HTTP/2 Denial of Service Attack Vulnerability Alert

The Netflix security team teamed up with Google, CERT/CC to disclose the problem of the Distributed-Denial of Service attack vulnerability in the implementation of the HTTP/2 protocol in various middleware services. Many of the attack vectors we’ve discovered (repaired today) are a key variant: a malicious client asks the server to do some response behavior, but the client refuses to read the response. This will test the server’s queue management code. Depending on how the server processes the queue, the client can force it to consume excessive memory and CPU when processing the request.

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.

Nginx has been confirmed to be affected and has released an update for this vulnerability.