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How to Find AWS EC2 Network ACLs Allowing All IPv4 Inbound Traffic

Even having a perimeter firewall, having security groups open allows any user or malware with vpc access to scan for well known and sensitive ports and gain access to instance.

info

This security check is part of the CIS Amazon Web Services Benchmarks and is rated severity high.

Prerequisites​

This guide assumes that you have already installed and configured Fix Inventory to collect your AWS resources.

Directions​

  1. Execute the following search command in Fix Inventory Shell:

    > search is(aws_ec2_network_acl) and acl_entries[*].{(egress=false and cidr_block="0.0.0.0/0" and rule_action=allow and protocol=-1) } | jq --no-rewrite  'if (( [.reported.acl_entries[]? | contains({egress:false, cidr_block:"0.0.0.0/0", protocol:"-1", rule_action:"deny"}) ] | any | not ) or ((.reported.acl_entries | sort_by(.rule_number) | .[]? | select(.egress==false) | select(.protocol=="-1") |select(.cidr_block=="0.0.0.0/0") | select(.rule_action=="allow") | .rule_number) < (.reported.acl_entries | sort_by(.rule_number) | .[]? | select(.egress==false) | select(.protocol=="-1") | select(.cidr_block=="0.0.0.0/0") | select(.rule_action=="deny") | .rule_number ))) then [.] else [] end' | flatten
    ​kind=aws_ec2_network_acl, ..., region=fixinventory-poweruser
    ​kind=aws_ec2_network_acl, ..., account=poweruser-team

  2. Pipe the search command into the dump command:

    > search is(aws_ec2_network_acl) and acl_entries[*].{(egress=false and cidr_block="0.0.0.0/0" and rule_action=allow and protocol=-1) } | jq --no-rewrite  'if (( [.reported.acl_entries[]? | contains({egress:false, cidr_block:"0.0.0.0/0", protocol:"-1", rule_action:"deny"}) ] | any | not ) or ((.reported.acl_entries | sort_by(.rule_number) | .[]? | select(.egress==false) | select(.protocol=="-1") |select(.cidr_block=="0.0.0.0/0") | select(.rule_action=="allow") | .rule_number) < (.reported.acl_entries | sort_by(.rule_number) | .[]? | select(.egress==false) | select(.protocol=="-1") | select(.cidr_block=="0.0.0.0/0") | select(.rule_action=="deny") | .rule_number ))) then [.] else [] end' | flatten | dump
    ​reported:
    ​  id: /aws/ec2/123
    ​  name: some-name
    ​  ctime: '2022-12-05T22:53:14Z'
    ​  kind: aws_ec2_network_acl
    ​  age: 2mo28d

    The command output will list the details of all non-compliant aws_ec2_network_acl resources.

Remediation​

  • Apply Zero Trust approach.
  • Implement a process to scan and remediate unrestricted or overly permissive network ACLs.
  • Recommended best practices is to narrow the definition for the minimum ports required.
note

Please refer to the AWS EC2 documentation for details.

Further Reading​