[FFmpeg-devel] [PATCH] avformat/hls: Check file extensions

[Hendrik Leppkes]

On Fri, Jun 2, 2017 at 9:19 PM, Michael Niedermayer
<michael at niedermayer.cc> wrote:
> This reduces the attack surface of local file-system and local network
> information leaking.
>
> It prevents the existing exploit leading to an information leak. As
> well as similar hypothetical attacks.
>
> Leaks of information from files and symlinks ending in common multimedia extensions
> are still possible. But files with sensitive information like private keys and passwords
> generally do not use common multimedia filename extensions.
>
> The existing exploit depends on a specific decoder as well.
> It does appear though that the exploit should be possible with any decoder.
> The problem is that as long as sensitive information gets into the decoder,
> the output of the decoder becomes sensitive as well.
> The only obvious solution is to prevent access to sensitive information. Or to
> disable hls or possibly some of its feature. More complex solutions like
> checking the path to limit access to only subdirectories of the hls path may
> work as an alternative. But such solutions are fragile and tricky to implement
> portably and would not stop every possible attack nor would they work with all
> valid hls files.
>
> Developers have expressed their dislike / objected to disabling hls by default as well
> as disabling hls with local files. This here is a less robust but also lower
> inconvenience solution.
> It can be applied stand alone or together with other solutions.
>

One of the most important things is that at the very least HLS keeps
working out of the box without magic options for actual HTTP HLS
streams, ie. their primary domain.
One aspect of this is that HLS streams hosted by CDNs often don't have
an actual extension, since they get generated by various dynamic URLs,
and this would break that, so its not a good idea.

No matter what security fixes get applied, if HLS basically stops
working its basic functionality, then its going too far.

- Hendrik