Microsoft Defender Live Response: Signed Script Execution Issue

The Importance of Security: Keeping Unsigned Scripts Off

In the realm of cybersecurity, especially when dealing with robust platforms like Microsoft Defender, maintaining a secure posture is paramount. One critical aspect of this is the control over script execution. Our team has been focused on ensuring that the “Live Response unsigned script execution” option in Microsoft Defender remains disabled. This isn't just a matter of preference; it's a fundamental security measure designed to prevent the inadvertent or malicious execution of unauthorized code on your systems. Allowing unsigned scripts to run freely opens up a significant attack surface, making systems vulnerable to malware, ransomware, and other forms of cyber threats. By keeping this setting off, we ensure that only scripts that have gone through a vetting process, typically indicated by a digital signature, are permitted to run. This is a proactive step that aligns with best practices in endpoint security and helps maintain the integrity of the environment. The implications of an improperly secured script execution policy can be dire, ranging from data breaches to complete system compromise. Therefore, the decision to keep unsigned script execution disabled is a deliberate and necessary one for robust security.

Navigating the Challenges: Signing Scripts for Compatibility

To reconcile our security requirement of keeping unsigned script execution disabled with the need to run custom scripts, our team explored the process of signing PowerShell scripts. The primary objective was to make our script, SubmitEvidencesToVMRay.ps1, compatible with Microsoft Defender's security policies. By digitally signing the script, we aimed to provide a verifiable mark of authenticity and integrity, essentially telling Defender, "This script is known and trusted." The initial results seemed promising; after signing, the script no longer triggered the immediate error associated with running unsigned code. This was a crucial step forward, demonstrating that the signing process itself was technically feasible and correctly applied. The image provided clearly shows the absence of the initial "unsigned script" error, indicating that the digital signature was recognized by the system. This success, however, was short-lived, as we soon discovered that the execution process itself continued to falter, revealing deeper complexities within the Live Response mechanism. The ability to sign scripts is a valuable tool for security-conscious administrators, allowing for a balance between enhanced security and operational flexibility. It’s a process that requires careful attention to detail, from choosing the right certificate to ensuring proper implementation within the execution environment. The challenges we encountered highlight that while signing is a necessary step, it is not always sufficient on its own when dealing with complex integrated systems.

The Unexpected Roadblock: Live Response Job Failures

Despite successfully signing the SubmitEvidencesToVMRay.ps1 script, we encountered a persistent and frustrating roadblock: the Live Response job continued to fail during execution. This was perplexing because, from a security policy standpoint, the script should have been allowed to run. The fact that it failed after the initial unsigned script check pointed towards an issue occurring later in the pipeline, specifically during the script's actual execution within the Live Response environment. The error message, though brief, indicated a critical failure during the job's operation. This unexpected behavior led us to investigate further, suspecting that the way the script was being handled after being signed might be the culprit. The signing process is intended to guarantee the script's integrity from the point of creation to its execution. If the execution fails despite a valid signature, it suggests that something in the intermediary steps is compromising the script or its execution context. This could be due to how the script is transferred, how it's interpreted, or how its components are managed within the Live Response system. The implications are significant, as it hinders our ability to automate security tasks and collect critical evidence efficiently. This failure means our security workflows are not operating as intended, leaving potential gaps in our ability to respond to threats effectively. We needed to understand precisely where the process was breaking down to find a viable solution.

Unraveling the Mystery: The Role of Python and SAS Tokens

Our investigation into the persistent Live Response job failures led us to suspect a deeper issue involving the Python Microsoft Defender class, MicrosoftDefender.py, and its interaction with the PowerShell script and Shared Access Signature (SAS) tokens. We theorized that the upload function or the parameter injection process within this Python script might be inadvertently altering the PowerShell script file itself. Such alterations, even subtle ones, could invalidate the digital signature that was so carefully applied. A digital signature is like a tamper-evident seal; if the file is changed in any way after signing, the seal is broken, and the signature becomes invalid. This could happen if the Python script, in its process of preparing the script for execution or transmitting it, modifies line endings, adds or removes metadata, or otherwise changes the file's binary content. Furthermore, the use of SAS tokens, typically employed for secure access to resources, could potentially play a role if not handled correctly during the script transfer or execution setup. We needed to meticulously examine the MicrosoftDefender.py code to understand how it prepares and transmits the script and associated tokens. This involved looking at how files are read, written, and passed between processes. If the script is being modified, it means that the integrity assurance provided by the digital signature is being undermined after the signing step, leading to the observed execution failures. This line of inquiry is critical because it points to a potential bug or oversight in the script uploading and handling mechanism, rather than an issue with the script's content or the signing process itself.

Recreating the Problem: Steps to Reproduce

To systematically address the failure of our signed PowerShell script in Microsoft Defender's Live Response, we've outlined a clear set of steps to reproduce the issue. This process is essential for both our internal troubleshooting and for providing clear information to development teams. First, ensure that the “Live Response unsigned script execution” option within your Microsoft Defender configuration is set to off. This is the foundational security setting that necessitates script signing. Second, take the PowerShell script, SubmitEvidencesToVMRay.ps1, and sign it using a valid digital certificate. This step verifies the script's authenticity and integrity prior to execution. Third, to test the service's behavior under controlled conditions, use an EICAR test file. The EICAR file is a standard, harmless file designed specifically to test antivirus and security software, confirming whether they correctly detect and react to a known test signature. Finally, after completing these steps, observe that the Live Response job still fails. This consistent failure, even with a signed script and a controlled test file, confirms the problem we are trying to resolve. By following these steps, anyone can reliably replicate the scenario, allowing for focused debugging and a clearer understanding of where the execution pipeline is breaking down. This structured approach is vital for isolating the root cause and developing an effective solution that ensures signed scripts can execute as expected.

The Desired Outcome: Seamless Signed Script Execution

Our expected behavior is straightforward yet critical for our security operations: when the “Live Response unsigned script execution” setting in Microsoft Defender is disabled, a signed PowerShell script should execute successfully. The digital signature is our guarantee of the script's authenticity and integrity. Therefore, upon presenting a properly signed script to the Live Response feature, we anticipate it being recognized as trusted and allowed to run without interruption. This means that the script should perform its intended function, whether it's collecting evidence, running diagnostics, or implementing a specific security measure, without encountering execution errors related to its signature or an unrecognized status. This successful execution would validate our security policy of disallowing unsigned scripts while still enabling the use of necessary automation tools. It’s about achieving a balance where security is not compromised, and operational efficiency is maintained. The goal is to have a robust system where signed scripts are inherently trusted and seamlessly integrated into the Live Response workflow, providing reliability and confidence in our security incident response capabilities. This is the benchmark against which the current functionality falls short.

The Current Reality: Persistent Live Response Failures

Our actual behavior starkly contrasts with our desired outcome. Despite diligently signing the SubmitEvidencesToVMRay.ps1 script with a valid certificate, the Live Response job continues to fail. This indicates that the signing process, while seemingly successful in its initial validation, is not sufficient to overcome the execution hurdles within the Microsoft Defender Live Response environment. The failure occurs after the initial checks, suggesting a problem in the subsequent stages of script handling or execution. This persistent failure means that our security automation workflows are not functioning as intended, potentially leaving us vulnerable or unable to gather critical data during an incident. The core issue seems to be that the system is not correctly processing or trusting the signed script during its operational phase within Live Response. This is a significant impediment to leveraging custom scripts for security tasks in an environment where unsigned script execution is intentionally disabled. The discrepancy between the expected and actual behavior points to a need for a deeper review of how signed scripts are managed within the Live Response execution pipeline.

Our Environment: The Technical Landscape

To provide context for the issue we are experiencing, here is a summary of our environment: Our Microsoft Defender configuration has the crucial setting “unsigned script execution” turned off. This is the core security policy driving our need for signed scripts. The script in question is SubmitEvidencesToVMRay.ps1, a custom PowerShell script developed to automate evidence submission. The mechanism used to upload and manage this script is a Python script, specifically our implementation of the MicrosoftDefender.py class. This class is responsible for interacting with the Microsoft Defender API to deploy and execute scripts within Live Response. Understanding these components is key to diagnosing the problem. The interplay between the Defender configuration, the signed PowerShell script, and the Python uploader creates the specific scenario where signed script execution is failing. The details of how MicrosoftDefender.py handles the script file and any associated tokens are central to our hypothesis about the cause of the failure. This technical setup represents a common approach for organizations seeking to integrate custom automation within managed security platforms, making this a relevant challenge for many.

The Request: Enhancing Signed Script Support

Our primary request is for a thorough review of the script upload and execution components within the Microsoft Defender Live Response solution. Specifically, we need the solution to better support signed script execution. This is not merely a bug report but a feature request, highlighting a capability that we believe is essential for users operating with strict security policies. We require the system to correctly handle and execute PowerShell scripts that have been properly signed with valid digital certificates, especially when unsigned script execution is disabled. This involves ensuring that the script file's integrity is maintained throughout the upload and execution process, so that the digital signature remains valid and is recognized by the Defender Live Response engine. We believe that addressing this will significantly enhance the utility and reliability of Microsoft Defender for organizations that prioritize security and need to leverage custom automation tools. Enhancing support for signed scripts would provide a more robust and secure way to deploy and manage scripts, aligning with modern cybersecurity best practices and empowering users to automate tasks with confidence. We are eager to see this capability strengthened.

For further insights into Microsoft Defender for Endpoint and its Live Response capabilities, you can visit the official Microsoft Learn documentation. Additionally, understanding PowerShell security best practices can be beneficial, and the PowerShell documentation offers valuable resources.