WAF (Web Application Firewall)
What is it?
The Web Application Firewall inspects every HTTP request coming to a customer's website and blocks malicious ones — like SQL injection, cross-site scripting (XSS), and other attacks that try to exploit vulnerabilities in web applications. Think of it as a bouncer that checks every visitor's intentions before letting them in.
What problem does it solve?
Modern web applications are constantly under attack. Common threats include:
- SQL Injection (SQLi): An attacker inserts malicious database commands into form fields or URLs to steal or corrupt data.
- Cross-Site Scripting (XSS): Injecting malicious JavaScript into a site to steal user sessions or redirect traffic.
- Remote Code Execution (RCE): Exploiting vulnerabilities to run arbitrary code on the server.
- Zero-day exploits: Brand-new vulnerabilities that haven't been patched yet.
Without a WAF, application developers must catch and fix every vulnerability themselves — an impossible task at scale.
How does it work?
Cloudflare's WAF runs on every request that passes through the network. It applies multiple layers of rules:
Managed Rulesets: Pre-built rules maintained by Cloudflare's security team that protect against known vulnerabilities. These are updated frequently, especially when new zero-days emerge (Cloudflare often rolls out protections within hours of a new vulnerability disclosure).
OWASP Core Ruleset: Rules based on the OWASP (Open Web Application Security Project) Top 10 — the most common web application security risks.
Custom Rules: Customers can write their own rules using Cloudflare's expression language. For example: "Block all requests from country X to my login page" or "Challenge any request with a suspicious User-Agent."
Rate Limiting Rules: Block or challenge clients that send too many requests in a given timeframe — useful against brute force attacks.
Leaked Credential Detection: Automatically checks login requests against databases of stolen credentials.
Why it matters strategically
The WAF is one of Cloudflare's highest-revenue products and a key differentiator. It's the primary reason many enterprise customers adopt Cloudflare. Because Cloudflare sees such a massive percentage of global web traffic, its threat intelligence and ability to deploy protections at scale is unmatched. When a new vulnerability like Log4Shell emerges, Cloudflare can protect millions of websites within hours — before most companies even know they're vulnerable.