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+# Serverless Security Best Practices
+
+Security best practices for serverless applications based on AWS Well-Architected Framework.
+
+## Table of Contents
+
+- [Shared Responsibility Model](#shared-responsibility-model)
+- [Identity and Access Management](#identity-and-access-management)
+- [Function Security](#function-security)
+- [API Security](#api-security)
+- [Data Protection](#data-protection)
+- [Network Security](#network-security)
+
+## Shared Responsibility Model
+
+### Serverless Shifts Responsibility to AWS
+
+With serverless, AWS takes on more security responsibilities:
+
+**AWS Responsibilities**:
+- Compute infrastructure
+- Execution environment
+- Runtime language and patches
+- Networking infrastructure
+- Server software and OS
+- Physical hardware and facilities
+- Automatic security patches (like Log4Shell mitigation)
+
+**Customer Responsibilities**:
+- Function code and dependencies
+- Resource configuration
+- Identity and Access Management (IAM)
+- Data encryption (at rest and in transit)
+- Application-level security
+- Secure coding practices
+
+### Benefits of Shifted Responsibility
+
+- **Automatic Patching**: AWS applies security patches automatically (e.g., Log4Shell fixed within 3 days)
+- **Infrastructure Security**: No OS patching, server hardening, or vulnerability scanning
+- **Operational Agility**: Quick security response at scale
+- **Focus on Code**: Spend time on business logic, not infrastructure security
+
+## Identity and Access Management
+
+### Least Privilege Principle
+
+**Always use least privilege IAM policies**:
+
+```typescript
+// ✅ GOOD - Specific grant
+const table = new dynamodb.Table(this, 'Table', {});
+const function = new lambda.Function(this, 'Function', {});
+
+table.grantReadData(function); // Only read access
+
+// ❌ BAD - Overly broad
+function.addToRolePolicy(new iam.PolicyStatement({
+  actions: ['dynamodb:*'],
+  resources: ['*'],
+}));
+```
+
+### Function Execution Role
+
+**Separate roles per function**:
+
+```typescript
+// ✅ GOOD - Each function has its own role
+const readFunction = new NodejsFunction(this, 'ReadFunction', {
+  entry: 'src/read.ts',
+  // Gets its own execution role
+});
+
+const writeFunction = new NodejsFunction(this, 'WriteFunction', {
+  entry: 'src/write.ts',
+  // Gets its own execution role
+});
+
+table.grantReadData(readFunction);
+table.grantReadWriteData(writeFunction);
+
+// ❌ BAD - Shared role with excessive permissions
+const sharedRole = new iam.Role(this, 'SharedRole', {
+  assumedBy: new iam.ServicePrincipal('lambda.amazonaws.com'),
+  managedPolicies: [
+    iam.ManagedPolicy.fromAwsManagedPolicyName('AdministratorAccess'), // Too broad!
+  ],
+});
+```
+
+### Resource-Based Policies
+
+Control who can invoke functions:
+
+```typescript
+// Allow API Gateway to invoke function
+myFunction.grantInvoke(new iam.ServicePrincipal('apigateway.amazonaws.com'));
+
+// Allow specific account
+myFunction.addPermission('AllowAccountInvoke', {
+  principal: new iam.AccountPrincipal('123456789012'),
+  action: 'lambda:InvokeFunction',
+});
+
+// Conditional invoke (only from specific VPC endpoint)
+myFunction.addPermission('AllowVPCInvoke', {
+  principal: new iam.ServicePrincipal('lambda.amazonaws.com'),
+  action: 'lambda:InvokeFunction',
+  sourceArn: vpcEndpoint.vpcEndpointId,
+});
+```
+
+### IAM Policies Best Practices
+
+1. **Use grant methods**: Prefer `.grantXxx()` over manual policies
+2. **Condition keys**: Use IAM conditions for fine-grained control
+3. **Resource ARNs**: Always specify resource ARNs, avoid wildcards
+4. **Session policies**: Use for temporary elevated permissions
+5. **Service Control Policies (SCPs)**: Enforce organization-wide guardrails
+
+## Function Security
+
+### Lambda Isolation Model
+
+**Each function runs in isolated sandbox**:
+- Built on Firecracker microVMs
+- Dedicated execution environment per function
+- No shared memory between functions
+- Isolated file system and network namespace
+- Strong workload isolation
+
+**Execution Environment Security**:
+- One concurrent invocation per environment
+- Environment may be reused (warm starts)
+- `/tmp` storage persists between invocations
+- Sensitive data in memory may persist
+
+### Secure Coding Practices
+
+**Handle sensitive data securely**:
+
+```typescript
+// ✅ GOOD - Clean up sensitive data
+export const handler = async (event: any) => {
+  const apiKey = process.env.API_KEY;
+
+  try {
+    const result = await callApi(apiKey);
+    return result;
+  } finally {
+    // Clear sensitive data from memory
+    delete process.env.API_KEY;
+  }
+};
+
+// ✅ GOOD - Use Secrets Manager
+import { SecretsManagerClient, GetSecretValueCommand } from '@aws-sdk/client-secrets-manager';
+
+const secretsClient = new SecretsManagerClient({});
+
+export const handler = async (event: any) => {
+  const secret = await secretsClient.send(
+    new GetSecretValueCommand({ SecretId: process.env.SECRET_ARN })
+  );
+
+  const apiKey = secret.SecretString;
+  // Use apiKey
+};
+```
+
+### Dependency Management
+
+**Scan dependencies for vulnerabilities**:
+
+```json
+// package.json
+{
+  "scripts": {
+    "audit": "npm audit",
+    "audit:fix": "npm audit fix"
+  },
+  "devDependencies": {
+    "snyk": "^1.0.0"
+  }
+}
+```
+
+**Keep dependencies updated**:
+- Run `npm audit` or `pip-audit` regularly
+- Use Dependabot or Snyk for automated scanning
+- Update dependencies promptly when vulnerabilities found
+- Use minimal dependency sets
+
+### Environment Variable Security
+
+**Never store secrets in environment variables**:
+
+```typescript
+// ❌ BAD - Secret in environment variable
+new NodejsFunction(this, 'Function', {
+  environment: {
+    API_KEY: 'sk-1234567890abcdef', // Never do this!
+  },
+});
+
+// ✅ GOOD - Reference to secret
+new NodejsFunction(this, 'Function', {
+  environment: {
+    SECRET_ARN: secret.secretArn,
+  },
+});
+
+secret.grantRead(myFunction);
+```
+
+## API Security
+
+### API Gateway Security
+
+**Authentication and Authorization**:
+
+```typescript
+// Cognito User Pool authorizer
+const authorizer = new apigateway.CognitoUserPoolsAuthorizer(this, 'Authorizer', {
+  cognitoUserPools: [userPool],
+});
+
+api.root.addMethod('GET', integration, {
+  authorizer,
+  authorizationType: apigateway.AuthorizationType.COGNITO,
+});
+
+// Lambda authorizer for custom auth
+const customAuthorizer = new apigateway.TokenAuthorizer(this, 'CustomAuth', {
+  handler: authorizerFunction,
+  resultsCacheTtl: Duration.minutes(5),
+});
+
+// IAM authorization for service-to-service
+api.root.addMethod('POST', integration, {
+  authorizationType: apigateway.AuthorizationType.IAM,
+});
+```
+
+### Request Validation
+
+**Validate requests at API Gateway**:
+
+```typescript
+const validator = new apigateway.RequestValidator(this, 'Validator', {
+  api,
+  validateRequestBody: true,
+  validateRequestParameters: true,
+});
+
+const model = api.addModel('Model', {
+  schema: {
+    type: apigateway.JsonSchemaType.OBJECT,
+    required: ['email', 'name'],
+    properties: {
+      email: {
+        type: apigateway.JsonSchemaType.STRING,
+        format: 'email',
+      },
+      name: {
+        type: apigateway.JsonSchemaType.STRING,
+        minLength: 1,
+        maxLength: 100,
+      },
+    },
+  },
+});
+
+resource.addMethod('POST', integration, {
+  requestValidator: validator,
+  requestModels: {
+    'application/json': model,
+  },
+});
+```
+
+### Rate Limiting and Throttling
+
+```typescript
+const api = new apigateway.RestApi(this, 'Api', {
+  deployOptions: {
+    throttlingRateLimit: 1000, // requests per second
+    throttlingBurstLimit: 2000, // burst capacity
+  },
+});
+
+// Per-method throttling
+resource.addMethod('POST', integration, {
+  methodResponses: [{ statusCode: '200' }],
+  requestParameters: {
+    'method.request.header.Authorization': true,
+  },
+  throttling: {
+    rateLimit: 100,
+    burstLimit: 200,
+  },
+});
+```
+
+### API Keys and Usage Plans
+
+```typescript
+const apiKey = api.addApiKey('ApiKey', {
+  apiKeyName: 'customer-key',
+});
+
+const plan = api.addUsagePlan('UsagePlan', {
+  name: 'Standard',
+  throttle: {
+    rateLimit: 100,
+    burstLimit: 200,
+  },
+  quota: {
+    limit: 10000,
+    period: apigateway.Period.MONTH,
+  },
+});
+
+plan.addApiKey(apiKey);
+plan.addApiStage({
+  stage: api.deploymentStage,
+});
+```
+
+## Data Protection
+
+### Encryption at Rest
+
+**DynamoDB encryption**:
+
+```typescript
+// Default: AWS-owned CMK (no additional cost)
+const table = new dynamodb.Table(this, 'Table', {
+  encryption: dynamodb.TableEncryption.AWS_MANAGED, // AWS managed CMK
+});
+
+// Customer-managed CMK (for compliance)
+const kmsKey = new kms.Key(this, 'Key', {
+  enableKeyRotation: true,
+});
+
+const table = new dynamodb.Table(this, 'Table', {
+  encryption: dynamodb.TableEncryption.CUSTOMER_MANAGED,
+  encryptionKey: kmsKey,
+});
+```
+
+**S3 encryption**:
+
+```typescript
+// SSE-S3 (default, no additional cost)
+const bucket = new s3.Bucket(this, 'Bucket', {
+  encryption: s3.BucketEncryption.S3_MANAGED,
+});
+
+// SSE-KMS (for fine-grained access control)
+const bucket = new s3.Bucket(this, 'Bucket', {
+  encryption: s3.BucketEncryption.KMS,
+  encryptionKey: kmsKey,
+});
+```
+
+**SQS/SNS encryption**:
+
+```typescript
+const queue = new sqs.Queue(this, 'Queue', {
+  encryption: sqs.QueueEncryption.KMS,
+  encryptionMasterKey: kmsKey,
+});
+
+const topic = new sns.Topic(this, 'Topic', {
+  masterKey: kmsKey,
+});
+```
+
+### Encryption in Transit
+
+**All AWS service APIs use TLS**:
+- API Gateway endpoints use HTTPS by default
+- Lambda to AWS service communication encrypted
+- EventBridge, SQS, SNS use TLS
+- Custom domains can use ACM certificates
+
+```typescript
+// API Gateway with custom domain
+const certificate = new acm.Certificate(this, 'Certificate', {
+  domainName: 'api.example.com',
+  validation: acm.CertificateValidation.fromDns(hostedZone),
+});
+
+const api = new apigateway.RestApi(this, 'Api', {
+  domainName: {
+    domainName: 'api.example.com',
+    certificate,
+  },
+});
+```
+
+### Data Sanitization
+
+**Validate and sanitize inputs**:
+
+```typescript
+import DOMPurify from 'isomorphic-dompurify';
+import { z } from 'zod';
+
+// Schema validation
+const OrderSchema = z.object({
+  orderId: z.string().uuid(),
+  amount: z.number().positive(),
+  email: z.string().email(),
+});
+
+export const handler = async (event: any) => {
+  const body = JSON.parse(event.body);
+
+  // Validate schema
+  const result = OrderSchema.safeParse(body);
+  if (!result.success) {
+    return {
+      statusCode: 400,
+      body: JSON.stringify({ error: result.error }),
+    };
+  }
+
+  // Sanitize HTML inputs
+  const sanitized = {
+    ...result.data,
+    description: DOMPurify.sanitize(result.data.description),
+  };
+
+  await processOrder(sanitized);
+};
+```
+
+## Network Security
+
+### VPC Configuration
+
+**Lambda in VPC for private resources**:
+
+```typescript
+const vpc = new ec2.Vpc(this, 'Vpc', {
+  maxAzs: 2,
+  natGateways: 1,
+});
+
+// Lambda in private subnet
+const vpcFunction = new NodejsFunction(this, 'VpcFunction', {
+  entry: 'src/handler.ts',
+  vpc,
+  vpcSubnets: {
+    subnetType: ec2.SubnetType.PRIVATE_WITH_EGRESS,
+  },
+  securityGroups: [securityGroup],
+});
+
+// Security group for Lambda
+const securityGroup = new ec2.SecurityGroup(this, 'LambdaSG', {
+  vpc,
+  description: 'Security group for Lambda function',
+  allowAllOutbound: false, // Restrict outbound
+});
+
+// Only allow access to RDS
+securityGroup.addEgressRule(
+  ec2.Peer.securityGroupId(rdsSecurityGroup.securityGroupId),
+  ec2.Port.tcp(3306),
+  'Allow MySQL access'
+);
+```
+
+### VPC Endpoints
+
+**Use VPC endpoints for AWS services**:
+
+```typescript
+// S3 VPC endpoint (gateway endpoint, no cost)
+vpc.addGatewayEndpoint('S3Endpoint', {
+  service: ec2.GatewayVpcEndpointAwsService.S3,
+});
+
+// DynamoDB VPC endpoint (gateway endpoint, no cost)
+vpc.addGatewayEndpoint('DynamoDBEndpoint', {
+  service: ec2.GatewayVpcEndpointAwsService.DYNAMODB,
+});
+
+// Secrets Manager VPC endpoint (interface endpoint, cost applies)
+vpc.addInterfaceEndpoint('SecretsManagerEndpoint', {
+  service: ec2.InterfaceVpcEndpointAwsService.SECRETS_MANAGER,
+  privateDnsEnabled: true,
+});
+```
+
+### Security Groups
+
+**Principle of least privilege for network access**:
+
+```typescript
+// Lambda security group
+const lambdaSG = new ec2.SecurityGroup(this, 'LambdaSG', {
+  vpc,
+  allowAllOutbound: false,
+});
+
+// RDS security group
+const rdsSG = new ec2.SecurityGroup(this, 'RDSSG', {
+  vpc,
+  allowAllOutbound: false,
+});
+
+// Allow Lambda to access RDS only
+rdsSG.addIngressRule(
+  ec2.Peer.securityGroupId(lambdaSG.securityGroupId),
+  ec2.Port.tcp(3306),
+  'Allow Lambda access'
+);
+
+lambdaSG.addEgressRule(
+  ec2.Peer.securityGroupId(rdsSG.securityGroupId),
+  ec2.Port.tcp(3306),
+  'Allow RDS access'
+);
+```
+
+## Security Monitoring
+
+### CloudWatch Logs
+
+**Enable and encrypt logs**:
+
+```typescript
+new NodejsFunction(this, 'Function', {
+  entry: 'src/handler.ts',
+  logRetention: logs.RetentionDays.ONE_WEEK,
+  logGroup: new logs.LogGroup(this, 'LogGroup', {
+    encryptionKey: kmsKey, // Encrypt logs
+    retention: logs.RetentionDays.ONE_WEEK,
+  }),
+});
+```
+
+### CloudTrail
+
+**Enable CloudTrail for audit**:
+
+```typescript
+const trail = new cloudtrail.Trail(this, 'Trail', {
+  isMultiRegionTrail: true,
+  includeGlobalServiceEvents: true,
+  managementEvents: cloudtrail.ReadWriteType.ALL,
+});
+
+// Log Lambda invocations
+trail.addLambdaEventSelector([{
+  includeManagementEvents: true,
+  readWriteType: cloudtrail.ReadWriteType.ALL,
+}]);
+```
+
+### GuardDuty
+
+**Enable GuardDuty for threat detection**:
+- Analyzes VPC Flow Logs, DNS logs, CloudTrail events
+- Detects unusual API activity
+- Identifies compromised credentials
+- Monitors for cryptocurrency mining
+
+## Security Best Practices Checklist
+
+### Development
+
+- [ ] Validate and sanitize all inputs
+- [ ] Scan dependencies for vulnerabilities
+- [ ] Use least privilege IAM permissions
+- [ ] Store secrets in Secrets Manager or Parameter Store
+- [ ] Never log sensitive data
+- [ ] Enable encryption for all data stores
+- [ ] Use environment variables for configuration, not secrets
+
+### Deployment
+
+- [ ] Enable CloudTrail in all regions
+- [ ] Configure VPC for sensitive workloads
+- [ ] Use VPC endpoints for AWS service access
+- [ ] Enable GuardDuty for threat detection
+- [ ] Implement resource-based policies
+- [ ] Use AWS WAF for API protection
+- [ ] Enable access logging for API Gateway
+
+### Operations
+
+- [ ] Monitor CloudTrail for unusual activity
+- [ ] Set up alarms for security events
+- [ ] Rotate secrets regularly
+- [ ] Review IAM policies periodically
+- [ ] Audit function permissions
+- [ ] Monitor GuardDuty findings
+- [ ] Implement automated security responses
+
+### Testing
+
+- [ ] Test with least privilege policies
+- [ ] Validate error handling for security failures
+- [ ] Test input validation and sanitization
+- [ ] Verify encryption configurations
+- [ ] Test with malicious payloads
+- [ ] Audit logs for security events
+
+## Summary
+
+- **Shared Responsibility**: AWS handles infrastructure, you handle application security
+- **Least Privilege**: Use IAM grant methods, avoid wildcards
+- **Encryption**: Enable encryption at rest and in transit
+- **Input Validation**: Validate and sanitize all inputs
+- **Dependency Security**: Scan and update dependencies regularly
+- **Monitoring**: Enable CloudTrail, GuardDuty, and CloudWatch
+- **Secrets Management**: Use Secrets Manager, never environment variables
+- **Network Security**: Use VPC, security groups, and VPC endpoints appropriately