Re: [FFmpeg-devel] [PATCH v10 1/2] avformat/imf: Demuxer

From: Anton Khirnov <anton@khirnov.net>
To: FFmpeg development discussions and patches <ffmpeg-devel@ffmpeg.org>
Subject: Re: [FFmpeg-devel] [PATCH v10 1/2] avformat/imf: Demuxer
Date: Fri, 17 Dec 2021 15:25:41 +0100
Message-ID: <163975114109.13029.10142418833883066194@lain.red.khirnov.net> (raw)
In-Reply-To: <CAF_7JxC7uLTL53bn7OA9G7H8brGAmPs-F89o4kz4KOBmMhh4CA@mail.gmail.com>

Quoting Pierre-Anthony Lemieux (2021-12-15 21:41:25)
> On Wed, Dec 15, 2021 at 12:20 PM Anton Khirnov <anton@khirnov.net> wrote:
> >
> > Quoting Pierre-Anthony Lemieux (2021-12-15 01:17:26)
> > > >
> > > > Now the question is whether a malicious attacker can craft those two
> > > > files to get access to anything they shouldn't. I suppose at the very
> > > > least the attacker can get information that the user opened the file (by
> > > > adding an asset on an attacker's server) but that will be a danger with
> > > > any playlists allowing network resources and can be controlled with
> > > > io_open(). Can you think of any other possible issues?
> > > >
> > >
> > > Some security considerations:
> > >
> > > - a DDoS can conceivably occur if a malicious CPL+ASSETMAP is widely
> > > distributed. Both an ASSETMAP and a CPL are required since (a) the CPL
> > > does not contain paths/hyperlinks and (b) only those resources
> > > referenced by the CPL are fetched using the ASSETMAP.
> > > - the CPL uses XML, which has its own security considerations. For
> > > example, XML parsing can result in entities being fetched over the
> > > network, but this is disabled by default in libxml AFAIK.
> >
> > This is concerning. From a brief glance at libxml2, it seems that you
> > need to pass XML_PARSE_NONET as the last parameter to xmlReadMemory() to
> > actually disabling network fetching.
> > But it is possible I'm misreading the code, so if you or anyone else
> > understands this better then clarifications are welcome.
> 
> I was referring to entity expansion and the loading of DTDs being
> disabled by default -- see XML_PARSE_NOENT and XML_PARSE_DTDLOAD at
> [1-2].

Okay then. If nobody has further comments, I will push your latest patch
in a few days.

-- 
Anton Khirnov
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