Introduction #

Microsoft Office applications can execute macros to automate routine tasks. However, macros can contain malicious code resulting in unauthorised access to sensitive information as part of a targeted cyber intrusion.

This publication has been developed to discuss approaches that can be applied by organisations to secure systems against malicious macros while balancing both their business and security requirements.

The names and locations of Group Policy settings used in this publication are taken from Microsoft Office 2016 and are equally applicable to Microsoft 365, Office 2021 and Office 2019. Some differences however may exist for earlier versions than Microsoft Office 2016.

Background #

An increasing number of attempts to compromise organisations using malicious macros have been observed. In particular, adversaries have been observed using social engineering techniques to entice users into executing malicious macros in Microsoft Office files. The purpose of these malicious macros can range from cybercrime to more sophisticated exploitation attempts.

By understanding the business requirements for the use of macros, and applying the recommendations in this publication, organisations can effectively manage the risk of allowing macros in their environments.

Macros explained #

What are macros?

Microsoft Office files can contain embedded code (known as a macro) written in the Visual Basic for Applications (VBA) programming language.

A macro can contain a series of commands that can be coded or recorded, and replayed at a later time to automate repetitive tasks. Macros are powerful tools that can be easily created by novice users to greatly improve their productivity. However, an adversary can also create macros to perform a variety of malicious activities, such as assisting in the compromise of workstations in order to exfiltrate or deny access to sensitive information.

How are macros verified and trusted?

Microsoft Office has both trusted document and trusted location functions. Once trusted documents or trusted locations are defined, macros in trusted documents or macros in Microsoft Office files in trusted locations automatically execute when the files are opened. While the use of trusted documents is discouraged, trusted locations when implemented in a controlled manner can allow organisations to appropriately balance both their business and security requirements.

Microsoft Office applications allow developers to include information about themselves by digitally signing their macros. The signing certificate that is used to create a signed macro confirms that the macro originated from the signatory, while the signature itself confirms that the macro has not been altered. Digital signing certificates can be self-generated by users, obtained from a commercial Certificate Authority or obtained from an organisation’s security administrator if they operate their own Certificate Authority.

How can I determine which macros to trust?

To manage the use of macros within an organisation, all macros created by users or third parties should be reviewed by an independent party to the developer and assessed to be safe before being approved for use within the organisation.

When assessing whether macros are safe or not, assessors should ask themselves the following questions:

• Is there a business requirement for a particular macro?
• Has the macro been developed or provided by a trusted party?
• Has the macro been validated by a trustworthy and technically skilled party?

Securing systems against malicious macros #

The following table displays the security benefit, business impact and implementation difficulty of different approaches to managing macros in Microsoft Office files.

Macros are disabled for users that do not have a demonstrated business requirement

If users do not have a demonstrated business requirement for macro use, support should be disabled across the Microsoft Office suite. Furthermore, to prevent users or an adversary from bypassing macro security controls, all support for trusted documents and trusted locations should be disabled.

To ensure users cannot knowingly or unintentionally alter macro security controls via a Microsoft Office application’s Trust Center interface, organisations should apply macro security controls using Group Policy settings.

Only macros from trusted locations are enabled

If organisations have a business requirement for macro use, approved macros in Microsoft Office files in trusted locations can be allowed to execute. However, only specific Microsoft Office applications for which there is a business requirement for macro use should be allowed to execute approved macros. All other Microsoft Office applications should have support for macros disabled.

To prevent users or an adversary from bypassing macro security controls, support for trusted documents should be disabled while trusted locations should prevent all users, except for approved users, from adding or modifying macros in Microsoft Office files in these locations. Using an appropriately secured network path as a trusted location can assist in the centralised management and control of macros in Microsoft Office files.

To ensure users cannot knowingly or unintentionally alter macro security controls via a Microsoft Office application’s Trust Center interface, organisations should apply macro security controls using Group Policy settings.

Only macros digitally signed by trusted publishers are enabled

If organisations have a business requirement for macro use, only macros digitally signed by trusted publishers can be allowed to execute. However, only specific Microsoft Office applications for which there is a business requirement for macro use should be allowed to execute digitally signed macros. All other Microsoft Office applications should have support for macros disabled.

To prevent users or an adversary from bypassing macro security controls, support for trusted documents and trusted locations should be disabled.

To further reduce the likelihood of an adversary signing a malicious macro and it being executed by users, the ability to enable macros signed by an untrusted publisher, or add additional trusted publishers, should be disabled for users. This includes via the trust bar, backstage view, Internet Options control panel and the use of certificate management tools.

Organisations should take care to sufficient test this approach before implementation to ensure that there are no unintended consequences of implementing the above restrictions on Microsoft Office and certificate management functionality within their environment.

To ensure users cannot knowingly or unintentionally alter macro security controls via a Microsoft Office application’s Trust Center interface, organisations should apply macro security controls using Group Policy settings.

Users decide which macros to enable on a case-by-case basis

If organisations have a business requirement for macro use, they should manage the use of macros in their environment using one of the recommended approaches discussed below. Relying on users to make correct security decisions one hundred percent of the time is not a realistic expectation considering the sophistication of many spear phishing attempts. As such, allowing users to decide which macros to enable on a case-by-case basis presents a significant risk and should not be implemented.

All macros are enabled

If organisations have a business requirement for macro use, they should manage the use of macros in their environment using one of the recommended approaches discussed below. Allowing unrestricting execution of all macros presents a serious risk and should never be implemented.

Recommended approaches

To protect themselves against malicious macros, organisations should implement one, or a combination, of the following approaches:

• macros are disabled for users that do not have a demonstrated business requirement
• only macros from trusted locations are enabled
• only macros digitally signed by trusted publishers are enabled.

In addition to the recommended approaches above, organisations should:

• implement application control to mitigate a malicious macro running unapproved applications
• implement email and web content filtering to inspect incoming Microsoft Office files for macros, and block or quarantine them as appropriate
• implement the Attack Surface Reduction rules outlined in the Hardening Microsoft 365, Office 2021, Office 2019 and Office 2016 publication
• implement macro execution logging to verify only approved macros are used
• ensure users assigned to assessing the safety of macros have appropriate VBA training.

Group Policy settings #

The following Group Policy settings can be implemented depending on an organisation’s desired approach to managing macros in Microsoft Office files.

Microsoft Windows

Microsoft Office 2016

Microsoft Access 2016

Microsoft Excel 2016

Microsoft Outlook 2016

Microsoft PowerPoint 2016

Microsoft Project 2016

Microsoft Publisher 2016

Microsoft Visio 2016

Microsoft Word 2016

Further information #

The Information Security Manual is a cyber security framework that organisations can apply to protect their systems and data from cyber threats. The advice in the Strategies to Mitigate Cyber Security Incidents, along with its Essential Eight, complements this framework.