RE: IDS evaluations procedures

["Nathan Davidson" <ndavidso () globix com>]

I think you are missing the point - by blocking the traffic we need take no further action. If you allow invalid 
traffic into the network you still need to inspect it further to see if it is malicious too!

 

I understand what you are saying about signature accuracy, but it just isn’t relevant. By reducing the number of 
packets that you inspect you can reduce the number of alerts – especially false positives.

 

Example:

 

10,000 PPS (packets per sec) of HTTP traffic flows into a network. 

100 PPS of these are malicious. An inline IPS discards all HTTP packets not containing www.xyz.com as a host header, 
this filters out all non-targeted worm nonsense – lets say 90% of the malicious traffic. 

We are now left with only 10% of the of malicious traffic = 10PPS

 

To make things easier to compare let us say that the IPS and IDS have the SAME signatures/policy and they both identify 
all of the malicious traffic:

 

The IPS will create 10 alerts/sec

 

The IDS will create 100 alerts/sec

 

 

Summary: IPS (in-line) and IDS units with the same signature policy will have the same number of false positives per 
1000 packets inspected. However, by pre-pending a blocking filter the IPS will have fewer packets to inspect and 
therefore create fewer false positive alerts.

 

Bottom line: I don’t care about the false positives that I never see!

 
 

        -----Original Message----- 
        From: Adam Powers [mailto:apowers () lancope com] 
        Sent: Sat 16/07/2005 14:57 
        To: Nathan Davidson; THolman () toplayer com; David.Sames () sparta com; focus-ids () securityfocus com 
        Cc: 
        Subject: Re: IDS evaluations procedures
        
        

        But white-listing or filtering of the kind you speak is available in most
        all commercial and even open source non-inline solutions.
        
        Take snort for instance.
        
        We could use a "pass" action to ignore all HTTP traffic that does NOT have a
        specific host-header or we could use the "activate" action to dynamically
        apply rules to packets that DO have a specific host-header thus satisfying
        the requirement you specify below WITHOUT the need for an inline technology.
        
        Don't get me wrong, I'm not doubting the usefulness of filtering out
        malicious traffic, just the fact that the rate of false positives is somehow
        different when a technology is inline. IMHO, it's more a function of how the
        detection engine is configured than anything else.
        
        
        
        On 7/16/05 7:29 AM, "Nathan Davidson" <ndavidso () globix com> wrote:
        
        > Hi Adam,
        >
        > 
        >
        > I am sure Tim can answer this one very well, but over the last 12 months I
        > have spent a lot of time working with IPS in an IDS orientated company. So I
        > thought I share my experiences.
        >
        > 
        >
        > When we deploy an in-line IPS solution we define a number of parameters in the
        > policy that should be present in ALL valid requests (White-listing). I use
        > this to filter out all traffic that I know must be malicious. From my
        > experience this is up to 95% of worm/scan traffic. We then apply IDS style
        > signatures based on known attack vectors (Black-listing) but only on the
        > remaining 5% of traffic. Thus we should have up to 95% less false positives
        > (and generally we do). Additional benefits can be gained by dropping all
        > subsequent packets from an abusing source IP address.
        >
        > 
        >
        > An example would be to use an IPS to force all HTTP requests to have the host
        > header www.xyz.com (your sites URL) this will stop a significant proportion of
        > HTTP noise before signature matching.
        >
        > 
        >
        > Conversely with IDS you just don¹t have the ability to white list traffic in
        > this way, I guess you could RST any request that didn¹t match the URL but I
        > think fragmented buffer overflows and the like could sneak through - so it¹s
        > risky.
        >
        > 
        >
        > As you alluded to, the IPS signatures tend to be less aggressive than those on
        > the IDS which I think reflects the much higher penalty of false positives on
        > an in-line blocking device. For this reason I do still deploy NIDS/HIDS on the
        > inside to collect forensic data, with the added benefit of having a second
        > manufacturers signatures.
        >
        > 
        >
        > 
        >
        > Internet
        >
        >      I
        >
        >    IPS
        >
        >      I
        >
        > Firewall
        >
        >      I
        >
        >      I
        >
        > Switch=== NIDS
        >
        >      I
        >
        >      I
        >
        > HIDS
        >
        > Server
        >
        > 
        >
        > 
        >
        > Hope that helps
        >
        > 
        >
        > Nathan Davidson
        >
        > Senior Architect
        >
        > Globix Corp.
        >
        > London
        >
        > 
        >
        > -----Original Message-----
        > From: Adam Powers [mailto:apowers () lancope com]
        > Sent: Wed 13/07/2005 19:00
        > To: THolman () toplayer com; David.Sames () sparta com; focus-ids () securityfocus com
        > Cc:
        > Subject: Re: IDS evaluations procedures
        >
        > Tim, I hate to stir up this whole can of worms (pun alert) and yes I know
        > this is off topic but can you please qualify this seemingly non sequitur
        > statement?
        >
        > "All IDS devices are subject to large numbers of false positives, which is
        > why if you're making a new investment you should consider IPS technology, as
        > this gives you a far lower TCO and real-world protection against zero-day
        > threats."
        >
        > How so?
        >
        > I really struggle with this whole "because it's inline it must be more
        > accurate" thing. Sure, if I turn off a bunch of sigs on the IPS that are
        > less reliable, accuracy will increase. But why not do the same thing on the
        > non-inline IDS?
        >
        > Is there something magical about being inline that makes the system less
        > prone to false positives? If so, what?
        >
        > ----------
        >
        > David, addressing your original question... (which, incidentally, was about
        > INTERNAL attack traffic, not Internet Storm Center quality stuff that's
        > randomly hitting the outside of your firewall), we'll need a few extra data
        > points.
        >
        > 1. What are you testing for? Traffic-based anomalies? Application level RFC
        > violations and anomalies? Relational-modeling anomalies?
        > Behavioral-anomalies?
        >
        > 2. What collection mechanism is employed? NetFlow? sFlow? Ethernet Frames?
        > Other?
        >
        > 3. Are you only interested in classic "attacks" (fire up Nessus, see what
        > happens) or other anomalies such as malfunctioning applications,
        > policy-driven anomalies, etc?
        >
        >
        >
        >
        >
        > On 7/13/05 3:33 AM, "THolman () toplayer com" <THolman () toplayer com> wrote:
        >
        >> Hi Dave,
        >>
        >> Take a peek at the Internet Storm Centre @ SANS -
        >>
        >> http://isc.sans.org/
        >>
        >> Gives you a good idea about what's going on.
        >> Which IDS devices are you looking at?  All IDS devices are subject to large
        >> numbers of false positives, which is why if you're making a new investment
        >> you should consider IPS technology, as this gives you a far lower TCO and
        >> real-world protection against zero-day threats.  It also saves you having to
        >> buy lots of IDS sensors, seeming a large proportion of the load will be
        >> absorbed and taken care of by the IPS.
        >> Just my 2 cents.. ;)
        >>
        >> Cheers,
        >>
        >> Tim
        >>
        >> -----Original Message-----
        >> From: Sames, David [mailto:David.Sames () sparta com]
        >> Sent: 13 July 2005 04:54
        >> To: THolman () toplayer com
        >> Subject: RE: IDS evaluations procedures
        >>
        >> Thanks for the info - those are exactly the kinds of characteristics I
        >> need to consider - at this point, I'm not evaluating a product per se -
        >> I'm evaluating some claims by some of our researchers :-) FP's are what
        >> I'm most concerned about. I'll check things out to see if I can get more
        >> stats - and of attempt to produce some data sets that may look like
        >> "anomalies" but are really traffic spikes and shouldn't be flagged.
        >>
        >>>>
        >>
        >> To specifically answer your question, look at current attack weather
        >> reports
        >> - you'll see approximately 15-20% of perimeter traffic is in fact worms
        >> trying to propagate.  Any evaluation should be designed with this in
        >> mind.
        >> ..but more importantly, make sure you're evaluating something that will
        >> do
        >> the job in hand and doesn't lead you up the garden path with inaccurate
        >> marketing collateral!  :)
        >> <<<
        >>
        >> That's exactly what I was looking for !
        >>
        >> Regards,
        >>
        >> Dave
        >>
        >> --------------------------------------------------------------------------
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        >>
        >
        >
        >
        > --------------------------------------------------------------------------
        > Test Your IDS
        >
        > Is your IDS deployed correctly?
        > Find out quickly and easily by testing it with real-world attacks from
        > CORE IMPACT.
        > Go to http://www.securityfocus.com/sponsor/CoreSecurity_focus-ids_040708
        > to learn more.
        > --------------------------------------------------------------------------
        >
        > 
        >
        
        
        --
        
        Adam  Powers
        Director of Technology
        Lancope, Inc.
        c. 678.725.1028
        f. 770.225.6501
        e. apowers () lancope com
        
        StealthWatch by Lancope - Security Through Network Intelligence