RE: Cisco IOS vulnerability
From: Abraham, Antony (Cognizant) (Antony_at_blr.cognizant.com)
Date: 07/18/03
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Date: Fri, 18 Jul 2003 12:31:30 +0530 To: <jvfields@tds.net>, <gkruel@openlink.com.br>
You are vulnerable unless you have deny statement which blocks all
packets other than say ICMP or IPSEC coming to the router interface
which is connected to the Internet.
Even though the packets targeted *at* the routers interface is only
dangerous for that router, you can block all the dangerous packets
*through* the router at your perimeter router which would avoid another
router down the line getting attacked.
Cisco has updated the security advisory and the same is available at
http://www.cisco.com/warp/public/707/cisco-sa-20030717-blocked.shtml
You can put lines like the one below given at the top of your existing
access list. This is not going to affect your ICMP or TCP Established
traffic.
access-list 101 deny 53 any any
access-list 101 deny 55 any any
access-list 101 deny 77 any any
access-list 101 deny 103 any any
Thanks,
Antony Abraham
-----Original Message-----
From: James Fields [mailto:jvfields@tds.net]
Sent: Friday, July 18, 2003 2:02 AM
To: gkruel@openlink.com.br
Cc: incidents@securityfocus.com
Subject: Re: Cisco IOS vulnerability
The vulnerability is based on a sequence of some number of special
packets targeted *at* the router's interface (i.e., not packets that are
going to be routed *through*).
The packets do not use the normal IP protocols such as TCP/UDP/ICMP, but
something different. So, first thing is that if you're only allowing
TCP, UDP, ICMP, and maybe IPSEC then you may be ok. Even so, you should
also as part of your ACLs be dropping packets targeted directly AT your
router's interface address unless you know what they do and where they
are from.
On Thu, 2003-07-17 at 10:14, Gustavo Kruel wrote:
> Hi all.
>
> I saw today the vulnerability alert on Cisco IOS. The workaround is to
> implement ACL?s that block packets from unknown sources directed to an
> exposed interface.
>
> Thinking about a perimeter router, i have one router with a "tcp any
any
> established" ACL. I also have ICMP opened in this same router, any ->
any.
> Are this lines enough to make this interface vulnerable to the
possible
> attack?
>
> What do you think about it?
>
>
>
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----------------------------------------------------------------------------
Attend the Black Hat Briefings & Training, July 28 - 31 in Las Vegas, the
world's premier technical IT security event! 10 tracks, 15 training sessions,
1,800 delegates from 30 nations including all of the top experts, from CSO's to
"underground" security specialists. See for yourself what the buzz is about!
Early-bird registration ends July 3. This event will sell out. www.blackhat.com
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