Re: Assessing Your Malware Exposure with Snort

Thanks for the clarification Brian.

I had started to combine these rules into multi-condition pcre. Making 
one rule out of 30 or 40 original single domain rules. And I can't 
remember where the heck I put them now or I'd link to them for you.

So my question is, will this be more efficient in a single comdin match 
per rule, or multiple?

Matt

Brian wrote:
>>I have written a few thousand Snort rules that are intended 
>>to detect successful HTTP communication with hosts known to 
>>be evil. They look for domain names in the Host string so 
>>they are not subject to evasion by changing IP addresses.
>>
>>If you would like to give them a try you can grab them from 
>>http://www.kgb.to/malware.html .
> 
> 
> Interesting.
> 
> The meat of one of your rules is:
>     flow:established;
>     pcre:"/(Host\:)\s[a-zA-Z0-9.-]+(\.whenyousearch.com\r\n)/";
> 
> Your rules are going to be slow.  Here are the few issues I see so
> far:
> 
>     * Your regular expression is matching when you don't need to, causing
>       pcre to do un-needed work. 
>     * By not using content, you are not making use of the multi-pattern 
>       match engine in Snort. 
>     * Since all of the rules I looked at are looking for traffic going
>       towards the web server, you should limit your flow to
>       "to_server" as well.
> 
> My tweaked "for speed" meat of the rule version is:
> 
>     flow:established,to_server; content:"whenyousearch.com"; nocase;
>     pcre:"/^Host\x3a\s*[a-z0-0\.-]+\.whenyousearch.com/smi";
> 
> Brian Caswell
> _______________________________________________
> Intrusions mailing list
> [email protected]
> http://www.dshield.org/mailman/listinfo/intrusions

-- 
--------------------------------------------
Matthew Jonkman, CISSP
Senior Security Engineer
Infotex
765-429-0398 Direct Anytime
765-448-6847 Office
866-679-5177 24x7 NOC
my.infotex.com
www.offsitefilter.com
www.bleedingsnort.com
--------------------------------------------


NOTICE: The information contained in this email is confidential
and intended solely for the intended recipient. Any use,
distribution, transmittal or retransmittal of information
contained in this email by persons who are not intended
recipients may be a violation of law and is strictly prohibited.
If you are not the intended recipient, please contact the sender
and delete all copies.
_______________________________________________
Intrusions mailing list
[email protected]
http://www.dshield.org/mailman/listinfo/intrusions