Securing Load Balancers: A Tale of Two CloudFront Approaches

Over the years i've lost count of how many times I have seen people stack a hundred layers of protection in front of a CloudFront endpoint, only to still allow traffic via the ALB.

Today, we explore two approaches to address this:

• The tried & true custom header validation method
• Amazon's new and much improved CloudFront VPC origins feature.

The Traditional Approach: Custom Header Validation

Historically, as an option to secure communication between CloudFront and Application Load Balancers, developers would implement custom header validation. This method, while functional, required careful implementation and maintenance. Here's how it typically works:

Setting Up Custom Header Validation

1. First, you configure CloudFront to add a secret header to all origin requests:

// CloudFront Origin Custom Headers Configuration
{
    "Quantity": 1,
    "Items": [
        {
            "HeaderName": "X-Origin-Verify",
            "HeaderValue": "your-secret-value-here"
        }
    ]
}

2. Then, you need to configure your application to validate this header. Here's an example using Node.js with Express:

app.use((req, res, next) => {
    const originVerifyHeader = req.header('X-Origin-Verify');
    
    if (originVerifyHeader !== process.env.EXPECTED_HEADER_VALUE) {
        // Reject requests without the correct header
        return res.status(403).send('Access Denied');
    }
    
    next();
});

This works because the user is never privvy to this injected secret, so you know that it had to have originated from a route that was secured.

Challenges with the Header-Based Approach

While this method certainly works, it comes with several drawbacks:

1. Secrets Management: You need to securely store and distribute the header value across your infrastructure.

2. Security Risks: The secret header could potentially be leaked, requiring regular rotation and monitoring.

3. Maintenance Overhead: You must implement the validation logic in your application code and maintain it across all services.

4. Network Complexity: Your ALB still needs to be internet-accessible, requiring public subnets and careful security group configuration.

The Path Forward: CloudFront VPC Origins

Amazon's new CloudFront VPC origins feature represents a massive shift in how we secure origin access. Now, instead of relying on application level validation, security can be handled at the infrastructure level.

Key Advantages of VPC Origins

1. True Private Origins: Your ALB can now reside in a private subnet with no internet accessibility, dramatically reducing your attack surface.

2. Simplified Configuration: No need for custom headers, validation logic, or complex networking rules (If you remember having to set up the rotating of CF's valid IP addresses, you're a real one). The security is now built into the infrastructure.

3. Cost Optimization: Eliminate the need for public IP addresses and NAT gateways, potentially reducing your AWS bill (it's $50/mo per NAT gateway).

4. Reduced Complexity: Remove the need for maintaining custom security code in your application.

Setting Up VPC Origins

The setup process is straightforward:

1. Navigate to the CloudFront console
2. Select "VPC origins"
3. Choose your ALB from the private subnet (make sure its in the same account)
4. Configure basic security options
5. Deploy and associate with your distribution

That's it! No additional code, secrets management, complex networking or arcane rituals required.

Real World Impact

Let's consider your typical microservices architecture with multiple ALBs. Under the previous approach, you would need:

• Custom header validation code in each service
• Secrets management for header values
• Public subnets for each ALB
• NAT gateways for outbound traffic
• Security groups with internet access rules

With VPC origins, you simply:

• Keep all ALBs in private subnets
• Configure VPC origins in CloudFront
• Let AWS handle the security (they are pretty good at that)

Keep In Mind

While VPC origins provides excellent infrastructure-level security, remember to maintain defence in depth:

• Use AWS WAF
• Implement AWS Shield for DDoS protection
• Maintain appropriate application-level security controls
• Use AWS SecurityHub for security posture monitoring

Conclusion

I cant tell you how often I have implemented the header based approach, and it certainly had its benefits. However for projects new & old, VPC origins should be the default choice for securing CloudFront to ALB communication. If you have existing projects using custom header validation, and you need a hand migrating over please get in touch!

For help with any other security related assistance reach out to us for a security assessment - we're always around to give you a hand!