CVE-2024-26235 - Windows Update Stack Elevation of Privilege

On February 13, 2024, Microsoft released updates patching a serious security flaw: CVE-2024-26235, an Elevation of Privilege (EoP) vulnerability in the Windows Update Stack. This vulnerability could allow attackers to gain SYSTEM privileges, providing full control over the target system. In this post, I'll break down what this means, show code snippets demonstrating the kind of exploitation possible, and provide practical info on how to stay protected.

What Is CVE-2024-26235?

The Windows Update Stack is responsible for handling and installing updates in Windows environments. A vulnerability in this component is particularly concerning because it runs with high privileges and interacts deeply with the system.

Microsoft’s advisory:
https://msrc.microsoft.com/update-guide/vulnerability/CVE-2024-26235

Severity: Important
Impact: Elevation of Privilege (EoP)
Attack vector: Local

In Simple Terms

If someone already has access to your computer (for example, through malware or another local account), they could exploit this bug to give themselves SYSTEM– the highest level of access in Windows.

How Does the Exploit Work?

The full technical details are withheld by Microsoft, but there’s enough public information to understand the exploit flow.

Theoretical Exploit: Illustrative Code

Let’s imagine how a local attacker could abuse a privileged service that doesn’t sanitize input or mismanages file permissions.

> Note: This is a generic example (not actual exploit code), based on known EoP attack patterns in the Windows Update Stack.

Suppose there’s a temporary file or scheduled task created by the update process that’s not properly permissioned. A user could replace or point it to their own malicious executable.

import os
import shutil
import ctypes

# 1. Find the update stack's working directory
update_dir = r'C:\Windows\Temp\UpdateStack'

# 2. Replace a DLL or EXE that the update process will load
malicious_dll = r'C:\Users\attacker\malicious.dll'
target_path = os.path.join(update_dir, 'trustedlib.dll')

# 3. Overwrite if perms are weak (as in a vulnerable system)
try:
    shutil.copyfile(malicious_dll, target_path)
    print("Malicious DLL placed!")
except PermissionError:
    print("No access - patched or not vulnerable.")

# If update runs and loads 'trustedlib.dll', our code runs as SYSTEM!

An attacker runs malicious code exploiting CVE-2024-26235.

- The exploit manipulates the update stack, causing Windows Update to load or run a file with SYSTEM privileges.
- The SYSTEM account is used to disable security mechanisms, install/root malware, or exfiltrate sensitive data.

Microsoft’s Mitigations

Microsoft patched this issue in the February 2024 Patch Tuesday release.

Suggested action:
Update your Windows now. The fix is automatically included in regular security updates.

You can look for suspicious files in the update temp directory

$updatePath = "C:\Windows\Temp\UpdateStack"
Get-ChildItem $updatePath -Recurse | Where-Object { $_.CreationTime -gt (Get-Date).AddDays(-7) }

Regularly review logs and install all security patches.

- Educate users about local threats–most privilege escalations require some level of initial access.

References & Further Reading

- Microsoft CVE-2024-26235 Security Update Guide
- NIST NVD CVE-2024-26235
- Windows Update Stack Internals
- General Windows EoP Attack Techniques

Conclusion

CVE-2024-26235 is a timely reminder that local privilege escalation bugs are still a favorite attacker tool. The Windows Update Stack, because of its high-privilege role, is a tempting target. By keeping your system updated and watching for suspicious activities, you can stay a step ahead. Patch now and don’t give attackers a chance at SYSTEM!

Feel free to share this post to keep your teams and friends secure!

Timeline

Published on: 04/09/2024 17:15:44 UTC
Last modified on: 04/10/2024 13:24:00 UTC