---

name: template-validator description: Validates CloudFormation templates for syntax, security, and best practices. Use when validating CloudFormation templates, checking for security issues, or ensuring compliance with best practices.

Template Validator

Quick Start

Validate CloudFormation templates for syntax errors, security issues, and adherence to best practices before deployment.

Instructions

Step 1: Validate template syntax

Copy# Basic validation
aws cloudformation validate-template \
  --template-body file://template.yaml

# Validation with parameters
aws cloudformation validate-template \
  --template-body file://template.yaml \
  --parameters ParameterKey=Param1,ParameterValue=Value1

Check for:

• Valid YAML/JSON syntax
• Required template sections
• Valid resource types
• Correct intrinsic function usage
• Parameter references

Step 2: Use cfn-lint for comprehensive checks

Copy# Install cfn-lint
pip install cfn-lint

# Validate template
cfn-lint template.yaml

# Validate with specific rules
cfn-lint template.yaml --ignore-checks W

# Output as JSON
cfn-lint template.yaml --format json

cfn-lint checks:

• Template structure
• Resource properties
• Best practices
• Security issues
• Regional availability

Step 3: Security validation

Check IAM policies:

Copy# Review for overly permissive policies
Resources:
  Role:
    Type: AWS::IAM::Role
    Properties:
      AssumeRolePolicyDocument:
        Statement:
          - Effect: Allow
            Principal:
              Service: ec2.amazonaws.com
            Action: sts:AssumeRole
      Policies:
        - PolicyName: AppPolicy
          PolicyDocument:
            Statement:
              # Avoid wildcards
              - Effect: Allow
                Action: s3:*  # Too permissive!
                Resource: '*'  # Too broad!

Better approach:

CopyPolicies:
  - PolicyName: AppPolicy
    PolicyDocument:
      Statement:
        - Effect: Allow
          Action:
            - s3:GetObject
            - s3:PutObject
          Resource: !Sub '${MyBucket.Arn}/*'

Check security groups:

Copy# Avoid open access
Resources:
  SecurityGroup:
    Type: AWS::EC2::SecurityGroup
    Properties:
      SecurityGroupIngress:
        # Don't allow 0.0.0.0/0 for SSH
        - IpProtocol: tcp
          FromPort: 22
          ToPort: 22
          CidrIp: 0.0.0.0/0  # Security risk!

Better approach:

CopySecurityGroupIngress:
  - IpProtocol: tcp
    FromPort: 22
    ToPort: 22
    CidrIp: 10.0.0.0/8  # Restrict to internal network

Step 4: Check resource dependencies

Verify DependsOn usage:

CopyResources:
  # Explicit dependency needed
  Instance:
    Type: AWS::EC2::Instance
    DependsOn: InternetGatewayAttachment
    Properties:
      # ...
  
  # Implicit dependency via Ref
  SecurityGroupIngress:
    Type: AWS::EC2::SecurityGroupIngress
    Properties:
      GroupId: !Ref SecurityGroup  # Implicit dependency

Check for circular dependencies:

• Review all DependsOn relationships
• Check Ref and GetAtt usage
• Verify no circular references

Step 5: Validate best practices

Use specific resource names:

Copy# Good
Resources:
  WebServerSecurityGroup:
    Type: AWS::EC2::SecurityGroup

# Avoid generic names
Resources:
  SecurityGroup1:
    Type: AWS::EC2::SecurityGroup

Add descriptions:

CopyAWSTemplateFormatVersion: '2010-09-09'
Description: Web application infrastructure with ALB and Auto Scaling

Parameters:
  InstanceType:
    Type: String
    Description: EC2 instance type for web servers

Use tags:

CopyResources:
  Instance:
    Type: AWS::EC2::Instance
    Properties:
      Tags:
        - Key: Name
          Value: !Sub '${AWS::StackName}-WebServer'
        - Key: Environment
          Value: !Ref Environment
        - Key: ManagedBy
          Value: CloudFormation

Common Validation Checks

Syntax Validation

Valid YAML structure:

CopyAWSTemplateFormatVersion: '2010-09-09'
Description: Template description

Parameters:
  # Parameters section

Resources:
  # Resources section (required)

Outputs:
  # Outputs section

Intrinsic functions:

Copy# Correct
Value: !Ref MyResource
Value: !GetAtt MyResource.Attribute
Value: !Sub '${MyResource}'

# Incorrect
Value: Ref: MyResource  # Wrong syntax
Value: !GetAtt MyResource  # Missing attribute

Security Validation

IAM policies:

• No wildcards in actions unless necessary
• Specific resources instead of '*'
• Least privilege principle
• No hardcoded credentials

Security groups:

• No 0.0.0.0/0 for sensitive ports (22, 3389, 3306, 5432)
• Specific port ranges
• Documented ingress rules

Encryption:

• Enable encryption for S3 buckets
• Enable encryption for EBS volumes
• Enable encryption for RDS instances
• Use KMS keys for sensitive data

Resource Validation

Required properties:

CopyResources:
  Bucket:
    Type: AWS::S3::Bucket
    Properties:
      # BucketName is optional but recommended
      BucketName: !Sub '${AWS::StackName}-bucket'

Valid property values:

CopyResources:
  Instance:
    Type: AWS::EC2::Instance
    Properties:
      InstanceType: t3.micro  # Must be valid instance type
      ImageId: ami-12345678  # Must be valid AMI ID

Validation Tools

AWS CLI

Copy# Validate template
aws cloudformation validate-template \
  --template-body file://template.yaml

# Create change set (validates before applying)
aws cloudformation create-change-set \
  --stack-name my-stack \
  --change-set-name my-changes \
  --template-body file://template.yaml

# Describe change set
aws cloudformation describe-change-set \
  --stack-name my-stack \
  --change-set-name my-changes

cfn-lint

Copy# Basic validation
cfn-lint template.yaml

# Ignore warnings
cfn-lint template.yaml --ignore-checks W

# Specific regions
cfn-lint template.yaml --regions us-east-1 us-west-2

# Custom rules
cfn-lint template.yaml --append-rules custom-rules/

cfn-nag

Copy# Install cfn-nag
gem install cfn-nag

# Scan template
cfn_nag_scan --input-path template.yaml

# Scan with rules
cfn_nag_scan --input-path template.yaml --deny-list-path rules.txt

TaskCat

Copy# Install taskcat
pip install taskcat

# Test template
taskcat test run

# Configuration in .taskcat.yml
project:
  name: my-project
  regions:
    - us-east-1
    - us-west-2
tests:
  default:
    template: template.yaml
    parameters:
      InstanceType: t3.micro

Validation Checklist

Template structure:

• Valid YAML/JSON syntax
• AWSTemplateFormatVersion present
• Description provided
• Resources section present

Parameters:

• Descriptive names
• Descriptions provided
• Validation constraints (AllowedValues, AllowedPattern)
• Appropriate defaults
• NoEcho for sensitive values

Resources:

• Descriptive logical IDs
• Required properties present
• Valid property values
• Appropriate DependsOn usage
• Tags applied

Security:

• IAM policies follow least privilege
• No hardcoded credentials
• Security groups restrict access
• Encryption enabled where appropriate
• No overly permissive policies

Outputs:

• Descriptive names
• Descriptions provided
• Appropriate exports
• Conditional outputs where needed

Best practices:

• Consistent naming convention
• Appropriate use of parameters
• Cross-stack references via exports
• Proper error handling
• Documentation in descriptions

Advanced

For detailed information, see:

• Security Best Practices - Comprehensive security validation guide
• Validation Rules - Complete list of validation rules and checks