Re: Intrusion Detection Evaluation Datasets

Ok, I will reply to both using this message.

> On Mar 18, 2009, at 2:31 PM, Paul Schmehl wrote:
>> I don't know if any IDS could do this. You'd have to capture the value
>> of Content-Length, insert that value into a variable, then compare
>> that variable against the number of bytes of a single value, all while
>> examining the same packet.

Ok...so, it's not easy to catch any attack variation right? :)
Btw, Snort did detect one attack instance, because a signature for IIS 
has something like 100 times the same byte value in it.
The problem is not only in catching the content length and storing it 
somewhere, for later comparison...it would be already difficult to 
detect that the same byte value is repeated over and over (and everytime 
is different). Why? Because Snort (and in general any other 
signature-based IDS) use regular expressions...in a regular expression 
you can only state that an expression must not occur at all, could 
occur, can occur once or more, or it can occur a number of times (but 
you cannot say how many times exactly).
That's what I referred to when I said that one should rewrite the 
regular expression engine. It's "easy" to match a regular expression (by 
building the equivalent finite automaton), but it's more difficult to 
validate expressions that contain the same value over and over. You can 
read the following Wikipedia's article:

http://en.wikipedia.org/wiki/Formal_grammar

> It would actually be easy to identify with Bro. The problem with your
> signature below is that it doesn't take into account the same byte value
> being repeated for the total Content-Length. It's a little more hacky to
> make Bro identify the repeating character, but still possible. You're
> also ignoring the bounds Damiano placed on the value of the
> Content-Length header. If I have some time tonight, I'll write a script
> to detect this situation and post it to the list.

I have to admit I have never looked at Bro signatures, although I know 
it approaches the problem differently. So, I'm really curious. :)

Cheers

-- 
Damiano Bolzoni

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