[FFmpeg-devel] [PATCH] avformat/hls: Disallow local file access by default

Michael Niedermayer michael at niedermayer.cc
Wed May 31 03:09:31 EEST 2017

On Wed, May 31, 2017 at 01:14:58AM +0200, Hendrik Leppkes wrote:
> On Wed, May 31, 2017 at 12:52 AM, Michael Niedermayer
> <michael at niedermayer.cc> wrote:
> > This prevents an exploit leading to an information leak
> >
> > 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.
> >
> > Found-by: Emil Lerner and Pavel Cheremushkin
> > Reported-by: Thierry Foucu <tfoucu at google.com>
> >
> >

> I don't particularly like this. Being able to dump a HLS stream (ie.
> all its file) onto disk and simply open it again is a good thing.
> Maybe it should just be smarter and only allow using the same protocol
> for the segments then it already used for the m3u8 file, so that a
> local m3u8 allows opening a local file (plus http(s), in case I only
> saved the playlist), but a http HLS playlist only allows http
> segments?

we already prevent every protocol except file and crypto for local
hls files. We also already block http* in local hls files by default
thorugh default whitelists (file,crypto for local files)

This is not sufficient, the exploit there is successfully puts the
content of a readable file choosen by the attacker into the output
video, which if its given back to the attacker leaks this information.

Michael     GnuPG fingerprint: 9FF2128B147EF6730BADF133611EC787040B0FAB

The worst form of inequality is to try to make unequal things equal.
-- Aristotle
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