CVE-2025-43858

YoutubeDLSharp is a wrapper for the command-line video downloaders youtube-dl and yt-dlp. In versions starting from 1.0.0-beta4 and prior to 1.1.2, an unsafe conversion of arguments allows the injection of a malicious commands when starting yt-dlp from a commands prompt running on Windows OS with the UseWindowsEncodingWorkaround value defined to true (default behavior). If a user is using built-in methods from the YoutubeDL.cs file, the value is true by default and a user cannot disable it from these methods. This issue has been patched in version 1.1.2.

YoutubeDLSharp is a wrapper for the command-line video downloaders youtube-dl and yt-dlp. In versions starting from 1.0.0-beta4 and prior to 1.1.2, an unsafe conversion of arguments allows the injection of a malicious commands when starting yt-dlp from a commands prompt running on Windows OS with the UseWindowsEncodingWorkaround value defined to true (default behavior). If a user is using built-in methods from the YoutubeDL.cs file, the value is true by default and a user cannot disable it from these methods. This issue has been patched in version 1.1.2.

Summary

This vulnerability only apply when running on a Windows OS. An unsafe conversion of arguments allows the injection of a malicous commands when starting yt-dlp from a commands prompt.

[!CAUTION] NOTE THAT DEPENDING ON THE CONTEXT AND WHERE THE LIBRARY IS USED, THIS MAY HAVE MORE SEVERE CONSEQUENCES. FOR EXAMPLE, A USER USING THE LIBRARY LOCALLY IS A LOT LESS VULNERABLE THAN AN ASP.NET APPLICATION ACCEPTING INPUTS FROM A NETWORK/INTERNET.

Details

The vulnerability have been implemented in a commit (https://github.com/Bluegrams/YoutubeDLSharp/commit/fdf3256da18d0e2da4a2f33ad4a1b72ff8273a50) 3 year ago to fix a issue with unicode characters on Windows. ( In the latest version at the time of writing this, the code seems to have moved here : https://github.com/Bluegrams/YoutubeDLSharp/blob/b2f7968a2ef06a9c7b2c212785cfeac0b187b6d8/YoutubeDLSharp/YoutubeDLProcess.cs#L87 ) In this commit, a new way of starting yt-dlp was implemented, method that was defined as the default behaviour.

When the internal method ConvertToArgs get called, the application will test multiples conditions to decide on how the yt-dlp application should be started. The condition we are interesed in, as well a the default one on Windows, is at line 99 . Inside the if statement, we can see that insead of directly calling the yt-dlp binary, a command prompt is opened to run yt-dlp.

The problem arises when you realize that both arguments in the ConvertToArgs method may be provided by an untrusted client. Since the documentation of YoutubeDLSharp does not warn developers about this behavior, they might assume that the library handles this safely by ensuring that the arguments are secure to run inside a command prompt. Instead, the two potentially malicious arguments are directly appended to the command string without any sanitization (see line 104 and 107).

PoC

For this example, I'm going to use the version 1.1.1 and a method inside YoutubeDL.cs. Assuming you are running on a Windows OS, this method will by default use a CMD to open yt-dlp.

using YoutubeDLSharp;

public async Task<RunResult<VideoData>> GetMediaInformation()
{
        YoutubeDL youtubeDl = new YoutubeDL();
	// Fetch media information using a badly crafted "url" (escaped)
	return await youtubeDl.RunVideoDataFetch("https://example.com/\" & start calc.exe");
}

At the call of GetMediaInformation, the method RunVideoDataFetch will be called, internally this method will call the vulnerable method [ConvertToArgs] resulting in the following string:

/C chcp 65001 >nul 2>&1 && "yt-dlp.exe"  --external-downloader "m3u8:native" --external-downloader-args "ffmpeg:-nostats -loglevel 0" -o "C:\Users\<hidden>\Documents\GitHub\<hidden>\<hidden>\bin\Release\net8.0\%(title)s [%(id)s]_%(epoch)s.%(ext)s" --force-overwrites --no-part -i --ignore-config --ffmpeg-location "ffmpeg.exe" --exec "echo outfile: {}" -- "https://example.com/" & start calc.exe"

[!NOTE] Some text have been replaced by <hidden> inside the command.

The important part here is at the end of the command, we can see "https://example.com/" & start calc.exe", if we compare it with our …

YoutubeDLSharp is a wrapper for the command-line video downloaders youtube-dl and yt-dlp. In versions starting from 1.0.0-beta4 and prior to 1.1.2, an unsafe conversion of arguments allows the injection of a malicious commands when starting yt-dlp from a commands prompt running on Windows OS with the UseWindowsEncodingWorkaround value defined to true (default behavior). If a user is using built-in methods from the YoutubeDL.cs file, the value is true by default and a user cannot disable it from these methods. This issue has been patched in version 1.1.2.

YoutubeDLSharp is a wrapper for the command-line video downloaders youtube-dl and yt-dlp. In versions starting from 1.0.0-beta4 and prior to 1.1.2, an unsafe conversion of arguments allows the injection of a malicious commands when starting yt-dlp from a commands prompt running on Windows OS with the UseWindowsEncodingWorkaround value defined to true (default behavior). If a user is using built-in methods from the YoutubeDL.cs file, the value is true by default and a user cannot disable it from these methods. This issue has been patched in version 1.1.2.

Summary

This vulnerability only apply when running on a Windows OS. An unsafe conversion of arguments allows the injection of a malicous commands when starting yt-dlp from a commands prompt.

[!CAUTION] NOTE THAT DEPENDING ON THE CONTEXT AND WHERE THE LIBRARY IS USED, THIS MAY HAVE MORE SEVERE CONSEQUENCES. FOR EXAMPLE, A USER USING THE LIBRARY LOCALLY IS A LOT LESS VULNERABLE THAN AN ASP.NET APPLICATION ACCEPTING INPUTS FROM A NETWORK/INTERNET.

Details

The vulnerability have been implemented in a commit (https://github.com/Bluegrams/YoutubeDLSharp/commit/fdf3256da18d0e2da4a2f33ad4a1b72ff8273a50) 3 year ago to fix a issue with unicode characters on Windows. ( In the latest version at the time of writing this, the code seems to have moved here : https://github.com/Bluegrams/YoutubeDLSharp/blob/b2f7968a2ef06a9c7b2c212785cfeac0b187b6d8/YoutubeDLSharp/YoutubeDLProcess.cs#L87 ) In this commit, a new way of starting yt-dlp was implemented, method that was defined as the default behaviour.

When the internal method ConvertToArgs get called, the application will test multiples conditions to decide on how the yt-dlp application should be started. The condition we are interesed in, as well a the default one on Windows, is at line 99 . Inside the if statement, we can see that insead of directly calling the yt-dlp binary, a command prompt is opened to run yt-dlp.

The problem arises when you realize that both arguments in the ConvertToArgs method may be provided by an untrusted client. Since the documentation of YoutubeDLSharp does not warn developers about this behavior, they might assume that the library handles this safely by ensuring that the arguments are secure to run inside a command prompt. Instead, the two potentially malicious arguments are directly appended to the command string without any sanitization (see line 104 and 107).

PoC

For this example, I'm going to use the version 1.1.1 and a method inside YoutubeDL.cs. Assuming you are running on a Windows OS, this method will by default use a CMD to open yt-dlp.

using YoutubeDLSharp;

public async Task<RunResult<VideoData>> GetMediaInformation()
{
        YoutubeDL youtubeDl = new YoutubeDL();
	// Fetch media information using a badly crafted "url" (escaped)
	return await youtubeDl.RunVideoDataFetch("https://example.com/\" & start calc.exe");
}

At the call of GetMediaInformation, the method RunVideoDataFetch will be called, internally this method will call the vulnerable method [ConvertToArgs] resulting in the following string:

/C chcp 65001 >nul 2>&1 && "yt-dlp.exe"  --external-downloader "m3u8:native" --external-downloader-args "ffmpeg:-nostats -loglevel 0" -o "C:\Users\<hidden>\Documents\GitHub\<hidden>\<hidden>\bin\Release\net8.0\%(title)s [%(id)s]_%(epoch)s.%(ext)s" --force-overwrites --no-part -i --ignore-config --ffmpeg-location "ffmpeg.exe" --exec "echo outfile: {}" -- "https://example.com/" & start calc.exe"

[!NOTE] Some text have been replaced by <hidden> inside the command.

The important part here is at the end of the command, we can see "https://example.com/" & start calc.exe", if we compare it with our …