Security Mistakes That Leave You Vulnerable To Compromise # 10: No Outbound Filtering


I’m always surprised at the number of organizations that have no outbound filtering at their perimeter. They have firewall rules that allow any traffic from the inside to go to the Internet on any port. Their reasoning is since the inside is trusted, they don’t need to block outgoing traffic.

But this is a big mistake for two reasons: first, there are lots of non-business applications, from file sharing sites to outright malware, which use non-standard ports.  Most of the time there is no legitimate reason for allowing traffic on these ports. To protect yourself and your Internet neighbors, you should block them.

Second, by blocking unnecessary ports, you greatly increase the chances of detecting malware or other unauthorized activity.   The access lists you create to block this traffic will create log events when traffic is blocked.  By examining your firewall logs, you can see when some malware is trying to “phone home” to its control server.  Now that you know where it is, you can go clean up the malware.

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Nick Kelly

Cybersecurity Engineer, Cisco

Nick has over 20 years of experience in Security Operations and Security Sales. He is an avid student of cybersecurity and regularly engages with the Infosec community at events like BSides, RVASec, Derbycon and more. The son of an FBI forensics director, Nick holds a B.S. in Criminal Justice and is one of Cisco’s Fire Jumper Elite members. When he’s not working, he writes cyberpunk and punches aliens on his Playstation.


Virgilio “BONG” dela Cruz Jr.

CCDP, CCNA V, CCNP, Cisco IPS Express Security for AM/EE
Field Solutions Architect, Tech Data

Virgilio “Bong” has sixteen years of professional experience in IT industry from academe, technical and customer support, pre-sales, post sales, project management, training and enablement. He has worked in Cisco Technical Assistance Center (TAC) as a member of the WAN and LAN Switching team. Bong now works for Tech Data as the Field Solutions Architect with a focus on Cisco Security and holds a few Cisco certifications including Fire Jumper Elite.


John Cavanaugh

CCIE #1066, CCDE #20070002, CCAr
Chief Technology Officer, Practice Lead Security Services, NetCraftsmen

John is our CTO and the practice lead for a talented team of consultants focused on designing and delivering scalable and secure infrastructure solutions to customers across multiple industry verticals and technologies. Previously he has held several positions including Executive Director/Chief Architect for Global Network Services at JPMorgan Chase. In that capacity, he led a team managing network architecture and services.  Prior to his role at JPMorgan Chase, John was a Distinguished Engineer at Cisco working across a number of verticals including Higher Education, Finance, Retail, Government, and Health Care.

He is an expert in working with groups to identify business needs, and align technology strategies to enable business strategies, building in agility and scalability to allow for future changes. John is experienced in the architecture and design of highly available, secure, network infrastructure and data centers, and has worked on projects worldwide. He has worked in both the business and regulatory environments for the design and deployment of complex IT infrastructures.