Re: Firewall Tester 0.6

From: Bennett Todd (
Date: 04/13/02

Date: Sat, 13 Apr 2002 11:21:26 -0400
From: Bennett Todd <>
To: "Marcus J. Ranum" <>

2002-04-11-11:48:40 Marcus J. Ranum:
> [...] We had a case where a tester played snort-sig fragments at
> an NFR and the NFR didn't generate a single false alarm. [...]
> A lesser system would have generated a ton of false alarms and
> irritated the heck out of the administrator, which would have been
> the wrong thing to do because there was no actual attack or even
> threat.

That statement carries an assumption to it. It sounds sorta
reasonable in the case where an IDS is placed outside the firewall
where it can see all the yammering of all the folks on the internet,
and where you're unconcerned about folks probing your security

Depending on local policies and on sensor placement, firing a wad of
signatures across the bow of the IDS could in fact be an actual
incident even if there were no software attack in progress.

E.g. consider IDSes placed inside the innermost firewall perimeter,
placed to detect incoming attacks that have succeeded in making it
through the firewalls, and outbound attacks of any sort. There the
only thing I'd call a real "false alarm" would be something like the
noted "AAA..." shellcode sled pattern match. Someone injecting a
stream of signatures through this perimeter is performing a security
probe of some sort, and nobody with access to that perimeter is
permitted to do any such thing --- except of course for the firewall
admins, one and the same as the recipients of the IDS alarms.

Given that sort of sensor placement and policy, any unexpected
alarms coming out of an IDS do reflect an incident, even if it's
just a signature stream.


Relevant Pages

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