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? GW On 17 Jul 2003 at 11:14, 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? ---------------------------------------------------------------------------- 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 ---------------------------------------------------------------------------- ---------------------------------------------------------------------------- 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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