[3.9] gh-101283: Improved fallback logic for subprocess with shell=True on Windows (GH-101286) (#101709)

Co-authored-by: Oleg Iarygin <oleg@arhadthedev.net>
Co-authored-by: Steve Dower <steve.dower@microsoft.com>

1 files changed, 17 insertions, 1 deletions

diff --git a/Lib/subprocess.py b/Lib/subprocess.py
index 4effc1d..8579d3f 100644
--- a/Lib/subprocess.py
+++ b/Lib/subprocess.py
@@ -1407,7 +1407,23 @@ class Popen(object):
             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: