How does overwhelming a server with rapid stream cancellations cause denial of service?

Question

The HTTP/2 protocol, designed to enhance web performance by allowing multiple data streams over a single connection, harbors a vulnerability that can be exploited for denial-of-service (DoS) attacks. This exploit, often termed 'Fast Reset,' leverages the protocol's ability for clients to cancel individual streams. Specifically, clients can send an RST_STREAM frame to abruptly terminate a previously initiated request. Attackers weaponize this by opening a multitude of HTTP/2 streams simultaneously and then, without waiting for server responses, immediately dispatching RST_STREAM frames to cancel these requests.

Accepted Answer

The core mechanism of the Fast Reset attack involves an aggressive cycle of request initiation followed by rapid cancellation. By packing multiple HEADERS frames (which initiate requests) and subsequent RST_STREAM frames within a single connection, malicious actors can dramatically increase the number of requests processed per second by the target server. This surge in activity, though composed of cancelled requests, places an immense burden on the server's resources.

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How does overwhelming a server with rapid stream cancellations cause denial of service?

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