What is the AppLocker bypass flaw and how can malicious apps exploit it in Windows systems?

Table of Contents

• What Happened in the Microsoft AppLocker Configuration Flaw?
• What Is the AppLocker Flaw in Simple Terms?
• How Attackers Can Exploit the Flaw
• Is This Vulnerability a Serious Threat?
• Why the Digital Signature Matters
• Where Did Microsoft Go Wrong?
• Security Best Practices for Enterprises
• Summary of the AppLocker Flaw
• Expert Quotes on the Flaw
• Conclusion
• Frequently Asked Questions (FAQs)

What Happened in the Microsoft AppLocker Configuration Flaw?

Security researchers from Varonis Threat Labs have uncovered a serious misconfiguration in Microsoft’s AppLocker block list policy. This flaw stems from an incorrect version limit (MaximumFileVersion) set in the block list, which could allow unauthorized executables to run, bypassing restrictions on enterprise systems.

What Is the AppLocker Flaw in Simple Terms?

AppLocker is Microsoft’s application control feature used to block or allow specific executable files. The flaw is due to a mismatch in the configured version limits. The configuration mistakenly uses:

65355.65355.65355.65355

Instead of the correct maximum:

65535.65535.65535.65535

This small numerical mistake allows attackers to craft or modify executables with a higher version number, slipping past the block list enforcement.

How Attackers Can Exploit the Flaw

An attacker can exploit this by:

1. Modifying the version metadata of a blocked executable.

2. Setting the version number just above the flawed limit (65355).

3. Bypassing the block list policy, allowing the malicious file to run.

   
   

Since AppLocker compares version numbers for blocking rules, anything with a version higher than 65355 is no longer restricted, even if it should be.

Is This Vulnerability a Serious Threat?

Yes — but only under certain conditions.

Exploit Severity Depends on Policy Implementation

• If code signing enforcement is enabled, modified executables will fail signature verification and still be blocked.

• If only block list policies are used without enforcing signed binaries, the system is vulnerable to this bypass.

Why the Digital Signature Matters

Modifying an executable’s metadata, such as the version number, will break its digital signature. In environments enforcing “signed binaries only”:

• The file will no longer pass signature checks.

• AppLocker will block the execution regardless of version.

This layered security model mitigates the practical impact — but only if implemented correctly.

Where Did Microsoft Go Wrong?

The error originated from Microsoft’s official documentation on the AppLocker Publish Page. Administrators commonly copy example configurations from this source. Unfortunately, the incorrect MaximumFileVersion value was replicated across many enterprise networks.

Microsoft’s Response

Following responsible disclosure by Varonis, Microsoft:

• Acknowledged and corrected the documentation.

• Recommended reviewing existing configurations for accuracy.

Security Best Practices for Enterprises

To protect against this flaw and future misconfigurations, enterprises should:

✅ Update Your Block List Configuration

• Replace the incorrect version value with the correct:

  65535.65535.65535.65535
  

✅ Enforce Code Signing Policies

• Allow only digitally signed executables to run.

• Prevent tampered files from bypassing controls.

✅ Audit and Validate All Security Configurations

• Avoid copy-pasting from vendor documentation blindly.

• Regularly review and test policy effectiveness.

Summary of the AppLocker Flaw

Expert Quotes on the Flaw

“It’s a textbook example of how a minor versioning mistake can break an entire security control.”
— Daniel Card, Security Researcher, PwnDefend

“Copying configurations from documentation without validation is a recipe for disaster.”
— Kaaviya Narayanan, Security Analyst

Conclusion: Why This Flaw Matters

This AppLocker flaw is a critical lesson in precision security management. A small misconfiguration in a widely adopted security feature can introduce gaps attackers are quick to exploit.

Key Takeaways:

• Defense-in-depth is essential — one layer alone is not enough.

• Security configs must be validated — even if they come from the vendor.

• Code signing + auditing + block listing = strong protection.

FAQs 

What is Microsoft AppLocker used for?

Microsoft AppLocker is a Windows feature that helps administrators control which applications can run on a system using rule-based policies.

What is the recent flaw in Microsoft AppLocker?

The flaw lies in a misconfigured MaximumFileVersion in block list rules, allowing malicious files with slightly higher versions to bypass security restrictions.

Who discovered the AppLocker bypass vulnerability?

The flaw was discovered by security researchers at Varonis Threat Labs.

How does the AppLocker vulnerability work?

It allows malicious files to bypass block rules by using a version number higher than the mistakenly configured maximum in the block list.

What is the incorrect version number set in the AppLocker configuration?

The incorrect value is 65355.65355.65355.65355.

What should be the correct version number?

The correct maximum should be 65535.65535.65535.65535.

Can attackers run unsigned executables using this flaw?

Yes, if code signing is not enforced, attackers can exploit this flaw to run unsigned executables.

Does modifying the version number break the digital signature?

Yes, modifying version metadata breaks the digital signature of an executable.

How can enterprises mitigate this vulnerability?

By correcting the version number and enforcing code signing policies.

What role does code signing play in preventing exploitation?

Code signing ensures that only verified, untampered executables are allowed to run, even if version metadata is altered.

Is this flaw a zero-day vulnerability?

No, it is a misconfiguration vulnerability, not an unpatched zero-day.

Is Microsoft responsible for the misconfiguration?

The flawed configuration appeared in Microsoft's own documentation, making it a vendor-introduced error.

Has Microsoft fixed the documentation?

Yes, after disclosure, Microsoft updated the documentation with the correct values.

Are systems with signed-executable policies still vulnerable?

No, if signed-binary enforcement is enabled, the vulnerability cannot be exploited.

Can this flaw be exploited remotely?

No, exploitation typically requires local access or the ability to drop a file onto the system.

What tools can detect improper AppLocker configurations?

Tools like Windows Event Viewer, Sysmon, and configuration auditing scripts can help.

Should organizations disable AppLocker?

No, AppLocker is still a powerful tool when properly configured.

What versions of Windows are affected?

Any Windows version using AppLocker with the flawed configuration can be affected.

How common is this misconfiguration?

It is widespread due to many administrators copying Microsoft’s documented example.

What are best practices to avoid such flaws?

Never blindly copy configurations; always verify parameters and test in sandboxed environments.

How can IT teams audit AppLocker policies?

Use PowerShell scripts to export and review current policy files and version filters.

What is the role of MaximumFileVersion in AppLocker?

It defines the upper limit of allowed file versions; incorrectly setting this allows newer versions to bypass policies.

Can this issue be exploited by malware?

Yes, malware authors can craft executables with higher version metadata to evade detection.

Is this issue logged in Windows Security logs?

If AppLocker is set to audit mode, logs will show allowed executions; otherwise, events may go unnoticed.

Can endpoint detection systems catch this?

Yes, if EDR systems are properly configured to monitor unusual executable behavior.

How long has this configuration error existed?

The flaw existed in documentation until it was recently corrected in response to Varonis’s findings.

What should be added to AppLocker for better protection?

Enable rules for signed apps and use path and publisher rules in combination.

What happens if the version number exceeds the maximum?

Executables with higher versions bypass block rules that rely on faulty version logic.

Are there similar flaws in other security products?

Yes, misconfigurations in filters, allow lists, and regex patterns can similarly create bypasses in other tools.

Where can I read the full Varonis report?

The full research is available on the Varonis Threat Labs blog and official advisory page.