gh-101283: Improved fallback logic for subprocess with shell=True on Windows (GH-101286)

1 files changed, 15 insertions, 1 deletions

diff --git a/Lib/subprocess.py b/Lib/subprocess.py
index 9cadd1b..fa527d5 100644
--- a/Lib/subprocess.py
+++ b/Lib/subprocess.py
@@ -1480,7 +1480,21 @@ class Popen:
             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
+
                 args = '{} /c "{}"'.format (comspec, args)
 
             if cwd is not None: