HTTP/2 SETTINGS frame bug and related registry keys

HTTP/2 protocol improves the performance and security of today’s digital world. It consists of several frames to carry requests between clients and servers. One of these frames is SETTINGS frame which may be used by attackers to increase CPU usage to 100% in IIS and eventually make the server unresponsive (Denial of Service – DoS).

In this post, we will discuss the root cause and the solution for this bug.

What is HTTP/2 SETTINGS frame?

It is part of the HTTP/2 request which contains 6 parameters to manage communication between peers. Here is how IETF HTTP Working Group explains the usage of this frame:

The SETTINGS frame (type=0x4) conveys configuration parameters that affect how endpoints communicate, such as preferences and constraints on peer behavior. The SETTINGS frame is also used to acknowledge the receipt of those parameters.

SETTINGS in Hypertext Transfer Protocol Version 2 (HTTP/2)
HTTP/2 SETTINGS frame (Source: IETF HTTP Working Group)