Software Engineering Institute

The Security Considerations section of RFC7540 discusses some of the considerations needed for HTTP/2 connections as they demand more resources to operate than HTTP/1.1 connections. While it generally covers expected behavior considerations, how to mitigate abnormal behavior is left to the implementer which can leave it open to the following weaknesses.

CVE-2019-9511, also known as Data Dribble
The attacker requests a large amount of data from a specified resource over multiple streams. They manipulate window size and stream priority to force the server to queue the data in 1-byte chunks. Depending on how efficiently this data is queued, this can consume excess CPU, memory, or both, potentially leading to a denial of service.

CVE-2019-9512, also known as Ping Flood
The attacker sends continual pings to an HTTP/2 peer, causing the peer to build an internal queue of responses. Depending on how efficiently this data is queued, this can consume excess CPU, memory, or both, potentially leading to a denial of service.

CVE-2019-9513, also known as Resource Loop
The attacker creates multiple request streams and continually shuffles the priority of the streams in a way that causes substantial churn to the priority tree. This can consume excess CPU, potentially leading to a denial of service.

CVE-2019-9514, also known as Reset Flood
The attacker opens a number of streams and sends an invalid request over each stream that should solicit a stream of RST_STREAM frames from the peer. Depending on how the peer queues the RST_STREAM frames, this can consume excess memory, CPU, or both, potentially leading to a denial of service.

CVE-2019-9515, also known as Settings Flood
The attacker sends a stream of SETTINGS frames to the peer. Since the RFC requires that the peer reply with one acknowledgement per SETTINGS frame, an empty SETTINGS frame is almost equivalent in behavior to a ping. Depending on how efficiently this data is queued, this can consume excess CPU, memory, or both, potentially leading to a denial of service.

CVE-2019-9516, also known as 0-Length Headers Leak
The attacker sends a stream of headers with a 0-length header name and 0-length header value, optionally Huffman encoded into 1-byte or greater headers. Some implementations allocate memory for these headers and keep the allocation alive until the session dies. This can consume excess memory, potentially leading to a denial of service.

CVE-2019-9517, also known as Internal Data Buffering
The attacker opens the HTTP/2 window so the peer can send without constraint; however, they leave the TCP window closed so the peer cannot actually write (many of) the bytes on the wire. The attacker then sends a stream of requests for a large response object. Depending on how the servers queue the responses, this can consume excess memory, CPU, or both, potentially leading to a denial of service.

CVE-2019-9518, also known as Empty Frame Flooding
The attacker sends a stream of frames with an empty payload and without the end-of-stream flag. These frames can be DATA, HEADERS, CONTINUATION and/or PUSH_PROMISE. The peer spends time processing each frame disproportionate to attack bandwidth. This can consume excess CPU, potentially leading to a denial of service.

These attacks can consume excessive system resources, potentially enough that a single end-system could cause issues on multiple servers that may lead to Distributed DoS (DDoS) attacks.

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