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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