Malicious Macros in Microsoft Office: Updated Guidance to Mitigate Risks

TL;DR

Malicious macros remain a significant threat, with Microsoft Office being a common attack vector.
Updated guidance from Microsoft and the UK’s National Cyber Security Centre (NCSC) provides strategies to mitigate risks.
Key recommendations: Disable macros by default, use trusted locations, and implement attack surface reduction rules.
Relevance to security professionals: Understanding macro-based threats is critical for both Red and Blue Teams to defend against or simulate attacks.

Despite advancements in cybersecurity, malicious macros in Microsoft Office documents continue to pose a significant threat. These macros, often embedded in seemingly harmless files, are a favorite tool for attackers to deliver malware, ransomware, and other malicious payloads. Andrew A, a cybersecurity expert, recently highlighted updated guidance from Microsoft and the NCSC on mitigating these risks¹. This article delves into the technical details of the updated guidance, its relevance to security professionals, and actionable steps to protect systems.

The Persistent Threat of Malicious Macros

Macros, small programs written in Visual Basic for Applications (VBA), are designed to automate repetitive tasks in Microsoft Office applications. While they can be incredibly useful, they are also easily weaponized by attackers. According to Microsoft, 98% of Office-targeted threats use macros². These macros often arrive via phishing emails, tricking users into enabling them by exploiting social engineering techniques.

How Macros Are Exploited

Phishing Campaigns: Attackers send emails with malicious attachments, often disguised as invoices or other legitimate documents.
Social Engineering: Users are prompted to “Enable Content” to view the document, unknowingly activating malicious macros.
Persistence: Once enabled, macros can execute arbitrary code, download additional payloads, or exfiltrate data.

Updated Guidance from Microsoft and NCSC

Microsoft has introduced several measures to combat macro-based threats, including blocking macros from the internet by default in Office applications³. The NCSC has also provided updated guidance, emphasizing the importance of disabling macros entirely where possible and using trusted locations for legitimate macros⁴.

Key Recommendations

Disable Macros by Default: Ensure macros are disabled in all Office applications unless explicitly required.
Use Trusted Locations: Only allow macros to run from designated trusted locations, such as internal network shares.
Implement Attack Surface Reduction (ASR) Rules: Use ASR rules to block high-risk macro capabilities, such as creating child processes or injecting code⁵.
Educate Users: Train employees to recognize phishing attempts and avoid enabling macros in suspicious documents.

Technical Steps for Implementation

Group Policy: Administrators can enforce macro restrictions using Group Policy settings. For example:

Path: User Configuration > Administrative Templates > Microsoft Excel 2016 > Excel Options > Security > Trust Center
Policy: Block macros from running in Office files from the Internet

Microsoft Intune: Use Intune to deploy configuration profiles that block macros in Office applications⁶.

Relevance to Security Professionals

For Red Teams

Simulating Attacks: Red Teams can use macro-based payloads to test an organization’s defenses and identify vulnerabilities.
Social Engineering: Understanding how macros are exploited can help in crafting realistic phishing simulations.

For Blue Teams

Detection and Prevention: Blue Teams should monitor for macro-enabled documents and implement controls to block malicious macros.
Incident Response: Develop playbooks for responding to macro-based incidents, including isolating affected systems and analyzing macro code.

For SOC Analysts

Threat Hunting: Look for indicators of compromise (IOCs) related to macro-enabled documents, such as unusual VBA code or suspicious file downloads.
Log Analysis: Use logs from Microsoft Defender or other endpoint protection tools to identify macro-related activity.

Remediation and Best Practices

Disable Macros: If macros are not needed, disable them entirely using Group Policy or Intune.
Use Code Signing: Only allow macros signed by trusted publishers to run.
Patch and Update: Ensure Office applications and Windows are up to date to benefit from the latest security features.
Monitor and Audit: Regularly review macro usage and investigate any anomalies.

Conclusion

Malicious macros remain a potent tool for attackers, but with updated guidance from Microsoft and the NCSC, organizations can significantly reduce their risk. By disabling macros by default, using trusted locations, and implementing ASR rules, security professionals can protect their systems from this persistent threat. For Red and Blue Teams, understanding macro-based attacks is essential for both offensive and defensive operations.