AWS WAF Anti-DDoS Customization Real-World Scenarios and Best Practices

Introduction

In today’s web, threats are evolving fast, and application-layer (Layer 7) Distributed Denial of Service (DDoS) attacks are becoming more frequent and sophisticated. Simply relying on infrastructure-level protections (such as the network or transport layer) is no longer enough. That’s where AWS WAF’s new Anti-DDoS Amazon Managed Rule (AMR) group comes in, offering powerful, adaptive protection for AWS-hosted web apps. However, beyond the default setup, you may want to customize how your application responds to potential attacks.

In this post, we’ll explore:

• How Anti-DDoS AMR works under the hood
• Why customization matters, and when to do it
• Several real-world customization scenarios to match your app’s behavior and risk profile

How Anti-DDoS AMR Works (At a High Level)

• Once enabled for your web ACL (Web Access Control List), Anti-DDoS AMR begins to observe and learn normal traffic patterns for your application.
• When traffic behavior deviates significantly, indicating a potential request flood or attack, the rule group applies labels to suspect requests. Typical labels include:
• event-detected (overall anomaly detected)
• ddos-request (requests suspected to be part of the attack)
• high-suspicion-ddos-request (requests with high confidence of malicious intent)
• Once labeled, subsequent AWS WAF rules, whether default or custom, can inspect these labels and take action (such as blocking or challenging), providing fine-grained control over mitigation.
• Default mitigation by Anti-DDoS AMR employs a combination of silent challenges (e.g., JavaScript-based challenges for browser clients) and hard blocking (for highly suspicious requests) to protect applications while minimizing disruption to legitimate traffic.

This labeling and layered rule logic provides a great foundation, but it also opens up opportunities for customization.

Why You Might Want to Customize (Not Just Use Defaults)?

The default protections are often fine for standard web applications. But real-world apps aren’t always standard. Consider:

• APIs consumed by mobile apps or non-browser clients that can’t handle browser-based challenges.
• Single-page applications (SPAs) or background fetch requests, where a challenge disrupts the functionality.
• Sensitive workflows where you want stricter blocking, or situationally more lenient behavior.
• Regulatory or compliance requirements that demand alerting, logging, or custom response codes rather than silent challenges.

In these cases, customizing the AWS WAF + AMR rules can help you strike the right balance between security, usability, and user experience.

Customization Techniques: 3 Common Scenarios

Below are three patterns you can use to adapt Anti-DDoS AMR to your environment.

Scenario 1: API-Only Clients (No Browser Challenge)

Problem: Your application has mobile apps or backend services calling APIs. The browser-based JS challenge will fail.

Solution: In your Web ACL, configure the Anti-DDoS AMR rule group.

Immediately after it, add a custom rule that matches ddos-request (or other relevant labels) and blocks or rate-limits these requests, instead of issuing a browser challenge.

Benefit: Clients don’t get broken by browser-only challenges; API calls are defended via rate limiting or blocking as appropriate.

Scenario 2: Gentle Handling for First-Time Spikes, Strict on Sustained / High-Suspicion Traffic

Problem: Legitimate traffic spikes (e.g., marketing campaigns) may appear to be DDoS attacks. You don’t want false positives, but you also don’t want attackers to exploit the spike window.

Solution:

Keep Anti-DDoS AMR’s built-in labeling and default challenge for low-suspicion requests.

Add custom WAF rules for high-suspicion labels (e.g., high-suspicion-ddos-request) that block outright or send stricter rate limits.

Benefit: Good user experience under legitimate load, while still catching real attacks under high suspicious traffic.

Scenario 3: Custom Response Codes, Logging & Alerting for Analytics / Security Audit

Problem: For compliance or internal monitoring purposes, you want to track all mitigation events, log them with metadata, and possibly return custom response codes (e.g., 429 Too Many Requests, 403 Forbidden) instead of the default block.

Solution:

Use AWS WAF’s custom rule functionality, after the managed rule group, add a rule matching the AMR labels (ddos-request, etc.).

In that custom rule, choose a Block or Count + Custom response code action.

Attach logging via CloudWatch or your logging pipeline to capture metadata for each blocked request.

Benefit: Good observability and audit trail; smoother integration with security analytics or compliance systems.

Additional Best Practices & Considerations

• Give the rule group time to learn baseline traffic, typically 10–15 minutes after enabling. Without a baseline, detection can misidentify legitimate traffic as an attack.
• Order your Web ACL rules carefully. Managed rule-group should usually sit high in the rule list (but below any explicit Allow rules). This ensures it sees all traffic first, but doesn’t block trusted known traffic prematurely.
• Use combined protection layers:
• Network layer protection (e.g., AWS Shield / Shield Standard) for Layer 3/4 attacks,
• WAF + AMR for application layer (Layer 7), possibly behind a CDN (like CloudFront) for additional edge-level protection.
• Test under load / simulate traffic spikes to help you fine-tune thresholds and avoid false positives during legitimate traffic bursts. AWS supports controlled simulation testing of DDoS for resilience testing.
• Monitor and log events, use AWS WAF logs + Amazon CloudWatch metrics to detect patterns, investigate false positives or evaded attacks, and continuously refine your rules.

When to Use AMR vs When to Use Full DDoS Response (AWS Shield Advanced + AWS WAF)?

• AMR is great for many typical web apps and API backends, it’s fast, pay-as-you-go, and often enough for Layer 7 request floods.
• For mission-critical workloads, or when you need guaranteed protection, global edge mitigation, or 24/7 DDoS response team (DRT) support, pairing Shield Advanced + AWS WAF with AMR gives the most robust defense.

Conclusion

The real strength lies in thinking beyond default, customizing rule logic, and integrating logging/monitoring for long-term resilience.

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