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Network Security Snort-Signatures
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Re: [Snort-sigs] PNG vulnerabilities and more

from [Joe Stewart] Network Security [Permanent Link]
Subject: Re: [Snort-sigs] PNG vulnerabilities and more
Date: Thu, 19 Aug 2004 17:28:12 -0400 
On Thursday 19 August 2004 4:10 pm, Joseph Gama wrote:

I can see that you based your rule on the source code
with the else if vulnerability. I based mine on the
code and comments from the exploit which is why you
look for PLTE while I look for the 0x03 because:

* to 0x03, byte 10 of the IHDR data. that signfies
that a PALLETE chunk should
 * be present. but we dont have one, and that is how
the len check is bypassed.

I tested my rule and it detects pngtest_bad.png, plus
I browsed through hundreds of PNG's and got no false
positives. I haven't tested yours, I didn't see it
before.


Based on that it sounds like yours will false-positive on many 
transparent PNGs, since you're checking to see if the PLTE is required, 
but not checking to see if it is actually missing. Attached is a normal 
transparent PNG that gives a false positive on your sig in my tests.

-Joe

-- 
Joe Stewart, GCIH 
Senior Security Researcher
LURHQ http://www.lurhq.com/

Attachment: test-transparent.png
Description: PNG image

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