cis-aws-foundations-6-5

name	cis-aws-foundations-6.5
description	Ensure the default security group of every VPC restricts all traffic
category	cis-networking
version	7.0.0
author	cyberstrike-official
tags	["cis","aws","networking","security-groups","vpc","default-security-group"]
cis_id	6.5
cis_benchmark	CIS AWS Foundations Benchmark v7.0.0
tech_stack	["aws"]
cwe_ids	[]
chains_with	["cis-aws-foundations-6.3","cis-aws-foundations-6.4","cis-aws-foundations-6.6"]
prerequisites	[]
severity_boost	{}

Ensure the default security group of every VPC restricts all traffic

Description

A VPC comes with a default security group whose initial settings deny all inbound traffic, allow all outbound traffic, and allow all traffic between instances assigned to the security group. If a security group is not specified when an instance is launched, it is automatically assigned to this default security group. Security groups provide stateful filtering of ingress/egress network traffic to AWS resources. It is recommended that the default security group restrict all traffic, both inbound and outbound.

The default VPC in every region should have its default security group updated to comply with the following:

• No inbound rules.
• No outbound rules.

Any newly created VPCs will automatically contain a default security group that will need remediation to comply with this recommendation.

Note: When implementing this recommendation, VPC flow logging is invaluable in determining the least privilege port access required by systems to work properly, as it can log all packet acceptances and rejections occurring under the current security groups. This dramatically reduces the primary barrier to least privilege engineering by discovering the minimum ports required by systems in the environment. Even if the VPC flow logging recommendation in this benchmark is not adopted as a permanent security measure, it should be used during any period of discovery and engineering for least privileged security groups.

Rationale

Configuring all VPC default security groups to restrict all traffic will encourage the development of least privilege security groups and promote the mindful placement of AWS resources into security groups, which will, in turn, reduce the exposure of those resources.

Impact

Implementing this recommendation in an existing VPC that contains operating resources requires extremely careful migration planning, as the default security groups are likely enabling many ports that are unknown. Enabling VPC flow logging (for accepted connections) in an existing environment that is known to be breach-free will reveal the current pattern of ports being used for each instance to communicate successfully. The migration process should include:

• Analyzing VPC flow logs to understand current traffic patterns.
• Creating least privilege security groups based on the analyzed data.
• Testing the new security group rules in a staging environment before applying them to production.

Audit Procedure

Using AWS Console

Security Group State:

1. Login to the AWS VPC Console at https://console.aws.amazon.com/vpc/home.
2. Repeat the following steps for all VPCs, including the default VPC in each AWS region: