From: Octavio Novoa (ONOVOAdiveo.net.pe)
Date: Fri Jul 18 2003 - 15:41:28 CDT

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I've tried myself the undefinied ACL as "not of the others rule" but never
worked out. I always specify the last acl as "permit everything I do not
want to deny".

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-----Mensaje original-----
De: Mitchell Rowton [mailto:mitchellattackprevention.com]
Enviado el: Viernes, 18 de Julio de 2003 02:24 p.m.
Para: wangwsingnet.com.sg; Gustavo Kruel; incidentssecurityfocus.com
Asunto: Re: Cisco IOS vulnerability

I just looked this up to verify my memory, in Managing Cisco Network
Security by Michael Wenstrom (p.713) it says that undefined access list
equals permit any. I'm not saying it is true... just referencing that
book.

But in this case a "tcp established" ACL isn't empty anyway. Yeah,
this should be fine.

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>
> My understanding of the basic way cisco ACL works are: if your ACL is
not
> empty, then any unmatched packet (with ACL list) will be dropped,
like a
> default deny all. So in your case, the supposedly attack packets all
use
> protocol 53, 55 etc, thus won't match anything in your ACL list, thus
shall be
> dropped. So for this particular attack, it shall be OK (provided the
ACL has
> applied to the external interface for external attacks).
>
> Any cisco expert has any comment / confirmation on this?
>

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