PYSEC-2026-3433

Summary

If aria2c is used as an external downloader for a fragmented manifest format (such as an HLS/DASH stream), yt-dlp passes insufficiently sanitized input to aria2c that allows an attacker to perform an arbitrary file write. On Windows platforms, this can lead to immediate arbitrary code execution. On non-Windows platforms, this can lead to arbitrary code execution upon the next invocation of yt-dlp.

Details

When downloading a fragmented manifest format such as an HLS or DASH stream, yt-dlp first extracts a list of all fragment URLs from the stream's manifest. If the user has selected aria2c as an external downloader, yt-dlp then constructs an input file for aria2c from the fragment URL list and passes its filepath as the argument to aria2c's -i option.

aria2c's -i (or --input-file) option allows for downloading a list of URIs from the given text file. The text file must be formatted as a list of URIs separated by newlines. aria2c's format permits configuration lines for each URI, which can contain command-line options to be given to aria2c. These optional lines follow each URI line and are signified only by leading whitespace. yt-dlp constructs the input file with these optional lines so that it's able to specify the output filename for each fragment using the out= option.

yt-dlp's utilization of the aria2c input file format presents two known attack vectors:

1. An attacker can craft a malicious DASH manifest with one or more fragment URLs that contain 
, which is the HTML escape sequence for a newline. yt-dlp interprets this escape sequence as an actual newline character when writing the fragment URLs to the aria2c input file, which allows for an attacker to inject arbitrary aria2c options into the input file. With option injection, the attacker can achieve arbitrary file writes. This attack vector is possible only via downloads of DASH formats, since their manifests are an XML format which necessitates unescaping of HTML special...