In late 2022, security researchers discovered a major vulnerability in Microsoft’s Sysmon utility that could allow attackers to elevate their user privileges. Cataloged as CVE-2022-41120, this bug affected many Windows systems and caught the attention of both security teams and malicious actors. In this post, I’ll break down what this flaw is about, show you how attackers might exploit it, and provide resources for learning more.

What is Sysmon?

Before jumping into the bug, let’s look at Sysmon. Sysmon (System Monitor) is a Windows system service and driver that logs system activity for security monitoring. System administrators and security professionals use it frequently to track events like process creations, network connections, and file changes. It’s part of the Microsoft Sysinternals suite.

Sysmon is usually run with System privileges and is designed to help with detecting intrusions.

What’s vulnerable?

According to Microsoft’s official advisory, a local attacker (already on your machine) could use this vulnerability to gain elevated (SYSTEM) privileges.

The flaw is actually in how Sysmon processes configuration updates. If a user can replace or tamper with the Sysmon config file, it may lead to privilege escalation.

The Vulnerable Logic

Sysmon supports updating its config via a file. In some scenarios, if file permissions aren’t properly set, a standard user can replace the config. Sysmon runs as SYSTEM, so the config is later loaded—or handled—with elevated rights.

A regular (non-admin) user has write access to the Sysmon config file at C:\Sysmon\sysmon.xml.

2. They replace the XML with their own, inserting a malicious script or a reference to a file they control.
3. Next time Sysmon reloads its config (maybe via an admin running an update, or a system reboot), the service parses the attacker’s file as SYSTEM.
4. If the crafted XML carries malicious payloads or points to locations that trigger SYSTEM-privilege commands, the attacker can escalate privileges.

Simulated Environment

Disclaimer: Do not run this on production systems or machines you do not own.

Suppose Sysmon config is stored here

C:\Sysmon\sysmon.xml

1. Replace the Config File

# Attacker has write permission
Copy-Item "C:\Users\User\malicious_sysmon.xml" "C:\Sysmon\sysmon.xml" -Force

Admins or the system often reload the config

sysmon64.exe -c C:\Sysmon\sysmon.xml

3. Malicious Payload in Config

An attacker might try to insert a configuration like this (hypothetical, the real attack needs tailoring):

<Sysmon schemaversion="4.50">
  <EventFiltering>
    <RuleGroup name="EvilRule" groupRelation="or">
      <ProcessCreate onmatch="include">
        <Image condition="contains">evil.exe</Image>
      </ProcessCreate>
    </RuleGroup>
  </EventFiltering>
</Sysmon>


Or more powerfully, point to system files or scripts, triggering further actions as SYSTEM.

Why is This Dangerous?

- Runs as SYSTEM: Anything executed by Sysmon (directly or via included files in the config) happens with maximum privileges.

Security Fix

To patch, update Sysmon to version 14.16 or later. You can download the current installer from the official Sysmon page.

Additionally, set strict permissions on your Sysmon directory and config file

icacls C:\Sysmon\sysmon.xml /inheritance:r /grant SYSTEM:F /grant Administrators:F


This removes inherited permissions, grants full control just to SYSTEM and Administrators.

References

- Microsoft's CVE-2022-41120 advisory
- Sysmon official download and documentation
- Security Researcher Write-up (Bleeping Computer)
- Sysinternals Blog Announcement

Conclusion

CVE-2022-41120 reminds us that even legitimate, widely used security tools can become dangerous if their privileges are abused. A simple file permission misconfiguration is all it takes for an attacker to seize SYSTEM-level access. Stay on top of updates and always tighten permissions—especially on security monitoring tools like Sysmon.

Timeline

Published on: 11/09/2022 22:15:00 UTC
Last modified on: 11/10/2022 00:33:00 UTC