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[3.7] gh-101283: Improved fallback logic for subprocess with shell=True on Windows (GH-101286) #101713

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ambv merged 2 commits into from
Feb 9, 2023
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[3.7] gh-101283: Improved fallback logic for subprocess with shell=True on Windows (GH-101286) #101713

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gh-101283: Improved fallback logic for subprocess with shell=True on …
arhadthedev Feb 8, 2023
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ambv Feb 9, 2023
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43 changes: 43 additions & 0 deletions Doc/library/subprocess.rst
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Original file line number Diff line number Diff line change
Expand Up @@ -111,6 +111,14 @@ compatibility with older versions, see the :ref:`call-function-trio` section.
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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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.
Expand Down Expand Up @@ -442,6 +450,17 @@ functions.
:program:`ps`. If ``shell=True``, on POSIX the *executable* argument
specifies a replacement shell for the default :file:`/bin/sh`.

.. versionchanged:: 3.6
*executable* parameter accepts a :term:`path-like object` on POSIX.
Comment on lines +453 to +454
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@arhadthedev arhadthedev Feb 9, 2023

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Was it added by mistake?

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@ambv ambv Feb 9, 2023

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It was probably added to resolve the conflict but it belongs in the 3.7 docs as well. For some reason this doc bit wasn't ported back to 3.6 and 3.7 when Serhiy landed GH-5914.

I would leave it in.


.. versionchanged:: 3.11.2
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ambv marked this conversation as resolved.
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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
Expand Down Expand Up @@ -1032,6 +1051,14 @@ calls these functions.
.. versionchanged:: 3.3
*timeout* was added.

.. versionchanged:: 3.11.2
Comment thread
ambv marked this conversation as resolved.
Outdated

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)
Expand Down Expand Up @@ -1062,6 +1089,14 @@ calls these functions.
.. versionchanged:: 3.3
*timeout* was added.

.. versionchanged:: 3.11.2
Comment thread
ambv marked this conversation as resolved.
Outdated

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, \
Expand Down Expand Up @@ -1116,6 +1151,14 @@ calls these functions.
.. versionadded:: 3.7
*text* was added as a more readable alias for *universal_newlines*.

.. versionchanged:: 3.11.2
Comment thread
ambv marked this conversation as resolved.
Outdated

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:

Expand Down
18 changes: 17 additions & 1 deletion Lib/subprocess.py
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Expand Up @@ -1192,7 +1192,23 @@ 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)

# Start the process
Expand Down
3 changes: 3 additions & 0 deletions Misc/NEWS.d/next/Security/2023-01-24-16-12-00.gh-issue-101283.9tqu39.rst
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@@ -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.