youtube-dl vulnerable to file system modification and RCE through improper file-extension sanitization

Summary

#### Description This advisory follows the security advisory [GHSA-79w7-vh3h-8g4j published by the _yt-dlp/yt-dlp_ project](https://github.com/yt-dlp/yt-dlp/security/advisories/GHSA-79w7-vh3h-8g4j) to aid remediation of the issue in the _ytdl-org/youtube-dl_ project.

### Vulnerability _youtube-dl_ 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).

### Impact Since _youtube-dl_ also reads config from the working directory (and, on Windows, executables will be executed from the _youtube-dl_ directory by default) the vulnerability could allow the unwanted execution of local code, including downloads masquerading as, eg, subtitles.

### Patches The versions of _youtube-dl_ listed as _Patched_ remediate this vulnerability by disallowing path separators and whitelisting allowed extensions. As a result, some very uncommon extensions might not get downloaded.

**Master code d42a222 or later and nightly builds tagged 2024-07-03 or later** contain the remediation.

### Workarounds Any/all of the below considerations may limit exposure in case it is necessary to use a vulnerable version * have `.%(ext)s` at the end of the output template * download from websites that you trust * do not download to a directory within the executable search `PATH` or other sensitive locations, such as your user directory or system directories * in Windows versions that support it, set [`NoDefaultCurrentDirectoryInExePath`](https://stackoverflow.com/a/50118548) to prevent the _cmd_ shell's executable search adding the default directory before `PATH` * consider that the path traversal vulnerability as a result of resolving `non_existent_dir\..\..\target` does not exist in Linux or macOS * ensure the extension of the media to download is a common video/audio/... one (use `--get-filename`) * omit any of the subtitle options (`--write-subs`/` --write-srt`, `--write-auto-subs`/`--write-automatic-subs`, `--all-subs`).

### References * [GHSA-79w7-vh3h-8g4j](https://github.com/yt-dlp/yt-dlp/security/advisories/GHSA-79w7-vh3h-8g4j) * https://github.com/ytdl-org/youtube-dl/pull/32830