A Web Application Firewall, or WAF, sits between your website and incoming traffic. Its job is to inspect requests and decide what should reach the application. The tricky part is how you tell it what looks safe and what doesn’t.
That’s where blocklists and allowlists come in. They take almost opposite approaches.
How a Blocklist WAF Works
A blocklist WAF starts by allowing traffic through. It then looks for requests that match known bad patterns and blocks them. Think of it as having a guest list for troublemakers rather than checking every person at the door.
A rule might block a request containing a known SQL injection pattern. Another rule could catch a familiar cross-site scripting attempt. Because new attack patterns appear all the time, these rules often need regular updates.
Where Blocklists Fit
• Broad access by default, which keeps the site convenient for unknown visitors
• Known attack patterns get stopped at the WAF, although new or cleverly changed attacks can slip past
• Less restrictive for growing applications, and that’s often exactly what a busy public website needs
How an Allowlist WAF Works
An allowlist WAF flips the idea around. Instead of asking, “Does this request look dangerous?” it asks, “Is this request supposed to be here?”
Only traffic that matches approved rules gets through. Everything else is rejected. So if an application has a small set of known users or predictable request patterns, this approach can be extremely strict.
Why Allowlists Are Stricter
Imagine an internal company application where employees access only a few specific functions. The security team already knows what valid requests should look like. An allowlist can restrict access to those expected patterns and shut out anything unusual.
Blocklist vs Allowlist in Practice
• Blocklist means traffic is accepted unless it matches something the WAF knows it should reject.
• Allowlist starts from a stricter position. Requests need to fit an approved pattern before they reach the application.
• Public websites usually need flexibility, while tightly controlled applications often benefit from stronger restrictions.
Which Approach Should You Use?
For a typical public website, I’d lean toward a well-maintained blocklist approach. It’s easier to handle unknown visitors and changing user behaviour without constantly updating firewall rules.
An allowlist is a better fit when you genuinely know what valid traffic looks like. Internal tools are a common example. So are APIs with tightly defined request formats.
And there’s no rule saying a security setup has to rely on only one idea. A WAF can combine broad blocking rules with specific allow rules for sensitive parts of an application.