CVE-2022-41723: Understanding the HPACK Decoder CPU Exhaustion Vulnerability and Exploit in HTTP/2 Streams

Summary: With the recent release of CVE-2022-41723, security researchers have identified a critical vulnerability within the HPACK decoder used in HTTP/2 streams. By maliciously crafting an HTTP/2 request, an attacker can exploit this vulnerability to cause excessive CPU consumption, ultimately leading to a denial of service (DoS) attack on the targeted server.

Background: HTTP/2 is a widely-used protocol for delivering web content efficiently and securely. One of its features, HPACK header compression, allows for more efficient communication by compressing and replacing headers in place. However, this functionality also creates an opportunity for bad actors to exploit poorly-implemented HPACK decoders.

In this long-read post, we will be diving into the details of CVE-2022-41723, examining how the vulnerability can be exploited through a maliciously crafted HTTP/2 stream, and discussing possible mitigations and patches that can be applied to prevent this type of attack.

Exploit Details: At the core of the vulnerability is the use of the HPACK decoder in processing HTTP/2 requests. An attacker can craft an HTTP/2 request to send a malicious stream to a web server, causing the server's HPACK decoder to consume an excessive amount of CPU resources. This high CPU usage can ultimately lead to a DoS attack on the targeted server, negatively impacting its availability and responsiveness.

Consider the following code snippet as an example of a maliciously crafted HTTP/2 stream

:h2_stream {
    prepend: "\x82\x87\x86\x85\x84\x83\x82\x81\x80",
    append: "\x80\x81\x82\x83\x84\x85\x86\x87",
}

This example shows a specially crafted, malicious HTTP/2 request that uses the :h2_stream keyword to begin a new stream. The prepend and append parameters are used to define the starting and ending positions of the string, effectively causing the HPACK decoder to enter a loop that results in high CPU usage.

Original References

1. HTTP/2 Specification
2. HPACK: Header Compression for HTTP/2
3. CVE-2022-41723 Official Details

Potential Impact: The implications of this vulnerability are significant, as it allows an attacker to perform a DoS attack on a web server with a relatively small number of maliciously crafted requests. This can result in a loss of availability or responsiveness for the targeted server, potentially crippling its ability to serve web content to users.

Available Mitigations: There are several options for mitigating this vulnerability. First, organizations should ensure that they are using the latest version of the software implementing their HPACK decoder, as updates and patches may have been released to address this specific CVE. Second, organizations can implement rate limiting to protect against the sudden influx of malicious requests, mitigating the impact of a potential DoS attack. Third, organizations can monitor their servers for high CPU usage and act accordingly if an abnormal spike in usage is detected.

In conclusion, CVE-2022-41723 is a critical vulnerability that highlights the importance of appropriate HPACK decoder implementation and robust security measures for HTTP/2-based web servers. By understanding the details of how this vulnerability can be exploited and following mitigation best practices, organizations can protect themselves against potential attacks that may leverage this weakness.

Stay informed and vigilant to ensure the safety of your online infrastructure!

Timeline

Published on: 02/28/2023 18:15:00 UTC
Last modified on: 04/12/2023 03:15:00 UTC