -711,6 +711,40 @@ class CmdLineTest(unittest.TestCase):
         self.assertEqual(proc.returncode, 0, proc)
         self.assertEqual(proc.stdout.strip(), b'0')
 
+    def test_int_max_str_digits(self):
+        code = "import sys; print(sys.flags.int_max_str_digits, sys.get_int_max_str_digits())"
+
+        assert_python_failure('-X', 'int_max_str_digits', '-c', code)
+        assert_python_failure('-X', 'int_max_str_digits=foo', '-c', code)
+        assert_python_failure('-X', 'int_max_str_digits=100', '-c', code)
+
+        assert_python_failure('-c', code, PYTHONINTMAXSTRDIGITS='foo')
+        assert_python_failure('-c', code, PYTHONINTMAXSTRDIGITS='100')
+
+        def res2int(res):
+            out = res.out.strip().decode("utf-8")
+            return tuple(int(i) for i in out.split())
+
+        res = assert_python_ok('-c', code)
+        self.assertEqual(res2int(res), (-1, sys.get_int_max_str_digits()))
+        res = assert_python_ok('-X', 'int_max_str_digits=0', '-c', code)
+        self.assertEqual(res2int(res), (0, 0))
+        res = assert_python_ok('-X', 'int_max_str_digits=4000', '-c', code)
+        self.assertEqual(res2int(res), (4000, 4000))
+        res = assert_python_ok('-X', 'int_max_str_digits=100000', '-c', code)
+        self.assertEqual(res2int(res), (100000, 100000))
+
+        res = assert_python_ok('-c', code, PYTHONINTMAXSTRDIGITS='0')
+        self.assertEqual(res2int(res), (0, 0))
+        res = assert_python_ok('-c', code, PYTHONINTMAXSTRDIGITS='4000')
+        self.assertEqual(res2int(res), (4000, 4000))
+        res = assert_python_ok(
+            '-X', 'int_max_str_digits=6000', '-c', code,
+            PYTHONINTMAXSTRDIGITS='4000'
+        )
+        self.assertEqual(res2int(res), (6000, 6000))
+
+
 @unittest.skipIf(interpreter_requires_environment(),
                  'Cannot run -I tests when PYTHON env vars are required.')
 class IgnoreEnvironmentTest(unittest.TestCase):
diff --git a/Lib/test/test_compile.py b/Lib/test/test_compile.py
index 5ac1c5f271..b8f04d93ba 100644
--- a/Lib/test/test_compile.py
+++ b/Lib/test/test_compile.py
@@ -189,6 +189,19 @@ if 1:
         self.assertEqual(eval("0o777"), 511)
         self.assertEqual(eval("-0o0000010"), -8)
 
+    def test_int_literals_too_long(self):
+        n = 3000
+        source = f"a = 1\nb = 2\nc = {'3'*n}\nd = 4"
+        with support.adjust_int_max_str_digits(n):
+            compile(source, "<long_int_pass>", "exec")  # no errors.
+        with support.adjust_int_max_str_digits(n-1):
+            with self.assertRaises(SyntaxError) as err_ctx:
+                compile(source, "<long_int_fail>", "exec")
+            exc = err_ctx.exception
+            self.assertEqual(exc.lineno, 3)
+            self.assertIn('Exceeds the limit ', str(exc))
+            self.assertIn(' Consider hexadecimal ', str(exc))
+
     def test_unary_minus(self):
         # Verify treatment of unary minus on negative numbers SF bug #660455
         if sys.maxsize == 2147483647:
diff --git a/Lib/test/test_decimal.py b/Lib/test/test_decimal.py
index 1f37b5372a..cfa9e17051 100644
--- a/Lib/test/test_decimal.py
+++ b/Lib/test/test_decimal.py
@@ -2446,6 +2446,15 @@ class CUsabilityTest(UsabilityTest):
 class PyUsabilityTest(UsabilityTest):
     decimal = P
 
+    def setUp(self):
+        super().setUp()
+        self._previous_int_limit = sys.get_int_max_str_digits()
+        sys.set_int_max_str_digits(7000)
+
+    def tearDown(self):
+        sys.set_int_max_str_digits(self._previous_int_limit)
+        super().tearDown()
+
 class PythonAPItests(unittest.TestCase):
 
     def test_abc(self):
@@ -4503,6 +4512,15 @@ class CCoverage(Coverage):
 class PyCoverage(Coverage):
     decimal = P
 
+    def setUp(self):
+        super().setUp()
+        self._previous_int_limit = sys.get_int_max_str_digits()
+        sys.set_int_max_str_digits(7000)
+
+    def tearDown(self):
+        sys.set_int_max_str_digits(self._previous_int_limit)
+        super().tearDown()
+
 class PyFunctionality(unittest.TestCase):
     """Extra functionality in decimal.py"""
 
diff --git a/Lib/test/test_int.py b/Lib/test/test_int.py
index c048b712da..98ba847e7d 100644
--- a/Lib/test/test_int.py
+++ b/Lib/test/test_int.py
@@ -1,4 +1,5 @@
 import sys
+import time
 
 import unittest
 from test import support
@@ -516,5 +517,200 @@ class IntTestCases(unittest.TestCase):
         self.assertEqual(int('1_2_3_4_5_6_7', 32), 1144132807)
 
 
+class IntStrDigitLimitsTests(unittest.TestCase):
+
+    int_class = int  # Override this in subclasses to reuse the suite.
+
+    def setUp(self):
+        super().setUp()
+        self._previous_limit = sys.get_int_max_str_digits()
+        sys.set_int_max_str_digits(2048)
+
+    def tearDown(self):
+        sys.set_int_max_str_digits(self._previous_limit)
+        super().tearDown()
+
+    def test_disabled_limit(self):
+        self.assertGreater(sys.get_int_max_str_digits(), 0)
+        self.assertLess(sys.get_int_max_str_digits(), 20_000)
+        with support.adjust_int_max_str_digits(0):
+            self.assertEqual(sys.get_int_max_str_digits(), 0)
+            i = self.int_class('1' * 20_000)
+            str(i)
+        self.assertGreater(sys.get_int_max_str_digits(), 0)
+
+    def test_max_str_digits_edge_cases(self):
+        """Ignore the +/- sign and space padding."""
+        int_class = self.int_class
+        maxdigits = sys.get_int_max_str_digits()
+
+        int_class('1' * maxdigits)
+        int_class(' ' + '1' * maxdigits)
+        int_class('1' * maxdigits + ' ')
+        int_class('+' + '1' * maxdigits)
+        int_class('-' + '1' * maxdigits)
+        self.assertEqual(len(str(10 ** (maxdigits - 1))), maxdigits)
+
+    def check(self, i, base=None):
+        with self.assertRaises(ValueError):
+            if base is None:
+                self.int_class(i)
+            else:
+                self.int_class(i, base)
+
+    def test_max_str_digits(self):
+        maxdigits = sys.get_int_max_str_digits()
+
+        self.check('1' * (maxdigits + 1))
+        self.check(' ' + '1' * (maxdigits + 1))
+        self.check('1' * (maxdigits + 1) + ' ')
+        self.check('+' + '1' * (maxdigits + 1))
+        self.check('-' + '1' * (maxdigits + 1))
+        self.check('1' * (maxdigits + 1))
+
+        i = 10 ** maxdigits
+        with self.assertRaises(ValueError):
+            str(i)
+
+    def test_denial_of_service_prevented_int_to_str(self):
+        """Regression test: ensure we fail before performing O(N**2) work."""
+        maxdigits = sys.get_int_max_str_digits()
+        assert maxdigits < 50_000, maxdigits  # A test prerequisite.
+        get_time = time.process_time
+        if get_time() <= 0:  # some platforms like WASM lack process_time()
+            get_time = time.monotonic
+
+        huge_int = int(f'0x{"c"*65_000}', base=16)  # 78268 decimal digits.
+        digits = 78_268
+        with support.adjust_int_max_str_digits(digits):
+            start = get_time()
+            huge_decimal = str(huge_int)
+        seconds_to_convert = get_time() - start
+        self.assertEqual(len(huge_decimal), digits)
+        # Ensuring that we chose a slow enough conversion to measure.
+        # It takes 0.1 seconds on a Zen based cloud VM in an opt build.
+        if seconds_to_convert < 0.005:
+            raise unittest.SkipTest('"slow" conversion took only '
+                                    f'{seconds_to_convert} seconds.')
+
+        # We test with the limit almost at the size needed to check performance.
+        # The performant limit check is slightly fuzzy, give it a some room.
+        with support.adjust_int_max_str_digits(int(.995 * digits)):
+            with self.assertRaises(ValueError) as err:
+                start = get_time()
+                str(huge_int)
+            seconds_to_fail_huge = get_time() - start
+        self.assertIn('conversion', str(err.exception))
+        self.assertLess(seconds_to_fail_huge, seconds_to_convert/8)
+
+        # Now we test that a conversion that would take 30x as long also fails
+        # in a similarly fast fashion.
+        extra_huge_int = int(f'0x{"c"*500_000}', base=16)  # 602060 digits.
+        with self.assertRaises(ValueError) as err:
+            start = get_time()
+            # If not limited, 8 seconds said Zen based cloud VM.
+            str(extra_huge_int)
+        seconds_to_fail_extra_huge = get_time() - start
+        self.assertIn('conversion', str(err.exception))
+        self.assertLess(seconds_to_fail_extra_huge, seconds_to_convert/8)
+
+    def test_denial_of_service_prevented_str_to_int(self):
+        """Regression test: ensure we fail before performing O(N**2) work."""
+        maxdigits = sys.get_int_max_str_digits()
+        assert maxdigits < 100_000, maxdigits  # A test prerequisite.
+        get_time = time.process_time
+        if get_time() <= 0:  # some platforms like WASM lack process_time()
+            get_time = time.monotonic
+
+        digits = 133700
+        huge = '8'*digits
+        with support.adjust_int_max_str_digits(digits):
+            start = get_time()
+            int(huge)
+        seconds_to_convert = get_time() - start
+        # Ensuring that we chose a slow enough conversion to measure.
+        # It takes 0.1 seconds on a Zen based cloud VM in an opt build.
+        if seconds_to_convert < 0.005:
+            raise unittest.SkipTest('"slow" conversion took only '
+                                    f'{seconds_to_convert} seconds.')
+
+        with support.adjust_int_max_str_digits(digits - 1):
+            with self.assertRaises(ValueError) as err:
+                start = get_time()
+                int(huge)
+            seconds_to_fail_huge = get_time() - start
+        self.assertIn('conversion', str(err.exception))
+        self.assertLess(seconds_to_fail_huge, seconds_to_convert/8)
+
+        # Now we test that a conversion that would take 30x as long also fails
+        # in a similarly fast fashion.
+        extra_huge = '7'*1_200_000
+        with self.assertRaises(ValueError) as err:
+            start = get_time()
+            # If not limited, 8 seconds in the Zen based cloud VM.
+            int(extra_huge)
+        seconds_to_fail_extra_huge = get_time() - start
+        self.assertIn('conversion', str(err.exception))
+        self.assertLess(seconds_to_fail_extra_huge, seconds_to_convert/8)
+
+    def test_power_of_two_bases_unlimited(self):
+        """The limit does not apply to power of 2 bases."""
+        maxdigits = sys.get_int_max_str_digits()
+
+        for base in (2, 4, 8, 16, 32):
+            with self.subTest(base=base):
+                self.int_class('1' * (maxdigits + 1), base)
+                assert maxdigits < 100_000
+                self.int_class('1' * 100_000, base)
+
+    def test_underscores_ignored(self):
+        maxdigits = sys.get_int_max_str_digits()
+
+        triples = maxdigits // 3
+        s = '111' * triples
+        s_ = '1_11' * triples
+        self.int_class(s)  # succeeds
+        self.int_class(s_)  # succeeds
+        self.check(f'{s}111')
+        self.check(f'{s_}_111')
+
+    def test_sign_not_counted(self):
+        int_class = self.int_class
+        max_digits = sys.get_int_max_str_digits()
+        s = '5' * max_digits
+        i = int_class(s)
+        pos_i = int_class(f'+{s}')
+        assert i == pos_i
+        neg_i = int_class(f'-{s}')
+        assert -pos_i == neg_i
+        str(pos_i)
+        str(neg_i)
+
+    def _other_base_helper(self, base):
+        int_class = self.int_class
+        max_digits = sys.get_int_max_str_digits()
+        s = '2' * max_digits
+        i = int_class(s, base)
+        if base > 10:
+            with self.assertRaises(ValueError):
+                str(i)
+        elif base < 10:
+            str(i)
+        with self.assertRaises(ValueError) as err:
+            int_class(f'{s}1', base)
+
+    def test_int_from_other_bases(self):
+        base = 3
+        with self.subTest(base=base):
+            self._other_base_helper(base)
+        base = 36
+        with self.subTest(base=base):
+            self._other_base_helper(base)
+
+
+class IntSubclassStrDigitLimitsTests(IntStrDigitLimitsTests):
+    int_class = IntSubclass
+
+
 if __name__ == "__main__":
     unittest.main()
diff --git a/Lib/test/test_json/test_decode.py b/Lib/test/test_json/test_decode.py
index fdb9e62124..0c3e98db72 100644
--- a/Lib/test/test_json/test_decode.py
+++ b/Lib/test/test_json/test_decode.py
@@ -2,6 +2,7 @@ import decimal
 from io import StringIO
 from collections import OrderedDict
 from test.test_json import PyTest, CTest
+from test import support
 
 
 class TestDecode:
@@ -95,5 +96,12 @@ class TestDecode:
         d = self.json.JSONDecoder()
         self.assertRaises(ValueError, d.raw_decode, 'a'*42, -50000)
 
+    def test_limit_int(self):
+        maxdigits = 5000
+        with support.adjust_int_max_str_digits(maxdigits):
+            self.loads('1' * maxdigits)
+            with self.assertRaises(ValueError):
+                self.loads('1' * (maxdigits + 1))
+
 class TestPyDecode(TestDecode, PyTest): pass
 class TestCDecode(TestDecode, CTest): pass
diff --git a/Lib/test/test_sys.py b/Lib/test/test_sys.py
index 0478f20cd3..90304520d3 100644
--- a/Lib/test/test_sys.py
+++ b/Lib/test/test_sys.py
@@ -447,11 +447,17 @@ class SysModuleTest(unittest.TestCase):
         self.assertIsInstance(sys.executable, str)
         self.assertEqual(len(sys.float_info), 11)
         self.assertEqual(sys.float_info.radix, 2)
-        self.assertEqual(len(sys.int_info), 2)
+        self.assertEqual(len(sys.int_info), 4)
         self.assertTrue(sys.int_info.bits_per_digit % 5 == 0)
         self.assertTrue(sys.int_info.sizeof_digit >= 1)
+        self.assertGreaterEqual(sys.int_info.default_max_str_digits, 500)
+        self.assertGreaterEqual(sys.int_info.str_digits_check_threshold, 100)
+        self.assertGreater(sys.int_info.default_max_str_digits,
+                           sys.int_info.str_digits_check_threshold)
         self.assertEqual(type(sys.int_info.bits_per_digit), int)
         self.assertEqual(type(sys.int_info.sizeof_digit), int)
+        self.assertIsInstance(sys.int_info.default_max_str_digits, int)
+        self.assertIsInstance(sys.int_info.str_digits_check_threshold, int)
         self.assertIsInstance(sys.hexversion, int)
 
         self.assertEqual(len(sys.hash_info), 9)
@@ -554,7 +560,7 @@ class SysModuleTest(unittest.TestCase):
                  "inspect", "interactive", "optimize", "dont_write_bytecode",
                  "no_user_site", "no_site", "ignore_environment", "verbose",
                  "bytes_warning", "quiet", "hash_randomization", "isolated",
-                 "dev_mode", "utf8_mode")
+                 "dev_mode", "utf8_mode", "int_max_str_digits")
         for attr in attrs:
             self.assertTrue(hasattr(sys.flags, attr), attr)
             attr_type = bool if attr == "dev_mode" else int
diff --git a/Lib/test/test_xmlrpc.py b/Lib/test/test_xmlrpc.py
index 0e002ec4ef..9cbbd2128e 100644
--- a/Lib/test/test_xmlrpc.py
+++ b/Lib/test/test_xmlrpc.py
@@ -283,6 +283,16 @@ class XMLRPCTestCase(unittest.TestCase):
         check('<bigdecimal>9876543210.0123456789</bigdecimal>',
               decimal.Decimal('9876543210.0123456789'))
 
+    def test_limit_int(self):
+        check = self.check_loads
+        maxdigits = 5000
+        with support.adjust_int_max_str_digits(maxdigits):
+            s = '1' * (maxdigits + 1)
+            with self.assertRaises(ValueError):
+                check(f'<int>{s}</int>', None)
+            with self.assertRaises(ValueError):
+                check(f'<biginteger>{s}</biginteger>', None)
+
     def test_get_host_info(self):
         # see bug #3613, this raised a TypeError
         transp = xmlrpc.client.Transport()
diff --git a/Misc/NEWS.d/next/Security/2022-08-07-16-53-38.gh-issue-95778.ch010gps.rst b/Misc/NEWS.d/next/Security/2022-08-07-16-53-38.gh-issue-95778.ch010gps.rst
new file mode 100644
index 0000000000..8eb8a34884
--- /dev/null
+++ b/Misc/NEWS.d/next/Security/2022-08-07-16-53-38.gh-issue-95778.ch010gps.rst
@@ -0,0 +1,14 @@
+Converting between :class:`int` and :class:`str` in bases other than 2
+(binary), 4, 8 (octal), 16 (hexadecimal), or 32 such as base 10 (decimal) now
+raises a :exc:`ValueError` if the number of digits in string form is above a
+limit to avoid potential denial of service attacks due to the algorithmic
+complexity. This is a mitigation for `CVE-2020-10735
+<https://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2020-10735>`_.
+
+This new limit can be configured or disabled by environment variable, command
+line flag, or :mod:`sys` APIs. See the :ref:`integer string conversion length
+limitation <int_max_str_digits>` documentation.  The default limit is 4300
+digits in string form.
+
+Patch by Gregory P. Smith [Google] and Christian Heimes [Red Hat] with feedback
+from Victor Stinner, Thomas Wouters, Steve Dower, Ned Deily, and Mark Dickinson.
diff --git a/Modules/main.c b/Modules/main.c
index be0807b6a9..b646fe52e6 100644
--- a/Modules/main.c
+++ b/Modules/main.c
@@ -3,6 +3,7 @@
 #include "Python.h"
 #include "osdefs.h"
 #include "internal/import.h"
+#include "internal/pycore_long.h"
 #include "internal/pygetopt.h"
 #include "internal/pystate.h"
 
@@ -142,6 +143,9 @@ static const char usage_3[] = "\
          -X utf8: enable UTF-8 mode for operating system interfaces, overriding the default\n\
              locale-aware mode. -X utf8=0 explicitly disables UTF-8 mode (even when it would\n\
              otherwise activate automatically)\n\
+         -X int_max_str_digits=number: limit the size of int<->str conversions.\n\
+             This helps avoid denial of service attacks when parsing untrusted data.\n\
+             The default is sys.int_info.default_max_str_digits.  0 disables.\n\
 \n\
 --check-hash-based-pycs always|default|never:\n\
     control how Python invalidates hash-based .pyc files\n\
@@ -167,6 +171,10 @@ static const char usage_6[] =
 "   to seed the hashes of str, bytes and datetime objects.  It can also be\n"
 "   set to an integer in the range [0,4294967295] to get hash values with a\n"
 "   predictable seed.\n"
+"PYTHONINTMAXSTRDIGITS: limits the maximum digit characters in an int value\n"
+"   when converting from a string and when converting an int back to a str.\n"
+"   A value of 0 disables the limit.  Conversions to or from bases 2, 4, 8,\n"
+"   16, and 32 are never limited.\n"
 "PYTHONMALLOC: set the Python memory allocators and/or install debug hooks\n"
 "   on Python memory allocators. Use PYTHONMALLOC=debug to install debug\n"
 "   hooks.\n"
@@ -1801,6 +1809,48 @@ config_init_tracemalloc(_PyCoreConfig *config)
     return _Py_INIT_OK();
 }
 
+static _PyInitError
+config_init_int_max_str_digits(_PyCoreConfig *config)
+{
+    int maxdigits;
+    int valid = 0;
+
+    const char *env = config_get_env_var("PYTHONINTMAXSTRDIGITS");
+    if (env) {
+        if (!pymain_str_to_int(env, &maxdigits)) {
+            valid = ((maxdigits == 0) || (maxdigits >= _PY_LONG_MAX_STR_DIGITS_THRESHOLD));
+        }
+        if (!valid) {
+#define STRINGIFY(VAL) _STRINGIFY(VAL)
+#define _STRINGIFY(VAL) #VAL
+            return _Py_INIT_USER_ERR(
+                    "PYTHONINTMAXSTRDIGITS: invalid limit; must be >= "
+                    STRINGIFY(_PY_LONG_MAX_STR_DIGITS_THRESHOLD)
+                    " or 0 for unlimited.");
+        }
+        _Py_global_config_int_max_str_digits = maxdigits;
+    }
+
+    const wchar_t *xoption = config_get_xoption(config, L"int_max_str_digits");
+    if (xoption) {
+        const wchar_t *sep = wcschr(xoption, L'=');
+        if (sep) {
+            if (!pymain_wstr_to_int(sep + 1, &maxdigits)) {
+                valid = ((maxdigits == 0) || (maxdigits >= _PY_LONG_MAX_STR_DIGITS_THRESHOLD));
+            }
+        }
+        if (!valid) {
+            return _Py_INIT_USER_ERR(
+                    "-X int_max_str_digits: invalid limit; must be >= "
+                    STRINGIFY(_PY_LONG_MAX_STR_DIGITS_THRESHOLD)
+                    " or 0 for unlimited.");
+#undef _STRINGIFY
+#undef STRINGIFY
+        }
+        _Py_global_config_int_max_str_digits = maxdigits;
+    }
+    return _Py_INIT_OK();
+}
 
 static void
 get_env_flag(int *flag, const char *name)
@@ -2020,6 +2070,12 @@ config_read_complex_options(_PyCoreConfig *config)
             return err;
         }
     }
+    if (_Py_global_config_int_max_str_digits < 0) {
+        _PyInitError err = config_init_int_max_str_digits(config);
+        if (_Py_INIT_FAILED(err)) {
+            return err;
+        }
+    }
     return _Py_INIT_OK();
 }
 
diff --git a/Objects/longobject.c b/Objects/longobject.c
index 202f652fc6..a481a16eb0 100644
--- a/Objects/longobject.c
+++ b/Objects/longobject.c
@@ -3,6 +3,8 @@
 /* XXX The functional organization of this file is terrible */
 
 #include "Python.h"
+#include "internal/pycore_long.h"
+#include "internal/pystate.h"  // _Py_global_config_int_max_str_digits
 #include "longintrepr.h"
 
 #include <float.h>
@@ -45,6 +47,9 @@ static PyLongObject small_ints[NSMALLNEGINTS + NSMALLPOSINTS];
 Py_ssize_t quick_int_allocs, quick_neg_int_allocs;
 #endif
 
+#define _MAX_STR_DIGITS_ERROR_FMT_TO_INT "Exceeds the limit (%d) for integer string conversion: value has %zd digits"
+#define _MAX_STR_DIGITS_ERROR_FMT_TO_STR "Exceeds the limit (%d) for integer string conversion"
+
 static PyObject *
 get_small_int(sdigit ival)
 {
@@ -1602,6 +1607,22 @@ long_to_decimal_string_internal(PyObject *aa,
     size_a = Py_ABS(Py_SIZE(a));
     negative = Py_SIZE(a) < 0;
 
+    /* quick and dirty pre-check for overflowing the decimal digit limit,
+       based on the inequality 10/3 >= log2(10)
+
+       explanation in https://github.com/python/cpython/pull/96537
+    */
+    if (size_a >= 10 * _PY_LONG_MAX_STR_DIGITS_THRESHOLD
+                  / (3 * PyLong_SHIFT) + 2) {
+        int max_str_digits = _PyRuntime.int_max_str_digits;
+        if ((max_str_digits > 0) &&
+            (max_str_digits / (3 * PyLong_SHIFT) <= (size_a - 11) / 10)) {
+            PyErr_Format(PyExc_ValueError, _MAX_STR_DIGITS_ERROR_FMT_TO_STR,
+                         max_str_digits);
+            return -1;
+        }
+    }
+
     /* quick and dirty upper bound for the number of digits
        required to express a in base _PyLong_DECIMAL_BASE:
 
@@ -1661,6 +1682,16 @@ long_to_decimal_string_internal(PyObject *aa,
         tenpow *= 10;
         strlen++;
     }
+    if (strlen > _PY_LONG_MAX_STR_DIGITS_THRESHOLD) {
+        int max_str_digits = _PyRuntime.int_max_str_digits;
+        Py_ssize_t strlen_nosign = strlen - negative;
+        if ((max_str_digits > 0) && (strlen_nosign > max_str_digits)) {
+            Py_DECREF(scratch);
+            PyErr_Format(PyExc_ValueError, _MAX_STR_DIGITS_ERROR_FMT_TO_STR,
+                         max_str_digits);
+            return -1;
+        }
+    }
     if (writer) {
         if (_PyUnicodeWriter_Prepare(writer, strlen, '9') == -1) {
             Py_DECREF(scratch);
@@ -2174,6 +2205,7 @@ PyLong_FromString(const char *str, char **pend, int base)
 
     start = str;
     if ((base & (base - 1)) == 0) {
+        /* binary bases are not limited by int_max_str_digits */
         int res = long_from_binary_base(&str, base, &z);
         if (res < 0) {
             /* Syntax error. */
@@ -2325,6 +2357,16 @@ digit beyond the first.
             goto onError;
         }
 
+        /* Limit the size to avoid excessive computation attacks. */
+        if (digits > _PY_LONG_MAX_STR_DIGITS_THRESHOLD) {
+            int max_str_digits = _PyRuntime.int_max_str_digits;
+            if ((max_str_digits > 0) && (digits > max_str_digits)) {
+                PyErr_Format(PyExc_ValueError, _MAX_STR_DIGITS_ERROR_FMT_TO_INT,
+                             max_str_digits, digits);
+                return NULL;
+            }
+        }
+
         /* Create an int object that can contain the largest possible
          * integer with this base and length.  Note that there's no
          * need to initialize z->ob_digit -- no slot is read up before
@@ -4807,6 +4849,7 @@ long_new_impl(PyTypeObject *type, PyObject *x, PyObject *obase)
         }
         return PyLong_FromLong(0L);
     }
+    /* default base and limit, forward to standard implementation */
     if (obase == NULL)
         return PyNumber_Long(x);
 
@@ -5430,6 +5473,8 @@ internal representation of integers.  The attributes are read only.");
 static PyStructSequence_Field int_info_fields[] = {
     {"bits_per_digit", "size of a digit in bits"},
     {"sizeof_digit", "size in bytes of the C type used to represent a digit"},
+    {"default_max_str_digits", "maximum string conversion digits limitation"},
+    {"str_digits_check_threshold", "minimum positive value for int_max_str_digits"},
     {NULL, NULL}
 };
 
@@ -5437,7 +5482,7 @@ static PyStructSequence_Desc int_info_desc = {
     "sys.int_info",   /* name */
     int_info__doc__,  /* doc */
     int_info_fields,  /* fields */
-    2                 /* number of fields */
+    4                 /* number of fields */
 };
 
 PyObject *
@@ -5452,6 +5497,17 @@ PyLong_GetInfo(void)
                               PyLong_FromLong(PyLong_SHIFT));
     PyStructSequence_SET_ITEM(int_info, field++,
                               PyLong_FromLong(sizeof(digit)));
+    /*
+     * The following two fields were added after investigating uses of
+     * sys.int_info in the wild: Exceedingly rarely used. The ONLY use found was
+     * numba using sys.int_info.bits_per_digit as attribute access rather than
+     * sequence unpacking. Cython and sympy also refer to sys.int_info but only
+     * as info for debugging. No concern about adding these in a backport.
+     */
+    PyStructSequence_SET_ITEM(int_info, field++,
+                              PyLong_FromLong(_PY_LONG_DEFAULT_MAX_STR_DIGITS));
+    PyStructSequence_SET_ITEM(int_info, field++,
+                              PyLong_FromLong(_PY_LONG_MAX_STR_DIGITS_THRESHOLD));
     if (PyErr_Occurred()) {
         Py_CLEAR(int_info);
         return NULL;
@@ -5503,6 +5559,10 @@ _PyLong_Init(void)
         if (PyStructSequence_InitType2(&Int_InfoType, &int_info_desc) < 0)
             return 0;
     }
+    _PyRuntime.int_max_str_digits = _Py_global_config_int_max_str_digits;
+    if (_PyRuntime.int_max_str_digits == -1) {
+        _PyRuntime.int_max_str_digits = _PY_LONG_DEFAULT_MAX_STR_DIGITS;
+    }
 
     return 1;
 }
diff --git a/Python/ast.c b/Python/ast.c
index 9d8a3544bd..d67e2b207a 100644
--- a/Python/ast.c
+++ b/Python/ast.c
@@ -9,6 +9,11 @@
 #include "ast.h"
 #include "token.h"
 #include "pythonrun.h"
+/* A Windows header defines its own Yield macro, so we don't use the one
+ * from Python-ast.h and instead call _Py_Yield() directly. [ugh] */
+#undef Yield
+#include "internal/pystate.h"
+#undef Yield
 
 #include <assert.h>
 #include <stdbool.h>
@@ -2138,8 +2143,32 @@ ast_for_atom(struct compiling *c, const node *n)
     }
     case NUMBER: {
         PyObject *pynum = parsenumber(c, STR(ch));
-        if (!pynum)
+        if (!pynum) {
+            PyThreadState *tstate = PyThreadState_GET();
+            // The only way a ValueError should happen in _this_ code is via
+            // PyLong_FromString hitting a length limit.
+            if (tstate->curexc_type == PyExc_ValueError &&
+                tstate->curexc_value != NULL) {
+                PyObject *type, *value, *tb;
+                // This acts as PyErr_Clear() as we're replacing curexc.
+                PyErr_Fetch(&type, &value, &tb);
+                Py_XDECREF(tb);
+                Py_DECREF(type);
+                PyObject *helpful_msg = PyUnicode_FromFormat(
+                    "%S - Consider hexadecimal for huge integer literals "
+                    "to avoid decimal conversion limits.",
+                    value);
+                if (helpful_msg) {
+                    const char* error_msg = PyUnicode_AsUTF8(helpful_msg);
+                    if (error_msg) {
+                        ast_error(c, ch, error_msg);
+                    }
+                    Py_DECREF(helpful_msg);
+                }
+                Py_DECREF(value);
+            }
             return NULL;
+        }
 
         if (PyArena_AddPyObject(c->c_arena, pynum) < 0) {
             Py_DECREF(pynum);
@@ -2678,7 +2707,7 @@ ast_for_expr(struct compiling *c, const node *n)
             }
             if (is_from)
                 return YieldFrom(exp, LINENO(n), n->n_col_offset, c->c_arena);
-            return Yield(exp, LINENO(n), n->n_col_offset, c->c_arena);
+            return _Py_Yield(exp, LINENO(n), n->n_col_offset, c->c_arena);
         }
         case factor:
             if (NCH(n) == 1) {
diff --git a/Python/clinic/sysmodule.c.h b/Python/clinic/sysmodule.c.h
index 3e1480513f..bc6c99a5d6 100644
--- a/Python/clinic/sysmodule.c.h
+++ b/Python/clinic/sysmodule.c.h
@@ -63,4 +63,52 @@ sys_get_coroutine_origin_tracking_depth(PyObject *module, PyObject *Py_UNUSED(ig
 exit:
     return return_value;
 }
-/*[clinic end generated code: output=4a3ac42b97d710ff input=a9049054013a1b77]*/
+
+PyDoc_STRVAR(sys_get_int_max_str_digits__doc__,
+"get_int_max_str_digits($module, /)\n"
+"--\n"
+"\n"
+"Set the maximum string digits limit for non-binary int<->str conversions.");
+
+#define SYS_GET_INT_MAX_STR_DIGITS_METHODDEF    \
+    {"get_int_max_str_digits", (PyCFunction)sys_get_int_max_str_digits, METH_NOARGS, sys_get_int_max_str_digits__doc__},
+
+static PyObject *
+sys_get_int_max_str_digits_impl(PyObject *module);
+
+static PyObject *
+sys_get_int_max_str_digits(PyObject *module, PyObject *Py_UNUSED(ignored))
+{
+    return sys_get_int_max_str_digits_impl(module);
+}
+
+PyDoc_STRVAR(sys_set_int_max_str_digits__doc__,
+"set_int_max_str_digits($module, /, maxdigits)\n"
+"--\n"
+"\n"
+"Set the maximum string digits limit for non-binary int<->str conversions.");
+
+#define SYS_SET_INT_MAX_STR_DIGITS_METHODDEF    \
+    {"set_int_max_str_digits", (PyCFunction)sys_set_int_max_str_digits, METH_FASTCALL|METH_KEYWORDS, sys_set_int_max_str_digits__doc__},
+
+static PyObject *
+sys_set_int_max_str_digits_impl(PyObject *module, int maxdigits);
+
+static PyObject *
+sys_set_int_max_str_digits(PyObject *module, PyObject *const *args, Py_ssize_t nargs, PyObject *kwnames)
+{
+    PyObject *return_value = NULL;
+    static const char * const _keywords[] = {"maxdigits", NULL};
+    static _PyArg_Parser _parser = {"i:set_int_max_str_digits", _keywords, 0};
+    int maxdigits;
+
+    if (!_PyArg_ParseStackAndKeywords(args, nargs, kwnames, &_parser,
+        &maxdigits)) {
+        goto exit;
+    }
+    return_value = sys_set_int_max_str_digits_impl(module, maxdigits);
+
+exit:
+    return return_value;
+}
+/*[clinic end generated code: output=c566fcdbb8f6ae2c input=a9049054013a1b77]*/
diff --git a/Python/pylifecycle.c b/Python/pylifecycle.c
index 55d1ba5734..ff0087e215 100644
--- a/Python/pylifecycle.c
+++ b/Python/pylifecycle.c
@@ -6,6 +6,7 @@
 #undef Yield /* undefine macro conflicting with winbase.h */
 #include "internal/context.h"
 #include "internal/hamt.h"
+#include "internal/pycore_long.h"
 #include "internal/pystate.h"
 #include "grammar.h"
 #include "node.h"
@@ -130,6 +131,9 @@ int Py_LegacyWindowsFSEncodingFlag = 0; /* Uses mbcs instead of utf-8 */
 int Py_LegacyWindowsStdioFlag = 0; /* Uses FileIO instead of WindowsConsoleIO */
 #endif
 
+/* Unusual name compared to the above for backporting from 3.12 reasons. */
+int _Py_global_config_int_max_str_digits = -1; /* -X int_max_str_digits or PYTHONINTMAXSTRDIGITS */
+
 /* Hack to force loading of object files */
 int (*_PyOS_mystrnicmp_hack)(const char *, const char *, Py_ssize_t) = \
     PyOS_mystrnicmp; /* Python/pystrcmp.o */
diff --git a/Python/sysmodule.c b/Python/sysmodule.c
index b953a00097..82e029fd38 100644
--- a/Python/sysmodule.c
+++ b/Python/sysmodule.c
@@ -15,6 +15,7 @@ Data members:
 */
 
 #include "Python.h"
+#include "internal/pycore_long.h"  // _PY_LONG_MAX_STR_DIGITS_THRESHOLD
 #include "internal/pystate.h"
 #include "code.h"
 #include "frameobject.h"
@@ -1218,6 +1219,43 @@ sys_mdebug(PyObject *self, PyObject *args)
 }
 #endif /* USE_MALLOPT */
 
+
+/*[clinic input]
+sys.get_int_max_str_digits
+
+Set the maximum string digits limit for non-binary int<->str conversions.
+[clinic start generated code]*/
+
+static PyObject *
+sys_get_int_max_str_digits_impl(PyObject *module)
+/*[clinic end generated code: output=0042f5e8ae0e8631 input=8dab13e2023e60d5]*/
+{
+    return PyLong_FromSsize_t(_PyRuntime.int_max_str_digits);
+}
+
+/*[clinic input]
+sys.set_int_max_str_digits
+
+    maxdigits: int
+
+Set the maximum string digits limit for non-binary int<->str conversions.
+[clinic start generated code]*/
+
+static PyObject *
+sys_set_int_max_str_digits_impl(PyObject *module, int maxdigits)
+/*[clinic end generated code: output=734d4c2511f2a56d input=d7e3f325db6910c5]*/
+{
+    if ((!maxdigits) || (maxdigits >= _PY_LONG_MAX_STR_DIGITS_THRESHOLD)) {
+        _PyRuntime.int_max_str_digits = maxdigits;
+        Py_RETURN_NONE;
+    } else {
+        PyErr_Format(
+            PyExc_ValueError, "maxdigits must be 0 or larger than %d",
+            _PY_LONG_MAX_STR_DIGITS_THRESHOLD);
+        return NULL;
+    }
+}
+
 size_t
 _PySys_GetSizeOf(PyObject *o)
 {
@@ -1605,6 +1643,8 @@ static PyMethodDef sys_methods[] = {
     {"getandroidapilevel", (PyCFunction)sys_getandroidapilevel, METH_NOARGS,
      getandroidapilevel_doc},
 #endif
+    SYS_GET_INT_MAX_STR_DIGITS_METHODDEF
+    SYS_SET_INT_MAX_STR_DIGITS_METHODDEF
     {NULL,              NULL}           /* sentinel */
 };
 
@@ -2051,6 +2091,7 @@ static PyStructSequence_Field flags_fields[] = {
     {"isolated",                "-I"},
     {"dev_mode",                "-X dev"},
     {"utf8_mode",               "-X utf8"},
+    {"int_max_str_digits",      "-X int_max_str_digits"},
     {0}
 };
 
@@ -2058,7 +2099,7 @@ static PyStructSequence_Desc flags_desc = {
     "sys.flags",        /* name */
     flags__doc__,       /* doc */
     flags_fields,       /* fields */
-    15
+    16
 };
 
 static PyObject*
@@ -2092,6 +2133,7 @@ make_flags(void)
     SetFlag(Py_IsolatedFlag);
     PyStructSequence_SET_ITEM(seq, pos++, PyBool_FromLong(core_config->dev_mode));
     SetFlag(Py_UTF8Mode);
+    SetFlag(_Py_global_config_int_max_str_digits);
 #undef SetFlag
 
     if (PyErr_Occurred()) {