[core] Prevent RCE when using --exec with %q (CVE-2024-22423)

The shell escape function now properly escapes `%`, `\\` and `\n`. `utils.Popen` as well as `%q` output template expansion have been patched accordingly.

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-hjq6-52gw-2g7p for more details.

Authored by: Grub4K

devscripts/changelog_override.json

@@ -142,5 +142,10 @@
         "when": "e3a3ed8a981d9395c4859b6ef56cd02bc3148db2",
         "short": "[cleanup:ie] No `from` stdlib imports in extractors",
         "authors": ["pukkandan"]
+    },
+    {
+        "action": "add",
+        "when": "9590cc6b4768e190183d7d071a6c78170889116a",
+        "short": "[priority] Security: [[CVE-2024-22423](https://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2024-22423)] [Prevent RCE when using `--exec` with `%q` on Windows](https://github.com/yt-dlp/yt-dlp/security/advisories/GHSA-hjq6-52gw-2g7p)\n    - The shell escape function now properly escapes `%`, `\\` and `\\n`.\n    - `utils.Popen` has been patched accordingly."
     }
 ]

test/test_utils.py

@@ -2069,6 +2069,10 @@ Line 1
 
         # Test escaping
         assert run_shell(['echo', 'test"&']) == '"test""&"\n'
+        assert run_shell(['echo', '%CMDCMDLINE:~-1%&']) == '"%CMDCMDLINE:~-1%&"\n'
+        assert run_shell(['echo', 'a\nb']) == '"a"\n"b"\n'
+        assert run_shell(['echo', '"']) == '""""\n'
+        assert run_shell(['echo', '\\']) == '\\\n'
         # Test if delayed expansion is disabled
         assert run_shell(['echo', '^!']) == '"^!"\n'
         assert run_shell('echo "^!"') == '"^!"\n'

yt_dlp/YoutubeDL.py

@@ -25,7 +25,7 @@ import unicodedata
 
 from .cache import Cache
 from .compat import functools, urllib  # isort: split
-from .compat import compat_os_name, compat_shlex_quote, urllib_req_to_req
+from .compat import compat_os_name, urllib_req_to_req
 from .cookies import LenientSimpleCookie, load_cookies
 from .downloader import FFmpegFD, get_suitable_downloader, shorten_protocol_name
 from .downloader.rtmp import rtmpdump_version
@@ -102,7 +102,6 @@ from .utils import (
     UserNotLive,
     YoutubeDLError,
     age_restricted,
-    args_to_str,
     bug_reports_message,
     date_from_str,
     deprecation_warning,
@@ -141,6 +140,7 @@ from .utils import (
     sanitize_filename,
     sanitize_path,
     sanitize_url,
+    shell_quote,
     str_or_none,
     strftime_or_none,
     subtitles_filename,
@@ -823,7 +823,7 @@ class YoutubeDL:
             self.report_warning(
                 'Long argument string detected. '
                 'Use -- to separate parameters and URLs, like this:\n%s' %
-                args_to_str(correct_argv))
+                shell_quote(correct_argv))
 
     def add_info_extractor(self, ie):
         """Add an InfoExtractor object to the end of the list."""
@@ -1355,7 +1355,7 @@ class YoutubeDL:
                 value, fmt = escapeHTML(str(value)), str_fmt
             elif fmt[-1] == 'q':  # quoted
                 value = map(str, variadic(value) if '#' in flags else [value])
-                value, fmt = ' '.join(map(compat_shlex_quote, value)), str_fmt
+                value, fmt = shell_quote(value, shell=True), str_fmt
             elif fmt[-1] == 'B':  # bytes
                 value = f'%{str_fmt}'.encode() % str(value).encode()
                 value, fmt = value.decode('utf-8', 'ignore'), 's'

yt_dlp/compat/__init__.py

@@ -27,12 +27,9 @@ def compat_etree_fromstring(text):
 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.replace('"', '""').join('""')
-else:
-    from shlex import quote as compat_shlex_quote  # noqa: F401
+    from ..utils import shell_quote
+    return shell_quote(s)
 
 
 def compat_ord(c):

yt_dlp/utils/_utils.py

@@ -50,7 +50,6 @@ from ..compat import (
     compat_expanduser,
     compat_HTMLParseError,
     compat_os_name,
-    compat_shlex_quote,
 )
 from ..dependencies import xattr
 
@@ -836,9 +835,11 @@ class Popen(subprocess.Popen):
 
         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)
+                args = shell_quote(args, shell=True)
             shell = False
-            args = f'{self.__comspec()} /Q /S /D /V:OFF /C "{args}"'
+            # Set variable for `cmd.exe` newline escaping (see `utils.shell_quote`)
+            env['='] = '"^\n\n"'
+            args = f'{self.__comspec()} /Q /S /D /V:OFF /E:ON /C "{args}"'
 
         super().__init__(args, *remaining, env=env, shell=shell, **kwargs, startupinfo=self._startupinfo)
 
@@ -1637,15 +1638,38 @@ def get_filesystem_encoding():
     return encoding if encoding is not None else 'utf-8'
 
 
-def shell_quote(args):
-    quoted_args = []
+_WINDOWS_QUOTE_TRANS = str.maketrans({'"': '\\"', '\\': '\\\\'})
+_CMD_QUOTE_TRANS = str.maketrans({
+    # Keep quotes balanced by replacing them with `""` instead of `\\"`
+    '"': '""',
+    # Requires a variable `=` containing `"^\n\n"` (set in `utils.Popen`)
+    # `=` should be unique since variables containing `=` cannot be set using cmd
+    '\n': '%=%',
+    # While we are only required to escape backslashes immediately before quotes,
+    # we instead escape all of 'em anyways to be consistent
+    '\\': '\\\\',
+    # Use zero length variable replacement so `%` doesn't get expanded
+    # `cd` is always set as long as extensions are enabled (`/E:ON` in `utils.Popen`)
+    '%': '%%cd:~,%',
+})
+
+
+def shell_quote(args, *, shell=False):
+    args = list(variadic(args))
+    if any(isinstance(item, bytes) for item in args):
+        deprecation_warning('Passing bytes to utils.shell_quote is deprecated')
         encoding = get_filesystem_encoding()
-    for a in args:
-        if isinstance(a, bytes):
-            # We may get a filename encoded with 'encodeFilename'
-            a = a.decode(encoding)
-        quoted_args.append(compat_shlex_quote(a))
-    return ' '.join(quoted_args)
+        for index, item in enumerate(args):
+            if isinstance(item, bytes):
+                args[index] = item.decode(encoding)
+
+    if compat_os_name != 'nt':
+        return shlex.join(args)
+
+    trans = _CMD_QUOTE_TRANS if shell else _WINDOWS_QUOTE_TRANS
+    return ' '.join(
+        s if re.fullmatch(r'[\w#$*\-+./:?@\\]+', s, re.ASCII) else s.translate(trans).join('""')
+        for s in args)
 
 
 def smuggle_url(url, data):
@@ -2849,7 +2873,7 @@ def ytdl_is_updateable():
 
 def args_to_str(args):
     # Get a short string representation for a subprocess command
-    return ' '.join(compat_shlex_quote(a) for a in args)
+    return shell_quote(args)
 
 
 def error_to_str(err):