AWS mTLS Configuration: Comparing ALB and API Gateway for Secure Client Authentication

In today’s increasingly security-conscious cloud environments, mutual TLS (mTLS) has become vital for authenticating and authorizing clients. AWS offers multiple services to implement mTLS, but the most commonly used are Application Load Balancer (ALB) and Amazon API Gateway. This article dives into how both options support mTLS and compares their configuration, scalability, security features, and use cases, helping you choose the best fit for your application.

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What is mTLS?

Mutual TLS is a security protocol in which the client and the server authenticate each other using certificates during the SSL/TLS handshake. Unlike one-way TLS (typical HTTPS), where only the server is authenticated, mTLS ensures that only verified clients can access resources, providing strong identity assurance.

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Implementing mTLS with AWS Application Load Balancer (ALB)

Features

• Support via TLS Listener on ALB (HTTPS): You can enable mTLS on ALB using a TLS listener with an HTTPS protocol.
  
  

• Trusted Certificate Authorities (CAs): ALB supports validating client certificates against a list of trusted CAs stored in AWS Certificate Manager (ACM).
  
  

• Header Forwarding: ALB injects the verified client certificate into the HTTP headers, allowing backend applications to consume it.
  
  

Configuration Steps

1. Create a TLS certificate using ACM.
   
   

2. Configure the ALB listener with mTLS enabled.
   
   

3. Upload the trusted client CA to ACM.
   
   

4. Enable client certificate forwarding.
   
   

Pros

• Suitable for microservices and container workloads (e.g., ECS, EKS, EC2).
  
  

• Scales horizontally with ease.
  
  

• Easier to integrate with existing network security policies.
  
  

Cons

• Requires application logic to parse forwarded certificates.
  
  

• Slightly more complex header validation on backend services.
  
  

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Implementing mTLS with Amazon API Gateway

Features

• Native mTLS Support: Built-in support for mTLS with custom domain names.
  
  

• No Need for Header Parsing: API Gateway handles validation and rejection of unauthenticated clients at the edge.
  
  

• Custom CA Integration: API Gateway supports specifying a truststore (S3 bucket) for client CA certificates.
  
  

Configuration Steps

1. Create and upload the client CA certificate to an S3 bucket.
   
   

2. Configure a custom domain name in API Gateway.
   
   

3. Enable mTLS and reference the S3 truststore.
   
   

Pros

• Strong edge-level protection.
  
  

• No need to parse certificates in your application logic.
  
  

• Ideal for serverless workloads (Lambda) and RESTful APIs.
  
  

Cons

• mTLS is only available for custom domain names, not default endpoints.
  
  

• Slightly higher latency due to gateway processing.
  
  

• API Gateway has hard limits (e.g., payload size, rate limits).
  
  

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Side-by-Side Comparison

Feature Comparison: ALB vs API Gateway

• mTLS Support
  
  

• ALB: Yes, via HTTPS listener
  
  

• API Gateway: Yes, but only for custom domains
  
  

• Certificate Truststore
  
  

• ALB: AWS Certificate Manager (ACM)
  
  

• API Gateway: S3 Bucket
  
  

• Client Certificate Forwarding
  
  

• ALB: Supported via headers
  
  

• API Gateway: Not supported (client certificates are validated and blocked if invalid)
  
  

• Ideal For
  
  

• ALB: Microservices, EKS, EC2
  
  

• API Gateway: Lambda, Serverless APIs
  
  

• Scalability
  
  

• ALB: High
  
  

• API Gateway: Moderate to High
  
  

• Application Parsing
  
  

• ALB: Required
  
  

• API Gateway: Not required

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Choosing the Right Option

Use ALB if:

• You are using ECS, EKS, or EC2 to host applications.
  
  

• You require layer-7 routing and direct access to certificate data.
  
  

• Your architecture involves ingress controllers or service mesh integrations.
  
  

Use API Gateway if:

• You're building serverless applications with AWS Lambda.
  
  

• You want simplified mTLS enforcement at the edge.
  
  

• You don't want to manage header parsing or downstream cert validation.
  
  

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Conclusion

AWS ALB and API Gateway provide robust mTLS support, each catering to different architectural needs. ALB offers flexibility and integration for container and VM-based workloads, while API Gateway simplifies edge protection for serverless APIs. By understanding the strengths and trade-offs of each, you can implement a secure and scalable client authentication mechanism tailored to your application’s needs.