Actually, no. What constitutes a useful alarm is mostly a matter of policy
for the user; the technical means used to detect an attack (or which fails
and alerts on an attack that never took place) doesn't define how useful the
data is when all is said and done.
-----Original Message-----
From: Zhuowei Li [mailto:zhuowei@gmail.com]
Sent: Thursday, September 16, 2004 9:22 PM
To: Rob Shein; focus-ids@securityfocus.com
Subject: Re: What is false alarm rate and false positive rate?
Hi,
Martin Roesch did a fantastic way of shedding light on this
question.
The short answer is "neither," but it comes down to this
question: If
the IDS sees an OpenSSL attack go towards an IIS server that isn't
using OpenSSL, is that a false alarm or not? It's
definitely not as
useful as it would be as an alert if the attack were aimed at an
actual OpenSSL listener, but it's not as useless as a
complete false
alarm that alerts on something that didn't happen at all.
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
Under such scenario, if it is in signature-based intrusion
detection, it is yes since one of its tasks is to identify
the intrusion correctly for the purpose of response. However,
in anomaly-based intrusion detection, there is no such task,
the only we can do for anomaly-based is to alert that there
is an anomaly occurs in the system. That's a true alarm, right?
Since Roesch's focus is on the signature-based, I think
his/her example is applicable only for his/her focus. For
anomaly-based intrusion detection, it is a different picture
we should draw. right?
Thanks.
Li
_______________________________________
http://www.cais.ntu.edu.sg/~zhuowei
-----Original Message-----
From: Zhuowei Li [mailto:zhuowei@gmail.com]
Sent: Wednesday, September 15, 2004 2:21 AM
To: focus-ids@securityfocus.com
Subject: What is false alarm rate and false positive rate?
Hi,
I am confused by the terms 'false positive rate' and 'false alarm
rate' within the context of intrusion detection. Does
anybody about
what's the exact definition for these two terms?
Some literatures said 'false positive rate = false alarm rate',
which the number of false alarms divided by the number of alarms
(true and false).
Other said false positive rate is not equal to false
alarm rate, the
false alarm rate is the same above definition, but the false
positive rate is "the total number of normal instances that were
incorrectly classified as intrusions divided by the total
number of
normal instances"
Who is true, who is wrong within the context of intrusion
detection?
Thanks.
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