WAF (web application firewall)

A filter in front of a web app that blocks malicious HTTP requests before they reach your code.

A WAF inspects requests against managed rules for known attack patterns and against your own rules: block a path, a country, a bot, a payload. When a vulnerability in a framework is published, a WAF rule can buy time until the patch ships. It reduces risk but does not fix the code; a request it misses still reaches the bug.

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Related terms

Rate limiting: Capping how many requests a user, key or IP can make in a time window, and answering HTTP 429 above it.
CSP (Content Security Policy): An HTTP header that tells the browser which scripts, styles and connections a page may use, limiting the damage of injected code.
Edge computing (Workers): Running code in data centers close to the user, on the CDN, instead of in one central region.
Zero-day: A vulnerability attackers know about or exploit before the vendor has a fix, so defenders have had zero days to patch.

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