Impact
yt-dlp allows the user to provide shell commands to be executed at various stages in its download process through the --exec flag. This flag allows output template expansion in its argument, so that video 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) did 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 version 2021.04.11.
> yt-dlp https://youtu.be/Jo66yyCpHcQ --exec "echo %(title)q"
[youtube] Extracting URL: https://youtu.be/Jo66yyCpHcQ
[youtube] Jo66yyCpHcQ: Downloading webpage
[youtube] Jo66yyCpHcQ: Downloading ios player API JSON
[youtube] Jo66yyCpHcQ: Downloading android player API JSON
[youtube] Jo66yyCpHcQ: Downloading m3u8 information
[info] Jo66yyCpHcQ: Downloading 1 format(s): 135+251
[download] Destination: ＂&echo(&echo(pwned&rem( [Jo66yyCpHcQ].f135.mp4
[download] 100% of 4.85KiB in 00:00:00 at 60.20KiB/s
[download] Destination: ＂&echo(&echo(pwned&rem( [Jo66yyCpHcQ].f251.webm
[download] 100% of 4.80KiB in 00:00:00 at 31.58KiB/s
[Merger] Merging formats into "＂&echo(&echo(pwned&rem( [Jo66yyCpHcQ].mkv"
Deleting original file ＂&echo(&echo(pwned&rem( [Jo66yyCpHcQ].f135.mp4 (pass -k to keep)
Deleting original file ＂&echo(&echo(pwned&rem( [Jo66yyCpHcQ].f251.webm (pass -k to keep)
[Exec] Executing command: echo "\"&echo(&echo(pwned&rem("
"\"
pwnedPatches
yt-dlp version 2023.09.24 fixes this issue by properly escaping each special character.
\n will be replaced by \r, as no way of escaping it has been found.
Workarounds
It is recommended to upgrade yt-dlp to version 2023.09.24 as soon as possible. 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:
- Avoid using any output template expansion in
--exec other than {} (filepath).
- If expansion in
--exec is needed, verify the fields you are using do not contain ", | or &.
- Instead of using
--exec, write the info json and load the fields from it instead.
References
Impact
yt-dlpallows the user to provide shell commands to be executed at various stages in its download process through the--execflag. This flag allows output template expansion in its argument, so that video metadata values may be used in the shell commands. The metadata fields can be combined with the%qconversion, 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'ssubprocesson Windows) did not properly escape special characters, which can allow for remote code execution if--execis used directly with maliciously crafted remote data. This vulnerability only impactsyt-dlpon Windows, and the vulnerability is present regardless of whetheryt-dlpis run fromcmdor fromPowerShell.Support for output template expansion in
--exec, along with this vulnerable behavior, was added toyt-dlpin version 2021.04.11.Patches
yt-dlp version 2023.09.24 fixes this issue by properly escaping each special character.
\nwill be replaced by\r, as no way of escaping it has been found.Workarounds
It is recommended to upgrade yt-dlp to version 2023.09.24 as soon as possible. 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:
--execother than{}(filepath).--execis needed, verify the fields you are using do not contain",|or&.--exec, write the info json and load the fields from it instead.References