Web Application Firewalls (WAF)

TL;DR:

A Web Application Firewall (WAF) acts as a security gate between your web service and incoming HTTP requests. It detects and blocks malicious HTTP traffic before vulnerabilities can be exploited. If your app is online, a WAF is one of the best first lines of defense against application layer attacks.

• Protects: Web applications, APIs, online services
• Type: Cloud Security Posture Management (CSPM)
• Fits in SDLC: Deploy and Runtime phases
• AKA: Web Firewall, Layer 7 Firewall
• Support: AWS WAF, Cloudflare WAF, Imperva, Fastly, Nginx WAF

What is a WAF?

A Web Application Firewall (WAF) is like a proxy server for your web app—it inspects incoming HTTP requests and blocks malicious HTTP traffic before reaching your backend. Unlike network firewalls that filter packets based on IP and port, WAFs focus on application layer attacks, stopping threats like:

• SQL Injection Attacks – Blocks attackers from injecting malicious database queries.
• Cross-Site Scripting (XSS) – Prevents scripts from being injected into web pages.
• DDoS Attacks – Detects and mitigates volumetric attacks before they take down your site.
• Cookie Poisoning – Protects against tampering with session cookies.
• Zero-Day Exploits – Provides real-time protection against new vulnerabilities.

Types of WAFs

There are three main types of WAFs, each suited for different environments:

• Network-Based WAFs – Deployed as hardware appliances or integrated into network infrastructure.
• Host-Based WAFs – Installed directly on a web server for granular control over security policies.
• Cloud-Based WAFs – Delivered as a managed service, providing scalability and simplified deployment.

Pros and Cons of WAFs

Pros:

• Real-time protection – Blocks attacks before they reach your app.
• Customizable security rules – Fine-tune protection for your specific use case.
• Helps meet compliance requirements – Required for PCI-DSS, GDPR, and other security frameworks.
• API security built-in – Many WAFs protect APIs as well as web apps.
• Centralized protection – Provides a single security layer across multiple applications.

Cons:

• Can generate false positives – Overly strict rules might block legitimate users.
• Latency concerns – Some WAFs add a slight delay to HTTP traffic.
• Needs regular updates – Attackers constantly evolve, so WAF rules need to keep up.

What Does a WAF Do Exactly?

A WAF filters and inspects HTTP requests based on:

• Known attack signatures – Blocks traffic that matches patterns of known threats.
• Behavioral analysis – Detects anomalies in traffic to prevent application layer attacks.
• Geo-blocking & rate limiting – Stops excessive requests and suspicious traffic sources.
• Bot mitigation – Identifies and blocks malicious bot traffic.

What Does a WAF Protect You From?

• SQL Injection Attacks – Prevents malicious database queries.
• Cross-Site Scripting (XSS) – Blocks script injections into web pages.
• API Abuse – Protects against unauthorized API access and credential stuffing.
• Distributed Denial-of-Service (DDoS) – Prevents attackers from overwhelming your servers.
• Cookie Poisoning – Defends against tampering with authentication cookies.

How Does a WAF Work?

A WAF operates by:

1. Intercepting incoming requests – All HTTP traffic flows through the WAF before reaching the app.
2. Analyzing request data – Inspects headers, payloads, and URLs for threats.
3. Applying security rules – Matches traffic against predefined attack patterns.
4. Blocking or allowing requests – Malicious requests get blocked, while safe traffic passes through.

Why and When Do You Need a WAF?

You need a WAF when:

• Your app is exposed to the internet – Publicly accessible web services and APIs are prime targets.
• You handle sensitive data – If your app processes personal or financial data, you need protection.
• You want to block automated threats – WAFs stop bot-driven attacks like credential stuffing and scraping.
• You need compliance requirements – WAFs help meet security standards like PCI-DSS, SOC 2, and more.

Where Does a WAF Fit in the SDLC Pipeline?

A WAF is primarily used in the Deploy and Runtime phases:

• Deploy Phase: Configure the WAF to protect HTTP traffic for web and API endpoints.
• Runtime Phase: Actively monitors HTTP requests and blocks attacks in real time.

How Do You Choose the Right WAF?

A good WAF should:

• Support your infrastructure – Works with cloud-based WAFs, host-based WAFs, and network-based WAFs.
• Use AI & behavioral detection – Detects application layer attacks, not just known threats.
• Provide real-time monitoring – Alerts you to attacks as they happen.
• Offer API protection – Ensures your API endpoints aren’t vulnerable to abuse.

Best WAFs 2025

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WAF FAQs

1. Do I need a WAF if I already use a firewall?

Yes. Traditional firewalls protect network traffic, while WAFs protect application-layer traffic. A regular firewall won’t stop SQL injection attacks, XSS, or API abuse—that’s the WAF’s job.

2. Can a WAF prevent all attacks?

No security tool is foolproof. A WAF significantly reduces risk, but it should be combined with other security measures like SAST, DAST, and API Security for robust application security.

3. Will a WAF slow down my website?

If configured properly, the impact is minimal. Many cloud-based WAFs use caching and optimized processing to keep latency low while still blocking attacks.

4. How does a WAF handle encrypted traffic (HTTPS)?

Most WAFs support SSL/TLS termination, meaning they decrypt HTTP requests, inspect them for threats, and then re-encrypt them before forwarding them to your app.

5. Is a cloud-based or on-prem WAF better?

Depends on your setup. Cloud-based WAFs (like AWS WAF and Cloudflare) are easy to manage and scale automatically. Host-based WAFs and network-based WAFs offer more customization and control but require manual maintenance.

6. What’s the difference between a WAF and an NGFW (Next-Gen Firewall)?

A WAF focuses on HTTP traffic and application-layer security, stopping attacks like SQL injection and XSS. An NGFW (Next-Gen Firewall) is broader—it includes intrusion prevention, deep packet inspection, and network-based filtering. While they overlap, a WAF is specialized for web application security, while an NGFW is a more general network security solution.