EC2 Subnet Auto-Assigns Public IPs

What this rule checks

Detects AWS::EC2::Subnet resources with MapPublicIpOnLaunch=true. Instances launched into the subnet inherit a public IP by default, which makes them internet-reachable without an explicit opt-in at the ENI level.

How to fix it

1. 1Set MapPublicIpOnLaunch to false unless this is intentionally a public subnet
2. 2Have instances opt into a public IP via the ENI configuration when needed
3. 3Use VPC endpoints / NAT for outbound access from private subnets

import { App, Stack } from 'aws-cdk-lib';
import { Construct } from 'constructs';
import * as ec2 from 'aws-cdk-lib/aws-ec2';

const vpc = new ec2.CfnVPC(this, 'Vpc', { cidrBlock: '10.0.0.0/16' });

// FLAGGED: every instance launched here inherits a public IP.
new ec2.CfnSubnet(this, 'Subnet', {
  vpcId: vpc.ref,
  cidrBlock: '10.0.1.0/24',
  mapPublicIpOnLaunch: true,
});

import { App, Stack } from 'aws-cdk-lib';
import { Construct } from 'constructs';
import * as ec2 from 'aws-cdk-lib/aws-ec2';

const vpc = new ec2.CfnVPC(this, 'Vpc', { cidrBlock: '10.0.0.0/16' });

// FIXED: subnet keeps the safe default (no auto-assigned public IPs).
new ec2.CfnSubnet(this, 'Subnet', {
  vpcId: vpc.ref,
  cidrBlock: '10.0.1.0/24',
  mapPublicIpOnLaunch: false,
});

CDK Insights pinpoints the exact file and line in your CDK source for every finding, so you can jump straight to the fix.

Affected resource types

Intentional? Suppress this finding

Sometimes a flag is deliberate — a genuinely public endpoint, say. You can dismiss ec2-subnet-auto-public-ip and the reason is kept in the report, not silently hidden.

In .cdk-insights.json:

{
  "ignoreRules": [
    { "id": "ec2-subnet-auto-public-ip", "reason": "Why this is intentional" }
  ]
}

Or inline in your CDK code:

Validations.of(scope).acknowledge({
  id: 'cdk-insights::ec2-subnet-auto-public-ip',
  reason: 'Why this is intentional',
});

Use the rule ID ec2-subnet-auto-public-ip shown above — not the CDK-* ID from SARIF / GitHub code scanning. To dismiss every finding on one construct instead, use ignorePaths. Suppression docs →

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