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Firewall scanning with Nessus

We have a Cisco ASA firewall that is configure to go. We want to do some scanning for possible vulnerability.

The firewall is currently set to allow TCP traffic to one specific port, say 1005, to one direction only (other way will be deny). All other traffics are deny using ACL.

What should I do to test for possible vulnerability? We have full version of Nessus and NMAP available.

A consultant suggested us to do both positive testing (making sure the firewall rule will allow the traffic you want) and negative testing (verifying that it will blocks traffic that is not defined).

He didn't mention the details....  Can someone help me out? thanks!
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SPERTW
Asked:
SPERTW
3 Solutions
 
Dave HoweCommented:
ok, you have two factors there.

you have a rulebase that says "for the range of addresses x.x.x.x to y.y.y.y, allow access on port zzz"

you can test that with suitable port scanning software, but to be honest it isn't worthwhile - if the rulebase says that, then outside of someone exploiting a bug in the ASA software, that is what you are going to get. now, an active exploit of ASA would be worth testing for, but pentesters (and obviously, blackhats) keep those close to their chest.

second, you can assume that an attacker has access to the open port, and configure nessus to throw every attack it knows how to run against the application behind that port against the outside firewall - usually by placing the testing machine outside the asa, making sure its IP is in the permitted range, then running the test.

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MultipathCommented:
In my experience scanning a firewall is never a good idea, the best thing to do is to track vulnerabilities to your version.  The reasons for this are simple, if someone outside scans the firewall they are going ot get locked out of traffic trough the content scanning engine.  What you can do to test if hte firewall is working is to test through know open ports to known applications behind the ports this will give you a good idea of you outside threat level.  I would however suggest that you give no special rules to the outside scanning engine as you want the firewall to do its job and stop the traffic if it sees it this will give you a win with your management because you can show the attacks against the services and the firewall stopping the attacks while still letting good traffic through.
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