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[3.11] gh-101283: Improved fallback logic for subprocess with shell=True on Windows (GH-101286) #101711

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40 changes: 40 additions & 0 deletions Doc/library/subprocess.rst
Original file line number Diff line number Diff line change
@@ -111,6 +111,14 @@ underlying :class:`Popen` interface can be used directly.
Added the *text* parameter, as a more understandable alias of *universal_newlines*.
Added the *capture_output* parameter.

.. versionchanged:: 3.11.2
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this version number is wrong everywhere. 3.11.2 was just released.

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Suggested change
.. versionchanged:: 3.11.2
.. versionchanged:: 3.11.3


Changed Windows shell search order for ``shell=True``. The current
directory and ``%PATH%`` are replaced with ``%COMSPEC%`` and
``%SystemRoot%\System32\cmd.exe``. As a result, dropping a
malicious program named ``cmd.exe`` into a current directory no
longer works.

.. class:: CompletedProcess

The return value from :func:`run`, representing a process that has finished.
@@ -488,6 +496,14 @@ functions.
*executable* parameter accepts a bytes and :term:`path-like object`
on Windows.

.. versionchanged:: 3.11.2
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Suggested change
.. versionchanged:: 3.11.2
.. versionchanged:: 3.11.3


Changed Windows shell search order for ``shell=True``. The current
directory and ``%PATH%`` are replaced with ``%COMSPEC%`` and
``%SystemRoot%\System32\cmd.exe``. As a result, dropping a
malicious program named ``cmd.exe`` into a current directory no
longer works.

*stdin*, *stdout* and *stderr* specify the executed program's standard input,
standard output and standard error file handles, respectively. Valid values
are :data:`PIPE`, :data:`DEVNULL`, an existing file descriptor (a positive
@@ -1160,6 +1176,14 @@ calls these functions.
.. versionchanged:: 3.3
*timeout* was added.

.. versionchanged:: 3.11.2
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Suggested change
.. versionchanged:: 3.11.2
.. versionchanged:: 3.11.3


Changed Windows shell search order for ``shell=True``. The current
directory and ``%PATH%`` are replaced with ``%COMSPEC%`` and
``%SystemRoot%\System32\cmd.exe``. As a result, dropping a
malicious program named ``cmd.exe`` into a current directory no
longer works.

.. function:: check_call(args, *, stdin=None, stdout=None, stderr=None, \
shell=False, cwd=None, timeout=None, \
**other_popen_kwargs)
@@ -1192,6 +1216,14 @@ calls these functions.
.. versionchanged:: 3.3
*timeout* was added.

.. versionchanged:: 3.11.2
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Suggested change
.. versionchanged:: 3.11.2
.. versionchanged:: 3.11.3


Changed Windows shell search order for ``shell=True``. The current
directory and ``%PATH%`` are replaced with ``%COMSPEC%`` and
``%SystemRoot%\System32\cmd.exe``. As a result, dropping a
malicious program named ``cmd.exe`` into a current directory no
longer works.


.. function:: check_output(args, *, stdin=None, stderr=None, shell=False, \
cwd=None, encoding=None, errors=None, \
@@ -1247,6 +1279,14 @@ calls these functions.
.. versionadded:: 3.7
*text* was added as a more readable alias for *universal_newlines*.

.. versionchanged:: 3.11.2
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Suggested change
.. versionchanged:: 3.11.2
.. versionchanged:: 3.11.3


Changed Windows shell search order for ``shell=True``. The current
directory and ``%PATH%`` are replaced with ``%COMSPEC%`` and
``%SystemRoot%\System32\cmd.exe``. As a result, dropping a
malicious program named ``cmd.exe`` into a current directory no
longer works.


.. _subprocess-replacements:

18 changes: 17 additions & 1 deletion Lib/subprocess.py
Original file line number Diff line number Diff line change
@@ -1480,7 +1480,23 @@ def _execute_child(self, args, executable, preexec_fn, close_fds,
if shell:
startupinfo.dwFlags |= _winapi.STARTF_USESHOWWINDOW
startupinfo.wShowWindow = _winapi.SW_HIDE
comspec = os.environ.get("COMSPEC", "cmd.exe")
if not executable:
# gh-101283: without a fully-qualified path, before Windows
# checks the system directories, it first looks in the
# application directory, and also the current directory if
# NeedCurrentDirectoryForExePathW(ExeName) is true, so try
# to avoid executing unqualified "cmd.exe".
comspec = os.environ.get('ComSpec')
if not comspec:
system_root = os.environ.get('SystemRoot', '')
comspec = os.path.join(system_root, 'System32', 'cmd.exe')
if not os.path.isabs(comspec):
raise FileNotFoundError('shell not found: neither %ComSpec% nor %SystemRoot% is set')
if os.path.isabs(comspec):
executable = comspec
else:
comspec = executable

args = '{} /c "{}"'.format (comspec, args)

if cwd is not None:
Original file line number Diff line number Diff line change
@@ -0,0 +1,3 @@
:class:`subprocess.Popen` now uses a safer approach to find
``cmd.exe`` when launching with ``shell=True``. Patch by Eryk Sun,
based on a patch by Oleg Iarygin.