Blocking SMB Traffic

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Blocking SMB Traffic

L4 Transporter

I was doing a review of some firewall policies and noticed the company I am consulting for is allowing all applications risk 1 through 3 from their trust to untrust zones.  Not sure why it's setup that way yet, but in doing so, SMB traffic is alllowed out. 


I want to immediately put a control in that blocks SMB traffic outbound. Is it recommended to create the policy using only ports, tcp/udp port 445, or should I block via SMB application?   My thought is block via ports, but I'll do whatever is the recommended way.


What about tcp/udp port 137 and 139?  Should these also be added to the blocked 'from trust to untrust' rule?


I'm curious to what you all are doing.





L7 Applicator

They are not mutually exclusive. You can do both. Just leave the rule where you define application SMB, set to application 'any', so if SMB is used evasively through different ports, it can also be blocked.

Not sure I'm following your response. Do you mean if you choose SMB for application, set the Service to Any?


Is it common practice to create multiple rules to block SMB?

Yes, SMB for application, Service to Any.


Not sure what common practice is, but some environments require that *no* packets leave out on specific ports.

The app-id engine needs to allow a few packets through before it identifies the application, so if you go solely with an app-id rule, a few packets will always leave the firewall before there's a block action enforced.

thanks. what method do you use on your firewalls (if you don't mind me asking)

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