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
```html
<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)](<
https://securitylab.github.com/advisories/GHSL-2024-090_yt-dlp>)
### 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