CVE-2025-26678 - How Improper Access Control in Windows Defender Application Control (WDAC) Enables Local Security Bypass

In early 2025, a new Windows vulnerability landed on the radar: CVE-2025-26678. This flaw exposes a weak spot in Windows Defender Application Control (WDAC), letting attackers bypass security checkpoints and run unauthorized code. Let’s break down what this vulnerability means, how it works, and what Microsoft and security enthusiasts are doing about it.

What is WDAC and Why Does It Matter?

WDAC is a security feature in Windows designed to let administrators lock down devices and only run trusted applications. By controlling which software can execute, WDAC is meant to stop malware and unwanted code from running—even if a user is tricked into trying.

But what if someone finds a way to wiggle past those controls?

The CVE-2025-26678 Vulnerability

CVE-2025-26678 stems from improper access control inside WDAC. In plain English, WDAC isn’t checking things as carefully as it should. This flaw means that local attackers—people with access to the computer but not full administrator rights—could slip past WDAC rules and execute files that should be blocked.

How the Flaw Can Be Exploited

The key problem lies in the way WDAC treats executable files and policy enforcement. Abusing this flaw typically involves:

1. Identifying WDAC Policy Gaps: The attacker checks which executables are allowed and which aren’t.
2. Crafting or Copying Malicious Files: The attacker prepares a malicious executable, renaming or placing it in locations that bypass WDAC controls due to misconfigured rules.
3. Running the Malicious Code: Thanks to the improper control, WDAC fails to block or scan the file, and it runs under the user’s privileges.

Example Exploit Snippet

Here's a simplified proof-of-concept (POC) that shows how an attacker might execute a forbidden program:

:: Assume notepad.exe is allowed by WDAC policy, attacker replaces it with malicious payload

copy C:\Malware\malicious.exe C:\Users\Public\notepad.exe
start C:\Users\Public\notepad.exe

This works when WDAC rules are too broad (e.g., allowing notepad.exe from any unsigned location), or if WDAC fails to check file integrity properly due to the vulnerability. Thus, malicious.exe runs as if it were trusted Notepad.

References and More Info

- Microsoft Security Update Guide - CVE-2025-26678

_(Official Microsoft vulnerability listing)_

- Microsoft Docs - WDAC Overview

- WDAC Policy Design Best Practices

Hide their activity from basic security auditing.

Organizations relying solely on WDAC without layering other protections are at particularly high risk.

Microsoft’s Fix

Microsoft released a patch to tighten the improper access control. All users should apply the latest Windows updates as soon as possible.

Audit your WDAC policies: Make sure rules aren’t too generic.

- Enable additional controls: Pair WDAC with tools like Microsoft Defender Antivirus and EDR (Endpoint Detection & Response).

Conclusion

CVE-2025-26678 proves that no security feature is bulletproof. Even WDAC, designed to be a fortress, can have gaps if not carefully managed. Administrators and users alike should review security policies, diligently apply updates, and stay alert for emerging threats.


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Timeline

Published on: 04/08/2025 18:15:52 UTC
Last modified on: 05/06/2025 17:03:33 UTC