Improper Neutralization of Special Elements used in an OS Command ('OS Command Injection')

Summary

yt-dlp is a youtube-dl fork with additional features and fixes. yt-dlp allows the user to provide shell command lines to be executed at various stages in its download steps through the "--exec" flag. This flag allows output template expansion in its argument, so that metadata values may be used in the shell commands. The metadata fields can be combined with the "%q" conversion, which is intended to quote/escape these values so they can be safely passed to the shell. However, the escaping used for "cmd" (the shell used by Python's "subprocess" on Windows) does not properly escape special characters, which can allow for remote code execution if "--exec" is used directly with maliciously crafted remote data. This vulnerability only impacts "yt-dlp" on Windows, and the vulnerability is present regardless of whether "yt-dlp" is run from "cmd" or from "PowerShell". Support for output template expansion in "--exec", along with this vulnerable behavior, was added to "yt-dlp" in versions 2021.04.11 prior to 2023.09.24. This issue was fixed by properly escaping each special character. "\n" will be replaced by "\r" as no way of escaping it has been found. Also, always be careful when using "--exec", because while this specific vulnerability has been patched, using unvalidated input in shell commands is inherently dangerous. For Windows users who are not able to upgrade: 1. Avoid using any output template expansion in --exec other than {} (filepath). 2. If expansion in --exec is needed, verify the fields you are using do not contain ", | or &. 3. Instead of using "--exec", write the info json and load the fields from it instead.