Full Report
Microsoft has announced that it will start disabling external workbook links to blocked file types by default between October 2025 and July 2026. [...]
Analysis Summary
# Best Practices: Mitigating Risks Associated with Legacy/Malicious Office File Features (e.g., XLM Macros, VBScript)
## Overview
These practices focus on hardening Microsoft Office applications, particularly Excel, by disabling or blocking inherently risky features such as external workbook links to blocked file types, legacy macros (XLM/Excel 4.0), VBScript execution, and untrusted add-ins, to curb malware distribution and exploitation.
## Key Recommendations
### Immediate Actions
1. **Verify Blocking of Untrusted XLL Add-ins:** Ensure that the configuration to block untrusted XLL (Excel Add-in) files by default is fully deployed across all Microsoft 365 tenants.
2. **Confirm XLM Macro Protection Status:** Verify that XLM macro protection has been enabled within your environment to prevent legacy Excel 4.0 macros from executing.
3. **Review Current VBA Macro Policies:** Confirm that default policies are actively blocking Office VBA macros originating from the internet or untrusted locations.
### Short-term Improvements (1-3 months)
1. **Implement Workbook Link Protection:** Configure systems to automatically disable external workbook links that point to file types designated as blocked, preventing potentially malicious linkages.
2. **Assess and Deprecate VBScript Usage:** Begin the process of identifying all reliance on VBScript within automated processes or user workflows, in preparation for its planned disabling by Microsoft in the second half of 2024.
3. **Audit Existing XLM Usage:** Conduct an internal audit to identify any existing, actively used legacy workbooks that rely on Excel 4.0 (XLM) macros and prioritize their modernization or controlled quarantine.
### Long-term Strategy (3+ months)
1. **Establish a Retirement Timeline for VBScript:** Finalize and execute the transition plan to eliminate all VBScript dependencies before the official end-of-life or forced disabling by Microsoft.
2. **Enhance Threat Intelligence Integration:** Integrate security tools capable of flagging or blocking file types known to be associated with malware distribution vectors mentioned in broader threat intelligence reports (like those relevant to the evolving Microsoft bounty programs).
3. **Develop a Comprehensive Application Control Strategy:** Expand application control beyond VBA/XLM to cover other high-risk scripting vectors and file types used programmatically in Office applications.
## Implementation Guidance
### For Small Organizations
- **Prioritize Built-in Settings:** Focus initially on ensuring that the latest security features released by Microsoft (which are often enabled by default in newer M365 subscriptions) are not being overridden by local group policies or user settings.
- **User Training Focus:** Conduct mandatory, short training sessions specifically covering the danger of enabling content (macros) from external or untrusted Excel files.
### For Medium Organizations
- **Group Policy Object (GPO) Rollout:** Use Group Policy Objects (GPO) or Microsoft Intune to centrally enforce macro blocking settings, XLM protection, and XLL add-in restrictions across the enterprise.
- **Controlled Phased Rollout:** Implement macro and script controls in a phased manner, testing impact on legitimate legacy files in a pilot group before a full deployment.
### For Large Enterprises
- **Advanced Endpoint Detection and Response (EDR):** Leverage EDR tools to monitor and block the *execution* paths associated with legacy content, even if the initial file opening is permitted under specific, highly controlled conditions.
- **Bug Bounty/Vulnerability Program Review:** Align internal security patching and vulnerability management programs with the proactive vulnerability disclosure incentives offered by Microsoft (e.g., the $40,000 payouts), ensuring critical vulnerabilities in .NET/ASP.NET Core components are immediately addressed.
- **Conditional Access/Zero Trust for Documents:** Implement strict policies dictating where and how files containing active content (like external links or macros) can be opened, potentially restricting them to managed, protected environments.
## Configuration Examples
*Note: Specific policy paths are not detailed here, but relate to administrative templates within Office/Microsoft 365 Security Center.*
- **Enable XLM Protection:** Configure settings to enforce the blocking of Excel 4.0 macros, treating them as macro-enabled workbooks when loaded.
- **Block Untrusted XLLs:** Configure the relevant setting (often via GPO or Intune) under Office \ Security Center \ Trust Center \ Add-ins to block or require explicit user trust for XLL files not signed by a trusted publisher.
- **Disable External Linking to Blocked Types:** Configure settings to prevent Excel from refreshing external data/links sourced from internally designated high-risk or blocked file extensions.
## Compliance Alignment
- **NIST SP 800-53 (Rev. 5):** SC-7 (Boundary Protection), SA-11 (Developer Testing), SI-4 (Information System Monitoring, focusing on execution monitoring).
- **CIS Controls V8:** Control 12 (Application Control: specifically blocking execution of unauthorized code).
- **ISO/IEC 27002:2022:** A.8.21 (Software acquisition – ensuring security requirements are embedded in software procurement/deployment).
## Common Pitfalls to Avoid
- **Ignoring Legacy Files:** Assuming that older, unreviewed Excel files do not utilize the deprecated XLM functionality. Auditing is essential.
- **Incomplete VBScript Removal:** Assuming VBScript is only used in older operating systems; it remains active in modern Windows versions unless explicitly disabled or blocked by application control.
- **Disabling Features Without Replacement:** Disabling macro functionality without providing users with secure, approved alternatives for data sharing or automation (e.g., migrating to Power Automate or modern JavaScript-based add-ins).
## Resources
- Microsoft Documentation regarding Office Security Settings (Referencing Microsoft 365 Defender/Intune documentation for exact policy settings related to Macro/XLL blocking).
- Microsoft Vulnerability Bounty Program Updates (For staying current on high-value attack vectors like .NET/ASP.NET Core weaknesses).