Securing the Edge: CloudFront, WAF, and DDoS Mitigation

Protecting web applications from malicious attacks, automated scrapers, vulnerability scanners, and Distributed Denial of Service (DDoS) attempts is a critical priority for site reliability engineers. If malicious requests reach your application servers or database, they will consume compute resources, inflate cloud bills, and potentially cause application outages.

The most effective way to secure workloads is to intercept and mitigate threats at the cloud edge—before they ever reach your origin network infrastructure.

Here is how to design edge security on AWS using Amazon CloudFront and AWS WAF.

Shielding the Origin with CloudFront

Amazon CloudFront is a Content Delivery Network (CDN) that caches content at global edge locations. While CDNs are primarily used for performance, they serve as a powerful security shield:

  • Hiding the Origin: By placing CloudFront in front of your Application Load Balancers (ALBs), you hide your origin IP addresses from the public internet. This prevents attackers from bypassing your edge rules and attacking your servers directly.
  • Enforcing Origin Access Control (OAC): For static content stored in S3, configure Origin Access Control so that the S3 buckets only accept requests signed by your CloudFront distribution, preventing public access to your backend files.
  • Protocol Downgrade Protection: CloudFront handles SSL/TLS termination at the edge, protecting your origin from resource intensive TLS handshakes and enforcing modern security protocols (like TLS 1.3).

Implementing AWS WAF (Web Application Firewall)

AWS WAF is deployed directly on CloudFront or Application Load Balancers. It inspects incoming HTTP requests and allows you to block, allow, or count requests based on customizable rules.

To establish basic edge security, SREs should implement these WAF rulesets:

1. AWS Managed Rule Groups

AWS provides pre-configured rulesets that protect against common attack vectors. SREs should enable:

  • Core Rule Set (CRS): Protects against OWASP Top 10 vulnerabilities, including SQL injection, cross-site scripting (XSS), and local file inclusion (LFI).
  • Known Bad Inputs: Blocks requests containing known malicious payloads or patterns.
  • Amazon IP Reputation List: Blocks requests originating from botnets, scanners, and compromised IPs.

2. Rate-Limiting Rules

One of the most effective ways to prevent application layer DDoS attacks and credential stuffing is rate-limiting.

  • Configure a rate-based rule that blocks or challenges (via CAPTCHA) any IP address that makes more than 300 requests in a 5-minute window to your login or checkout endpoints.

3. Geographic Blocking

If your business only operates in specific countries (such as Ghana or West Africa), configure WAF to block or prompt CAPTCHAs for traffic originating from regions where you have no customers. This drastically reduces scanner and bot traffic.

DDoS Mitigation: AWS Shield Standard & Advanced

AWS protects all infrastructure endpoints against infrastructure layer (Layer 3 and 4) DDoS attacks by default:

  • AWS Shield Standard: Included automatically at no extra cost. It defends against common DDoS attacks (like SYN floods, UDP reflection attacks, and state-exhaustion attempts) at the network layer.
  • AWS Shield Advanced: A paid tier that provides automated Layer 7 DDoS mitigation, 24/7 access to the AWS DDoS Response Team (DRT), and financial protection against cloud bill spikes caused by DDoS attacks.

Summary

Securing the edge is the first line of defense in modern SRE. By hiding your origins behind CloudFront, enforcing WAF rate limits and managed rules, and utilizing AWS Shield, you can mitigate malicious traffic at the edge, ensuring your application servers remain fast, cost-effective, and highly available for real users.