Web applications are constantly probed by automated tools looking for common vulnerabilities: injecting malicious SQL into a login form, embedding harmful scripts into a comment field, or simply hammering an endpoint with requests far faster than any real user could type. A traditional network firewall, which mostly looks at IP addresses and ports, cannot see far enough into these requests to recognize this kind of misuse — it operates at a lower level than the actual content of a web request.

AWS WAF (Web Application Firewall) is a managed service that inspects the actual content of HTTP and HTTPS requests — headers, query strings, request bodies, and more — against a set of rules you define or subscribe to, then allows, blocks, or counts each request based on the outcome.

A web ACL (Access Control List) is the top-level container attached to the resource you want to protect — such as a CloudFront distribution, an Application Load Balancer, or an API Gateway API. Inside a web ACL sit one or more rules, each defining a condition to match and an action to take. Rules can be grouped into reusable rule groups, including AWS-provided managed rule groups covering common threats.

flowchart TB
    A[Incoming Request] --> B[Web ACL]
    B --> C[Rule 1 - Managed Rule Group]
    C -->|No Match| D[Rule 2 - Rate-Based Rule]
    D -->|No Match| E[Rule 3 - Custom Rule]
    E -->|No Match| F[Default Action - Allow]
    C -->|Match - Block| G[Request Blocked]
    D -->|Match - Block| G
    E -->|Match - Block| G
    F --> H[Protected Resource]
        

Rules within a web ACL are evaluated in a defined priority order, and evaluation typically stops as soon as a blocking rule matches — meaning rule order matters just as much as the rules themselves.

When a request arrives at a protected resource, AWS WAF inspects it before the resource’s normal processing begins. Each rule defines a statement — the condition to check, such as “does the query string contain a known SQL injection pattern” — and AWS WAF evaluates these statements against the actual request content: headers, the URI path, query parameters, cookies, and even the request body.

Managed rule groups, maintained by AWS or AWS Marketplace security vendors, are continuously updated behind the scenes to reflect newly discovered attack patterns, meaning your protection can improve over time without you manually rewriting rules yourself.

This entire evaluation happens inline, adding minimal delay, before the protected resource ever begins its own normal processing — meaning a blocked request never consumes any backend compute or database resources at all.

The trade-off is broad, automated protection versus the need for ongoing tuning. AWS WAF stops a wide range of known attack patterns immediately, but achieving low false-positive rates for your specific application traffic takes deliberate observation and adjustment over time.

AWS WAF is a fully managed, serverless service — it scales automatically alongside the traffic reaching the resource it protects, with no capacity to provision or manage. Because it evaluates rules inline and close to the protected resource (especially when attached to CloudFront, at the network edge), the added latency for typical rule sets is generally minimal.

Each rule consumes a certain number of Web ACL Capacity Units (WCUs), a measure of the computational complexity a rule requires, and every web ACL has a maximum WCU budget — meaning very complex rule sets need to be balanced against this ceiling, encouraging efficient, well-targeted rules over excessive complexity.

As a fully managed AWS service, WAF’s own infrastructure runs redundantly across AWS’s underlying network without requiring you to configure or manage that redundancy. When attached to CloudFront, rule evaluation happens across CloudFront’s globally distributed edge network, inheriting that same broad resilience.

A key reliability practice is testing new or modified rules in count mode first — where WAF logs what it would have done without actually blocking traffic — before switching them to active blocking, avoiding the risk of an untested rule accidentally disrupting legitimate user traffic in production.

AWS WAF itself is a security service, but access to configure it still needs to be tightly controlled. IAM policies determine who can create, modify, or attach web ACLs, preventing unauthorized changes to your protection rules. WAF also integrates with AWS Firewall Manager for centrally enforcing consistent WAF policies across many accounts in an organization.

Amazon CloudWatch automatically tracks metrics per web ACL and per rule, including allowed, blocked, and counted request volumes — an essential dashboard for understanding what your rules are actually doing in production. For deeper detail, WAF logs can be streamed to Amazon S3, CloudWatch Logs, or Kinesis Data Firehose, capturing full details of every evaluated request.

Running new rules in count mode while reviewing these logs is the standard way to validate a rule’s real-world behavior before switching it to actively block traffic.

AWS WAF can be attached to several different AWS resource types, each suited to a different architecture. Attaching to Amazon CloudFront protects content at the network edge, closest to users globally. Attaching to an Application Load Balancer protects traffic reaching EC2 or container-based applications within a specific region. Attaching to API Gateway or AWS AppSync protects REST, HTTP, or GraphQL APIs directly.

Choosing where to attach WAF often comes down to where the traffic actually enters your architecture first — attaching as early as possible in the request path generally offers the strongest protection with the least wasted backend processing on malicious requests.

A common, layered pattern combines AWS Managed Rules for baseline coverage with a small number of custom rules tailored to the specific application’s known traffic patterns and edge cases. Another pattern uses rate-based rules alongside login or checkout endpoints specifically, since these are common targets for credential-stuffing or scripted abuse.