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Date: Fri, 15 Jul 2011 10:47:44 +1000
From: Erik de Castro Lopo <erikd@...a-nerd.com>
To: Jan Lieskovsky <jlieskov@...hat.com>
Cc: oss-security@...ts.openwall.com, "Steven M. Christey"
<coley@...us.mitre.org>, Secunia Research <vuln@...unia.com>
Subject: Re: CVE Request -- libsndfile -- Integer overflow by processing
certain PAF files
Please CC me on all mails regarding this bug. I am not on the list
where Dan Rosenberg wrote:
> On Thu, Jul 14, 2011 at 2:49 AM, Erik de Castro Lopo
> <erikd (AT) mega-nerd (DOT) com> wrote:
> > Jan Lieskovsky wrote:
> >
> >> * *an integer overflow, leading to heap-based buffer overflow flaw was
> >> found in the way libsndfile, library for reading and writing of sound
> >> files, processed certain PARIS Audio Format (PAF) audio files with
> >> crafted count of channels in the PAF file header. A remote attacker
> >> could provided a specially-crafted PAF audio file, which once opened by
> >> a local, unsuspecting user in an application, linked against libsndfile,
> >> could lead to that particular application crash (denial of service),
> >
> > I agree with everything up to here.
> >
> >> or, potentially arbitrary code execution with the privileges of the
> >> user running the application.
> >
> > but this is rubbish. The heap gets overwritten with zeros which would
> > certainly lead to the application segfaulting. However, there is
> > no way for arbitrary code to be executed on amy sane OS with proper
> > memory protection.
>
> This is not a sound assumption. Any sort of partially controlled heap
> corruption, even if the data that's being written isn't controllable
> by an attacker, should be considered potentially exploitable. Modern
> heap exploitation is alive and well - it's worth pointing out that a
> recent remote vulnerability in Microsoft IIS FTPD that allowed for a
> heap overflow of strictly 0xff bytes was shown to be exploitable,
> contradicting Microsoft's claims that it could only cause denial of
> service.
The code which caused the heap overflow was this:
memset (ppaf24->samples, 0, ppaf24->samplesperblock * ppaf24->channels) ;
where it was the ppaf24->channels value that was not validated (and
ppaf24->samplesperblock is always 10). In future versions of libsndfile
ppaf24->samplesperblock will be replaced by a compile time constant
value.
That means that the heap is overwritten in blocks that are a multiple
of 10 bytes which makes it significatly more difficult to exploit.
> Think about partially overwriting certain elements of heap
> metadata, or even heap data, with zeroes. Suppose an application with
> heavy function pointer usage was linked against libsndfile, and this
> overflow allowed overwriting the least significant bytes of a function
> pointer with zeroes and ultimately allowed for controlling execution
> flow.
For this instance of heap overflow (overwritten in multiples of 10 bytes
with the base being 4 byte aligned), its only possible to zero the lowest
2 bytes of a function pointer (assuming a little endian machine) if it
happens to lie in exactly the right place.
In terms of ease of exploitation, this one has to be in the very difficult
basket.
> It's better to be safe than sorry.
That's why I rushed out a new release. I do take this seriously, but
I do not like to see the threat exaggerated beyond reason.
Erik
--
----------------------------------------------------------------------
Erik de Castro Lopo
http://www.mega-nerd.com/
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