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Internet Explorer Help System RCE
From: Mike Allen <mallen7474 () yahoo com>
Date: Thu, 12 May 2005 22:15:55 -0700 (PDT)

Hi, quick question on decoding. A little background
first. Earlier tonight, one of my co-workers found
what might he thinks might be the first "in the wild"
example of the Internet Explorer/ms-its protocol
vulnerability. (Discussed at
gossamer-threads.com/lists/fulldisc/full-disclosure/30100.)
The exploit itself is based on a Proof-of-Concept by
ShredderSub7. This is another unpatched flaw that has
been known since at least December. This particular
exploit attempts to download a phony JPEG file from
iframedollars.biz/dl/adv622/JQTmudI.jpg (the SubSeven
PoC downloads a harmless htm.txt file instead). This
not a JPEG, but, rather, is a BASE64-encoded PHP
script. There is a decoding script at the end, but I
cannot make heads nor tails of what this decodes to. I
have very little PHP experience, but it seems to take
a character out of an encoded string, subtract that
from 127, and then convert it to Unicode. Here is the
decoding script:

for(i=0;i<568;i++)s=s.substr(1)+String.fromCharCode(127-s.charCodeAt(0));document.write(s);

I wrote a small decoder in assembly to perform what I
think this thing does, but all I got was the original
code. Am I missing something here? Is this really an
obfuscation script or am I chasing ghosts?

It's be happy to provide the munged data itself, if I
can cut 'n' paste it, but I'm basically interested in
knowing whether I am even on the right track.

Mike


                
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