Using the Parser in IAM Policies

Published: November 9, 2025

IAM policies are only as secure as the ARNs they reference. A single typo or malformed ARN can grant unintended access — or break your application. This guide shows how to use the ARN Parser to write bulletproof policies.

Why Validate ARNs in Policies?

AWS evaluates IAM policies at runtime. Invalid ARNs cause:

• Access denied errors (even with correct permissions)
• Overly broad access via wildcards
• Deployment failures in CI/CD

Common Policy Mistakes

// Wrong: Missing colon
"Resource": "arn:aws:s3::my-bucket"

// Wrong: Typos in service
"Resource": "arn:aws:ec3:us-east-1:123:instance/i-abc"

// Wrong: Hardcoded account
"Resource": "arn:aws:s3:::my-bucket" in shared templates

Best Practices

1. Always Validate Before Apply

Paste every ARN into the parser. Green table = safe to use.

2. Use Wildcards Strategically

// Good: Limit to prefix
"Resource": "arn:aws:s3:::prod-logs/app1/*"

// Bad: Too broad
"Resource": "*"

3. Leverage CloudFormation Parameters

Resource: !Sub "arn:aws:lambda:${AWS::Region}:${AWS::AccountId}:function:${FunctionName}"

4. Cross-Account ARNs

For cross-account access:

"Resource": "arn:aws:iam::456789012345:root"

Validate both source and target account ARNs.

Policy Testing Workflow

1. Write policy in JSON/YAML
2. Extract all ARNs
3. Paste into parser → confirm structure
4. Apply via CLI or console

Real-World Example

Lambda function reading from S3:

{
  "Effect": "Allow",
  "Action": ["s3:GetObject"],
  "Resource": "arn:aws:s3:::data-prod-2025/input/*.csv"
}

Parser confirms: service = s3, resourceType = bucket, resourceId = data-prod-2025/input/*.csv

FAQ

Never deploy an IAM policy without validating every ARN. One mistake = one breach.