skippy1hickorytech.net
Date: Tue Dec 21 2004 - 11:07:49 CST

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This is a fairly common idea, but its one that has some drawbacks to it.

In general, automatically blocking a IP based on an IDS alarm has the
advantage of needing less admin time but also has the potential of letting
an attacker block outside IPs from you network at will. If an attacker
figures out that your machine is protected this way and convinces your IDS
that your home IP is trying to exploit the box, you have a problem. If
you live half an hour from the box and don't have another way in, the
attacker gets a free half hour to do what he wants.

There are times when autoblocking is a worth while idea however. If you
don't mind the chance of the service being down, it certainly can increase
security. Added to the fact that spoofing TCP isn't that easy, and that
(in this case) the attacker would have to spoof the ssh handshake and it
sounds more feasible.

In the case of ssh specifically, it might be a bit tough to set up
autoblocking.

Assuming that you would want to block on failed login attempts or login
attempts using certain usernames, you would have to find a way to watch
the ssh traffic through its encryption. In theory an nIDS could watch the
handshake and given a copy of the server's private key could decrypt the
traffic on the fly. This is not a good idea in a bunch of ways.

A better way to do would be to set up a log watcher and block on the local
machine based on the ssh daemon logging the failed attempts. The
disadvantage is that you can only block one machine at a time that way.

Skippy

> Maybe this is a dumb question, but why not set up a honeynet or an IDS
> like snort and block addresses or networks as they begin scanning? Less
> administration needed and you don't have to block ranges larger than
> necessary...
>
> Also, I threw together a little C app and script which will quickly find
> passwords commonly used in brute force attacks. You may be able to use
> it with cron to locate users with easily-guessed passwords and reset
> them so brute force attacks aren't as successful.
> http://freshmeat.net/p/dumbass/
>
> Gerry Dalton wrote:
>
>>I have seen similar probes over the last 2 months. Most all have been
>> from APNIC address blocks. I got so tired of some of it I just went
>> ahead and blocked a full range of addresses from getting past our
>> border routers.
>>
>>So far these have just been a nuisance.
>>
>>Gerry
>>
>>
>>
>>At 09:21 AM 12/20/2004, Dejan Markovic wrote:
>>
>>
>>
>>>Hi Guys,
>>>
>>>Don't know whether this is the right list, but need to ask if others
>>> have the same entries in their logs for the past number of months. Let
>>> me take a step back, I maintain a number of networks on different IP
>>> ranges and they are all being probed by what looks like a tool, or
>>> maybe it is the same group/script. The originating computers range
>>> from open proxies to owned boxes and there are two distinct patterns
>>> I've seen so far. The following scan is a recent example where the
>>> root/password from x.x.x.x: 59 Time(s) caught my attention the first
>>> time a while back, and still getting the same scans on a daily basis:
>>>
>>>account/password from 210.245.168.28: 1 Time(s)
>>>adam/password from 210.245.168.28: 1 Time(s)
>>>adm/password from 210.245.168.28: 2 Time(s)
>>>alan/password from 210.245.168.28: 1 Time(s)
>>>apache/password from 210.245.168.28: 1 Time(s)
>>>backup/password from 210.245.168.28: 1 Time(s)
>>>cip51/password from 210.245.168.28: 1 Time(s)
>>>cip52/password from 210.245.168.28: 1 Time(s)
>>>cosmin/password from 210.245.168.28: 1 Time(s)
>>>cyrus/password from 210.245.168.28: 1 Time(s)
>>>data/password from 210.245.168.28: 1 Time(s)
>>>frank/password from 210.245.168.28: 1 Time(s)
>>>george/password from 210.245.168.28: 1 Time(s)
>>>henry/password from 210.245.168.28: 1 Time(s)
>>>horde/password from 210.245.168.28: 1 Time(s)
>>>iceuser/password from 210.245.168.28: 1 Time(s)
>>>irc/password from 210.245.168.28: 2 Time(s)
>>>jane/password from 210.245.168.28: 1 Time(s)
>>>john/password from 210.245.168.28: 1 Time(s)
>>>master/password from 210.245.168.28: 1 Time(s)
>>>matt/password from 210.245.168.28: 1 Time(s)
>>>mysql/password from 210.245.168.28: 1 Time(s)
>>>nobody/password from 210.245.168.28: 1 Time(s)
>>>noc/password from 210.245.168.28: 1 Time(s)
>>>operator/password from 210.245.168.28: 1 Time(s)
>>>oracle/password from 210.245.168.28: 1 Time(s)
>>>pamela/password from 210.245.168.28: 1 Time(s)
>>>patrick/password from 210.245.168.28: 2 Time(s)
>>>rolo/password from 210.245.168.28: 1 Time(s)
>>>root/password from 210.245.168.28: 59 Time(s)
>>>server/password from 210.245.168.28: 1 Time(s)
>>>sybase/password from 210.245.168.28: 1 Time(s)
>>>test/password from 210.245.168.28: 5 Time(s)
>>>user/password from 210.245.168.28: 3 Time(s)
>>>web/password from 210.245.168.28: 2 Time(s)
>>>webmaster/password from 210.245.168.28: 1 Time(s)
>>>www-data/password from 210.245.168.28: 1 Time(s)
>>>www/password from 210.245.168.28: 1 Time(s)
>>>wwwrun/password from 210.245.168.28: 1 Time(s)
>>>
>>>Regards,
>>>Dan
>>>
>>>
>>
>>

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