Web Cache Overflow: Exploiting Imprecise Keys for Cache Degradation and Beyond

Abstract

Web caches support the scalability needs of contemporary web applications by storing frequently accessed objects closer to clients. Web caches are conceptually associative arrays, tracking stored objects using cache keys consisting of HTTP request fields. However, these cache keys are often imprecisely defined by website operators. This allows clients to craft a multitude of requests that target the same object, but map to different cache keys. In this work, we show that request elements included unnecessarily in cache keys can be abused to create redundant cache entries. In susceptible deployments, sustained generation of such redundant copies reduces cache effectiveness and increases origin load, facilitating eviction-dependent attacks. Our experiments reproduce cache degradation across five stand-alone caching proxies and characterize how these parameters affect attacker cost and cache hit rate, potentially resulting in denial-of-service attacks. We conclude that precise cache-key design is the most direct mitigation against this abuse vector and should be recognized as a security best practice.