Microsoft AppLocker Flaw Lets Malicious Apps Bypass Restrictions

Security researchers at Varonis Threat Labs have identified a minor but noteworthy flaw in Microsoft’s recommended AppLocker block list policy that could potentially allow attackers to circumvent application restrictions.

The issue stems from an incorrect version number specification that creates a narrow window for exploitation, though its practical impact remains limited due to existing safeguards.

Technical Flaw Discovered in Version Control

The vulnerability centers around Microsoft’s AppLocker configuration documentation, where the MaximumFileVersion The field was incorrectly set to 65355.65355.65355.65355 instead of the expected maximum value of 65535.65535.65535.65535.

This discrepancy creates a gap in version control that could theoretically be exploited by malicious actors.

AppLocker, Microsoft’s enterprise-grade application control solution, uses XML-based rules to determine which applications can execute on Windows systems.

A typical deny rule in the block list appears as follows:

xml<Deny ID="ID_DENY_BASH" FriendlyName="bash.exe" FileName="bash.exe" 
MinimumFileVersion="0.0.0.0" MaximumFileVersion="65355.65355.65355.65355" />

The error is significant because 65535 represents the maximum value for an unsigned 16-bit integer, a fundamental limitation in computer systems.

Any executable with a version number falling between 65355.65355.65355.65355 and 65535.65535.65535.65535 could potentially bypass the block list restrictions, as it would fall outside the specified range while remaining within valid version numbering constraints.

Limited Security Impact Due to Existing Protections

Despite the technical flaw, security experts emphasize that the practical risk remains minimal.

AppLocker deployments typically implement a “signed executables only” policy alongside block lists, which serve as a critical secondary defense layer.

When an attacker modifies an executable’s version information to exploit this gap, the digital signature becomes invalid, triggering the signed executable restriction.

“While not a critical vulnerability, this highlights the importance of carefully reviewing and updating security policies,” noted the Varonis research team.

The flaw demonstrates how seemingly minor configuration errors can create unintended security gaps, even when comprehensive protection mechanisms are in place.

Organizations utilizing custom AppLocker policies based on Microsoft’s recommendations should audit their configurations to ensure proper version boundaries are implemented.

Microsoft Addresses Documentation Error

Investigation revealed that the erroneous version number originated from Microsoft’s Visual Studio documentation, specifically in the Publish Page reference materials.

Microsoft has since corrected the documentation following the security disclosure, updating both the source material and the AppLocker block list recommendations.

Security professionals recommend that organizations review their current AppLocker policies and update them MaximumFileVersion values to 65535.65535.65535.65535 where applicable.

Additionally, this incident underscores the importance of implementing layered security controls and avoiding direct copy-paste implementation of security policies without thorough review and testing in controlled environments.

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