Re: eSafe: Could this be exploited?

["Oliver () greyhat de" <Oliver () greyhat de>]

Hugo van der Kooij wrote:

> Hi,
>
> I had a bit of a chat with Aladdin support regarding the odd results I had
> with their network virusscanner (aka: eSafe). (see also:
> http://www.ealaddin.com/esafe/default.asp)
>
> Both as NitroEngine or CVP server they will push as much of 80% to the
> end-user before they stop a virus. Then they rely on the adding of the
> exact URL so that URL can be blocked in all next requests.
>
> If it is a first time hit you can get as much as 80% of the payload on
> your machine and while they may reset the tcp stream at least IE does
> store the 80% chunk as if the file was transfered correctly. (This part I
> tested with over 30 different virus files.)
>
> First off this is extremely confusing to the user who just thinks (s)he
> just had a virus passing their scanner. (And they are about 80% right.)
>
> Then the chunk may contain enough to trigger another scanner which may
> reside on the desktop of said user adding further to the belief this is
> not a good product.
>
> But what if I were to write a really small harmfull virus (say less then 2
> ethernet packets)? Or create it in such way that the last 20 to 25% is
> expendible without loosing it's sting?
>
> Is someone able to verify such a virus may work? (I am not a programmer so
> I can think of the potential breach but I can't verify it is exploitable.)
>
> I have a felling it is just a matter of time before such a scanner will be
> bypassed.
>
> Hugo.
>
>  
Hi, i saw this "feature" already on other vendors AV-proxes, where this 
80% thing is a side effect of http-comforting of the proxy-software.
Comforting is, that the http-client is not running into an timeout.
I think it is possible to generate an exefile, and attach some random 
data. Whereby the exe file is about 80% and the random data 20%.
And also i think, the problem is, that the AV does not exactly stop at 
80%. So you have to generate multiple "infected" files with 80/20, 
81/19, 82/18 and so on.
In Addition  you have to test if for example a scripting host file or a 
binary is still executable, if the last
few bytes at the end are garbage. AFAIK does the PE-header on windows 
.exe files also include a checksum/lof of the file..... if i remember 
right, this checksum is not utilizised by
Win95/Win98, but by W2K/NT- Windows OS. So, there are many circumstances 
to take care for, but i think it is possible in some cases..... lets try 
it :)

ok, just some ideas late at night.... :)