yt-dlp File system modification and RCE through improper file-extension sanitization

Summary

yt-dlp does not limit the extensions of downloaded files, which could lead to arbitrary filenames being created in the download folder (and path traversal on Windows). Since yt-dlp also reads config from the working directory (and on Windows executables will be executed from the yt-dlp directory) this could lead to arbitrary code being executed.

Patches

yt-dlp version 2024.07.01 fixes this issue by whitelisting the allowed extensions.
This means some very uncommon extensions might not get downloaded; however, it will also limit the possible exploitation surface.

Workarounds

It is recommended to upgrade yt-dlp to version 2024.07.01 as soon as possible, always have .%(ext)s at the end of the output template, and make sure you trust the websites that you are downloading from. Also, make sure to never download to a directory within PATH or other sensitive locations like your user directory, system32, or other binaries locations.

For users not able to upgrade:

• Make sure the extension of the media to download is a common video/audio/sub/… one
• Try to avoid the generic extractor (--ies default,-generic)
• Keep the default output template (-o "%(title)s [%(id)s].%(ext)s)
• Omit any of the subtitle options (--write-subs, --write-auto-subs, --all-subs, --write-srt)
• Use --ignore-config --config-location ... to not load config from common locations

Details

One potential exploitation might look like this:

From a mimetype we do not know, we default to trimming the leading bit and using the remainder. Given a webpage that contains

<script type="application/ld+json">
{
    "@context": "https://schema.org",
    "@type": "VideoObject",
    "name": "ffmpeg",
    "encodingFormat": "video/exe",
    "contentUrl": "https://example.com/video.mp4"
}
</script>

this will try and download a file called ffmpeg.exe (-o "%(title)s.%(ext)s).
ffmpeg.exe will be searched for in the current directory, and so upon the next run arbitrary code can be executed.

Alternatively, when engineering a file called yt-dlp.conf to be created, the config file could contain --exec ... and so would also execute arbitrary code.

Acknowledgement

A big thanks to @JarLob for independently finding a new application of the same underlying issue.
More can be read about on the dedicated GitHub Security Lab disclosure here: Path traversal saving subtitles (GHSL-2024-090)

References

• https://github.com/yt-dlp/yt-dlp/security/advisories/GHSA-79w7-vh3h-8g4j
• https://nvd.nist.gov/vuln/detail/CVE-2024-38519
• https://github.com/yt-dlp/yt-dlp/releases/tag/2024.07.01
• https://github.com/yt-dlp/yt-dlp/commit/5ce582448ececb8d9c30c8c31f58330090ced03a
• https://securitylab.github.com/advisories/GHSL-2024-090_yt-dlp