full-disclosure - Re: [Security Advisory] Backdoor Discovered inImmunity Debugger

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Date: Fri, 10 Aug 2007 18:53:31 +0200
From: monikerd <monikerd@...il.com>
To: Valdis.Kletnieks@...edu
Cc: full-disclosure@...ts.grok.org.uk
Subject: Re: [Security Advisory] Backdoor Discovered
 inImmunity Debugger

Valdis.Kletnieks@...edu wrote:
> On Fri, 10 Aug 2007 09:25:11 +0200, monikerd said:
>
>   
>> However off course the md5 hashes don't really
>> say anything. Except that you would be able to
>> verify that your copy is the one you obtained.
>>     
>
> If you are unsure if your copy is the one you obtained, you have bigger
> issues. ;)
>
> An MD5 hash *will* however verify (to a fair extent, modulo intended
> attacks) if the copy you have is identical to the copy that was originally
> shipped (if they don't match, it shows that the copy you obtained isn't
> the one originally shipped).
>
> Yes, I'm splitting hairs here - but in cryptography, such hairs are often
> the basis of the security.  So it's important to keep straight which bits
> are which....
>   
clearly that was a typo, which originated from restructuring the
sentence a few
times. the point is that md5 *is* broken, when used in this way. You can
not
verify that your copy is the one that was _originally_ shipped. You
can't easily
make a file with the same hash as a existing file. However you *can*
manipulate
and .exe into 2 executables that are different but hash to the same. One
could be shipped
to almost everybody, and be verified to be clean. The server could
however serve up the evil .exe to certain IP addresses, and if they actually
did spend time to check the MD5 it wouldn't make them any wiser

Basically this is a slightly far fetched scenario. Still it's not
impossible, an hence
we should use other algorithms for this task. Or use several hashes. Md5
is broken
in this way.


I'm sorry I keep repeating myself, and i promise i will not do it again.

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