RST_STREAM is an HTTP/2 frame used to cancel a stream immediately. It tells the peer that the current request or response on that stream should stop and be treated as reset. This is normal protocol behavior, not a sign of attack by itself.
It matters in security because stream resets change application state while data may still be in flight. Clients and servers must release buffers, stop parsing, and clean up objects safely when RST_STREAM arrives. If that cleanup is wrong, a routine protocol event can trigger crashes, use-after-free bugs, or other memory-safety failures. Attackers may abuse this by sending resets at precise times during uploads or long-lived requests to stress error paths. Defenders should treat repeated resets, crashes during HTTP/2 transfers, and unusual reset patterns as signals to inspect client and server handling code, especially in libraries embedded in other products.