Serverless Security Best Practices for SaaS Founders

Treat Every Function as a Potential Entry Point

Serverless functions are often exposed directly via API Gateway or event sources. Any vulnerability in a function can be exploited. Validate and sanitize all inputs, including HTTP parameters, headers, and event payloads. Use allowlists for expected values and reject unexpected ones. For example, if a function expects a user ID, enforce a UUID format and reject anything else.

Apply rate limiting and throttling at the API Gateway level to prevent abuse. Use AWS WAF or equivalent to block common attack patterns like SQL injection and XSS before they reach your functions.

Use Least-Privilege IAM Roles

Each function should have its own IAM role with only the permissions it needs. Avoid using a single role for all functions. If a function only reads from one DynamoDB table, grant only dynamodb:GetItem on that table. Never grant wildcard permissions like dynamodb:* or s3:*.

Regularly review and clean up unused roles and policies. Use tools like AWS IAM Access Analyzer to detect overly permissive roles.

Encrypt Data at Rest and in Transit

Enable encryption for all data stores: databases (RDS, DynamoDB), object storage (S3), and queues (SQS). Use AWS KMS to manage encryption keys. For data in transit, enforce TLS 1.2 or higher on all API endpoints. Configure API Gateway to only accept HTTPS requests.

For sensitive data like API keys or database passwords, use a secrets manager (AWS Secrets Manager, Parameter Store) and never hardcode secrets in function code or environment variables.

Scan Dependencies and Containers

Serverless functions often use third-party libraries. Regularly scan these dependencies for known vulnerabilities. Use tools like Snyk, npm audit, or OWASP Dependency-Check. Integrate scanning into your CI/CD pipeline to block deployments with critical vulnerabilities.

If you use container images for functions (e.g., AWS Lambda container support), scan the image for vulnerabilities using tools like Trivy or Amazon ECR scanning.

Implement Proper Logging and Monitoring

Log all function invocations, including input, output, and errors. Use structured logging (JSON) to make analysis easier. Send logs to a centralized service like CloudWatch Logs, and set up alerts for suspicious patterns: repeated failures, unexpected IPs, or unusual data access.

Enable AWS CloudTrail or equivalent to audit changes to your serverless resources. Monitor for privilege escalation attempts, such as a function modifying its own IAM role.

Secure Your CI/CD Pipeline

Your deployment pipeline is a prime target. Use short-lived credentials for deployments (e.g., OIDC-based roles). Sign your code artifacts and verify signatures during deployment. Run security tests (SAST, dependency scanning) as part of the pipeline. Ensure that only authorized users can trigger deployments.

Store deployment secrets (API keys, tokens) in a secrets manager and inject them at deploy time, not in the code repository.

Test Continuously, Not Just Before Launch

Serverless applications evolve fast. New functions, dependencies, and configurations can introduce vulnerabilities. Run automated security tests regularly. Use tools that can test your live APIs for broken access control, injection, and auth bypass. For example, Kyro is an AI penetration tester for SaaS. You point it at your app URL, it continuously hunts for real, reproducible vulnerabilities, reproduces each finding before alerting, and emails you. Pay as you go, free credits to start. Start a free scan.

Also consider periodic manual penetration tests, especially before major releases. For more on this, see our guide on do I need a penetration test.