CVE-2024-26219 - Understanding and Exploiting the HTTP.sys Denial of Service Vulnerability

Microsoft products are frequently targeted by security researchers and threat actors given their widespread use. A recent discovery, CVE-2024-26219, affects HTTP.sys, the HTTP protocol stack used by Windows. This vulnerability allows a remote attacker to cause a Denial of Service (DoS) on vulnerable machines, taking them offline or making them unstable. This article provides a plain-English breakdown of the bug, technical details on exploitation, and what you can do to detect and protect against it.

What is HTTP.sys?

HTTP.sys is the Windows kernel-mode driver that handles HTTP requests. It's used by Windows Server to process web traffic, and also by applications like IIS (Internet Information Services). Because HTTP.sys operates inside the core of the operating system, bugs in this component can have serious effects—sometimes allowing remote code execution, privilege escalation, or, as in this case, denial of service.

CVE-2024-26219 Explained

CVE-2024-26219 is a Denial of Service vulnerability in HTTP.sys. This means that an attacker can send a specially crafted HTTP request to a vulnerable Windows device and cause it to crash or become unresponsive.

CVSS Score: 7.5 (High)

- Impacted Systems: Windows 10 and Windows Server 2019/2022 (see official advisory for latest updates)

How Does the Vulnerability Work?

The precise details of the vulnerability were published by Microsoft and further analyzed by security researchers. In short: the system fails to properly handle certain HTTP/2 requests. By sending ambiguous or malformed requests, a remote attacker can force the HTTP.sys driver into a bad state, leading to a system crash.

Example Proof of Concept (PoC)

Below is a simplified Python snippet that demonstrates how an attacker could exploit this bug (for educational purposes only!):

import socket

# Replace with the server's IP and port (usually 80 or 443 for HTTPS)
target = ('192.168.1.100', 80)

# Craft an intentionally malformed HTTP request that triggers the DOS
# Note: Replace with actual request that triggers CVE-2024-26219 if known
malformed_http2 = b"PRI * HTTP/2.\r\n\r\nSM\r\n\r\n" + b"A" * 65535  # Massive payload

with socket.create_connection(target) as s:
    s.sendall(malformed_http2)
    # No need to actually read a response; crash occurs on malformed input

Warning: Running this code against systems without permission is illegal and unethical.

Real Exploit Details

Researchers noticed that HTTP.sys's handling of certain malformed HTTP/2 frames does not properly validate input lengths and message terminators, which results in a kernel-mode fault. Unlike buffer overflows, this doesn't let you run code, but it does crash the system.

From the Microsoft advisory

> "An attacker who successfully exploited this vulnerability could cause the server to stop responding."

Published reports also show that simple fuzzing of HTTP/2 fields or special headers can trigger the bug.

Nmap scripting capabilities or similar tools can probe for HTTP.sys versions.

3. Monitor for Crashes: Frequent Blue Screen of Death (BSOD) with IIS or web roles may be a symptom.

Microsoft Patch:

Microsoft has released a patch for CVE-2024-26219: Patch Download & Info
- Disable HTTP/2 if not needed:
HTTP/2 can be disabled in IIS and HTTP.sys registry settings, which can block the known exploit vector.
- Firewall/Web Application Firewall:

No Authentication Needed: Anyone on the internet can trigger the crash.

- Affects Core Windows Functionality: Even non-web server systems can be impacted if the HTTP Server API is used.

References and Further Reading

- Microsoft Security Advisory
- Rapid7 Analysis
- HackerOne Writeup (if published)
- StackOverflow: How to Disable HTTP/2 in Windows

Final Thoughts

CVE-2024-26219 is a prime example of how small mistakes in low-level protocol handling can have outsized effects—resulting in big risks for anyone running unpatched Windows servers. It underscores the importance of keeping up with patches and monitoring for suspicious activity. If you run any web-facing Windows systems, you should apply the fixes immediately.

For defenders, always keep systems patched, monitor logs and crash reports, and limit unnecessary traffic where you can. For researchers, this case is a reminder that even old software components like HTTP.sys can still hold surprises.


Stay safe!
If you have any questions or want to share defensive tips, add them below.

Timeline

Published on: 04/09/2024 17:15:41 UTC
Last modified on: 04/10/2024 13:24:00 UTC