A cache key is the identifier a cache uses to store and retrieve a response. It is usually built from request details such as the URL, query string, selected headers, cookies, or device and language hints. If two requests produce the same key, the cache may treat them as the same object and reuse one stored response for both.
In cyber security, the cache key matters because it helps define who gets which content. If an attacker can influence the key, they may cause cache poisoning, make private data appear in a public context, or trigger access-control bypasses by reusing a response that should have been checked again. Defenders reduce risk by separating authenticated and unauthenticated content, normalizing input, using correct Vary behavior, and avoiding caching sensitive responses in shared reverse proxies or CDNs.