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Date: Tue, 21 Aug 2018 11:21:15 -0400
From: Alex Gaynor <alex.gaynor@...il.com>
To: oss-security@...ts.openwall.com
Subject: Re: Re: More Ghostscript Issues: Should we disable PS
 coders in policy.xml by default?

A small note. Both ImageMagick and GraphicsMagick process various file
formats that can nest a different image file inside of them. These are very
frequently implemented with a call to ReadImage(), with no checking that
it's the expected file format. (As a result, the fuzzer finds various
impressive chains, with sometimes 3 different image formats nested inside
of each other).

The conclusion of this is that people _must not_ attempt to do their own
format detection and then pass the data to IM/GM, because this can be
bypassed with nested formats. It's imperative that GS truly be disabled
with either policy.xml or by uninstall GS.

Alex

On Tue, Aug 21, 2018 at 11:01 AM Bob Friesenhahn <
bfriesen@...ple.dallas.tx.us> wrote:

> On Tue, 21 Aug 2018, Tavis Ormandy wrote:
> >
> > I think those thumbnails should be disabled, but you've probably noticed
> I
> > think everything related to untrusted ghostscript should be disabled :-)
>
> I have posted to the GraphicsMagick Announcements mailing list
> regarding your findings (with a link to this list) and suggested that
> a fool-proof solution is that Ghostscript should be uninstalled.
>
> Uninstalling Ghostscript entirely might cause software using libgs to
> not execute at all unless a stub library is put in its place.
>
> Dependencies on Ghostscript are much larger than one would initially
> think due to Postscript being the traditional output from Unix
> software for "printing" and thus it is used as an intermediate format
> in order to convert between formats.  EPS content is also embedded in
> some other formats.
>
> Bob
> --
> Bob Friesenhahn
> bfriesen@...ple.dallas.tx.us, http://www.simplesystems.org/users/bfriesen/
> GraphicsMagick Maintainer,    http://www.GraphicsMagick.org/
>


-- 
All that is necessary for evil to succeed is for good people to do nothing.

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