Introduction to Rapid Reset Vulnerability
What is “Rapid Reset” vulnerability?
HTTP/2 Rapid Reset is a flaw in the HTTP/2 protocol that can be exploited to carry out DDoS attacks. Because 62% of the Internet traffic we see uses HTTP/2, this is a high severity vulnerability. It has been exploited to create the largest DDoS attack.
The ‘Rapid Reset’ technique leverages the ‘stream multiplexing’ feature of HTTP/2, wherein numerous requests and subsequent immediate cancellations cause substantial server-side workload with minimal client-side attacker cost. The attack takes advantage of a feature in HTTP/2 by repeatedly sending and canceling requests, which overwhelms the target website or application, causing it to stop working correctly. HTTP/2 has a safety feature that tries to limit the number of active streams to protect against DoS attacks, but it doesn’t always work effectively. The protocol allows the client to cancel streams without needing the server’s agreement, which is exploited in this attack.
HTTP/2 uses “streams”, bidirectional abstractions used to transmit various messages, or “frames”, between the endpoints. “Stream multiplexing” is the core HTTP/2 feature which allows higher utilization of each TCP connection. Streams are multiplexed in a way that can be tracked by both sides of the connection while only using one Layer 4 connection. Stream multiplexing enables clients to have multiple in-flight requests without managing multiple individual connections.
One of the main constraints when mounting a Layer 7 DoS attack is the number of concurrent transport connections. Each connection carries a cost, including operating system memory for socket records and buffers, CPU time for the TLS handshake, as well as each connection needing a unique four-tuple, the IP address and port pair for each side of the connection, constraining the number of concurrent connections between two IP addresses.
The HTTP/2 Rapid Reset attack
The HTTP/2 protocol allows clients to indicate to the server that a previous stream should be canceled by sending a RST_STREAM frame. The protocol does not require the client and server to coordinate the cancellation in any way, the client may do it unilaterally. The client may also assume that the cancellation will take effect immediately when the server receives the RST_STREAM frame, before any other data from that TCP connection is processed.
This attack is called Rapid Reset because it relies on the ability for an endpoint to send a RST_STREAM frame immediately after sending a request frame, which makes the other endpoint start working and then rapidly resets the request. The request is canceled, but leaves the HTTP/2 connection open.
The HTTP/2 Rapid Reset attack built on this capability is simple: The client opens a large number of streams at once as in the standard HTTP/2 attack, but rather than waiting for a response to each request stream from the server or proxy, the client cancels each request immediately.
The ability to reset streams immediately allows each connection to have an indefinite number of requests in flight. By explicitly canceling the requests, the attacker never exceeds the limit on the number of concurrent open streams. The number of in-flight requests is no longer dependent on the round-trip time (RTT), but only on the available network bandwidth.
How to mitigate “Rapid Reset” vulnerability?
In mitigating HTTP/2 Rapid Reset attacks, a multi-faceted approach is essential. Utilize comprehensive HTTP-flood protection tools and enhance DDoS defenses by employing various mitigative strategies. Implementing rate controls is pivotal in managing the effects of the attacks since they exploit the protocol directly and lack a one-size-fits-all solution. Critically, the underpinning preventive measure across all defenses is the vital and timely updating and patching of systems. Organizations should update their systems with available patches to strengthen against this vulnerability, ensuring a robust barrier against exploitative attacks.
