[core] Prevent RCE when using --exec with %q (CVE-2023-40581)

The shell escape function is now using `""` instead of `\"`. `utils.Popen` has been patched to properly quote commands.

Prior to this fix using `--exec` together with `%q` when on Windows could cause remote code to execute. See https://github.com/yt-dlp/yt-dlp/security/advisories/GHSA-42h4-v29r-42qg for reference.

Authored by: Grub4K

yt_dlp/compat/__init__.py

@@ -30,7 +30,7 @@ compat_os_name = os._name if os.name == 'java' else os.name
 if compat_os_name == 'nt':
     def compat_shlex_quote(s):
         import re
-        return s if re.match(r'^[-_\w./]+$', s) else '"%s"' % s.replace('"', '\\"')
+        return s if re.match(r'^[-_\w./]+$', s) else s.replace('"', '""').join('""')
 else:
     from shlex import quote as compat_shlex_quote  # noqa: F401
 

yt_dlp/postprocessor/exec.py

@@ -1,8 +1,6 @@
-import subprocess
-
 from .common import PostProcessor
 from ..compat import compat_shlex_quote
-from ..utils import PostProcessingError, encodeArgument, variadic
+from ..utils import Popen, PostProcessingError, variadic
 
 
 class ExecPP(PostProcessor):
@@ -27,10 +25,10 @@ class ExecPP(PostProcessor):
     def run(self, info):
         for tmpl in self.exec_cmd:
             cmd = self.parse_cmd(tmpl, info)
-            self.to_screen('Executing command: %s' % cmd)
-            retCode = subprocess.call(encodeArgument(cmd), shell=True)
-            if retCode != 0:
-                raise PostProcessingError('Command returned error code %d' % retCode)
+            self.to_screen(f'Executing command: {cmd}')
+            _, _, return_code = Popen.run(cmd, shell=True)
+            if return_code != 0:
+                raise PostProcessingError(f'Command returned error code {return_code}')
         return [], info
 
 

yt_dlp/utils/_utils.py

@@ -825,7 +825,7 @@ class Popen(subprocess.Popen):
         _fix('LD_LIBRARY_PATH')  # Linux
         _fix('DYLD_LIBRARY_PATH')  # macOS
 
-    def __init__(self, *args, env=None, text=False, **kwargs):
+    def __init__(self, args, *remaining, env=None, text=False, shell=False, **kwargs):
         if env is None:
             env = os.environ.copy()
         self._fix_pyinstaller_ld_path(env)
@@ -835,7 +835,21 @@ class Popen(subprocess.Popen):
             kwargs['universal_newlines'] = True  # For 3.6 compatibility
             kwargs.setdefault('encoding', 'utf-8')
             kwargs.setdefault('errors', 'replace')
-        super().__init__(*args, env=env, **kwargs, startupinfo=self._startupinfo)
+
+        if shell and compat_os_name == 'nt' and kwargs.get('executable') is None:
+            if not isinstance(args, str):
+                args = ' '.join(compat_shlex_quote(a) for a in args)
+            shell = False
+            args = f'{self.__comspec()} /Q /S /D /V:OFF /C "{args}"'
+
+        super().__init__(args, *remaining, env=env, shell=shell, **kwargs, startupinfo=self._startupinfo)
+
+    def __comspec(self):
+        comspec = os.environ.get('ComSpec') or os.path.join(
+            os.environ.get('SystemRoot', ''), 'System32', 'cmd.exe')
+        if os.path.isabs(comspec):
+            return comspec
+        raise FileNotFoundError('shell not found: neither %ComSpec% nor %SystemRoot% is set')
 
     def communicate_or_kill(self, *args, **kwargs):
         try: