SC-7(5)—Deny By Default -- Allow By Exception
MODERATE
HIGH
>Control Description
Deny network communications traffic by default and allow network communications traffic by exception ☑at managed interfaces; for [Assignment: organization-defined systems].
>Cross-Framework Mappings
>Programmatic Queries
Beta
Related Services
Security Groups
Network ACLs
WAF
CLI Commands
Create security group with deny-by-default ingress rules
aws ec2 create-security-group --group-name default-deny-sg --description 'Deny by default security group' --vpc-id vpc-12345678Add explicit allow rules for necessary exceptions
aws ec2 authorize-security-group-ingress --group-id sg-12345678 --protocol tcp --port 443 --cidr 10.0.0.0/8Configure default deny network ACL rules
aws ec2 create-network-acl-entry --network-acl-id acl-12345678 --rule-number 100 --protocol -1 --cidr 0.0.0.0/0 --egressDeploy AWS WAF with default deny rules for HTTP traffic
aws wafv2 create-web-acl --region us-east-1 --name default-deny-waf --scope REGIONAL --default-action Block={} --rules file://waf-rules.json>Supplemental Guidance
Denying by default and allowing by exception applies to inbound and outbound network communications traffic. A deny-all, permit-by-exception network communications traffic policy ensures that only those system connections that are essential and approved are allowed. Deny by default, allow by exception also applies to a system that is connected to an external system.
>Assessment Interview Topics
Questions assessors commonly ask
Process & Governance:
- •What policies govern the implementation of deny by default -- allow by exception?
- •How are system and communications protection requirements defined and maintained?
- •Who is responsible for configuring and maintaining the security controls specified in SC-7(5)?
Technical Implementation:
- •How is deny by default -- allow by exception technically implemented in your environment?
- •What systems, tools, or configurations enforce this protection requirement?
- •How do you ensure that deny by default -- allow by exception remains effective as the system evolves?
- •What network boundary protections are in place (firewalls, gateways, etc.)?
Evidence & Documentation:
- •What documentation demonstrates the implementation of SC-7(5)?
- •Can you provide configuration evidence or system diagrams showing this protection control?
- •What logs or monitoring data verify that this control is functioning correctly?
- •Can you provide network architecture diagrams and firewall rulesets?
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