Outbound network filtering is a free tactic to help improve protection against supply chain attacks, accomplished by limiting outbound network connections to known and approved flows. This ensures a server can only communicate with pre-defined destinations on the internet, preventing the server (or the software it hosts) from communicating with cybercriminals’ command and control servers. 

If a server can communicate without restriction, it's easy to exfiltrate large stores of data within a few days. This measure makes it harder for malware to "phone home" to report that they've gained a foothold in someone's environment. Blocking such connections will also create "noise," making it easier for your team to catch such attacks early.

In most cases, filtering can be implemented using existing tools and firewalls. Organisations typically start by establishing a baseline and identifying existing outbound connections. Then rules are implemented denying all unexpected connections to unknown destinations. 

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