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03.13.06Network communications - deny by default - allow by exceptionITSP.10.171System and communications protection

Official Requirement

Deny network communications traffic by default and allow network communications traffic by exception.

Source: Canadian Centre for Cyber Security ITSP.10.171 (2025), adapted from NIST SP 800-171 Rev. 3

In Plain English

All network traffic must be denied by default. Only explicitly authorized traffic should be allowed through firewall rules and access control lists. This 'default deny' posture ensures that only known, approved communications are permitted.

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The Trust Services Criteria referenced in this tool are published by the American Institute of Certified Public Accountants (AICPA) and incorporate principles from the Committee of Sponsoring Organizations of the Treadway Commission (COSO). Criteria descriptions are reproduced for reference purposes. All implementation guidance, evidence recommendations, and control language are original work by Truvo Cyber. ISO 27001 and ISO 42001 are standards published by the International Organization for Standardization (ISO). Framework and standard names are the property of their respective owners. This tool is not affiliated with, endorsed by, or certified by AICPA, COSO, or ISO.

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On-premises implementation and evidence

Default deny network policy

Implementation steps

  1. Configure all firewalls with a default deny-all rule as the last rule in every rule set
  2. Create explicit allow rules only for authorized traffic with specific source, destination, port, and protocol definitions
  3. Implement default deny on host-based firewalls (Windows Firewall, iptables) in addition to network firewalls
  4. Review firewall rules at a defined frequency and remove any overly broad or obsolete allow rules

Tools / systems

Evidence artifacts

Evidence frequency: Continuous enforcement; quarterly firewall rule review