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Hardening Microsoft Office 2013

First published 2015; Latest version April 2019

Introduction

Workstations are often targeted by adversaries using malicious web pages, malicious email attachments and removable media with malicious content in an attempt to extract sensitive information. Hardening applications on workstations is an important part of reducing this risk.

This document provides guidance on hardening commonly targeted Microsoft Office 2013 applications – specifically Microsoft Excel, Microsoft PowerPoint and Microsoft Word. Before implementing the recommendations in this document, testing should be undertaken to ensure the potential for unintended negative impacts on business processes is reduced as much as possible.

This document is intended for information technology and information security professionals within organisations looking to undertake risk assessments or vulnerability assessments as well as those wishing to develop a hardened Standard Operating Environment for workstations.

The Group Policy Administrative Templates for Microsoft Office 2013 referenced in this document can be obtained from Microsoft [1]. Once downloaded, the ADMX and associated ADML files can be placed in %SystemDrive%\Windows\SYSVOL\domain\Policies\PolicyDefinitions on the Domain Controller and they will automatically be loaded in the Group Policy Management Editor. As Group Policy Administrative Templates for Microsoft Office are periodically updated by Microsoft, care should be taken to ensure the latest version is always used.

High priorities

The following security controls, listed in alphabetical order, are considered to have an excellent effectiveness and should be treated as high priorities when hardening Microsoft Office deployments.

Attack Surface Reduction

Attack Surface Reduction (ASR) [2] is a new security feature introduced in Microsoft Windows 10, version 1709 as part of Windows Defender Exploit Guard. It is designed to combat the threat of malware exploiting legitimate functionality in Microsoft Office applications. In order to use ASR, Windows Defender Antivirus must be configured as the primary real-time antivirus scanning engine on workstations.

ASR offers a number of attack surface reduction rules, these include:

  • block executable content from email client and webmail
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  • block Office applications from creating child processes
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  • block Office applications from creating executable content
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  • block Office applications from injecting code into other processes
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  • block JavaScript and VBScript from launching downloaded executable content
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  • block execution of potentially obfuscated scripts
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  • block Win32 API calls from Office macro
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Organisations should either implement ASR using Windows Defender Antivirus or use third party antivirus solutions that offer similar functionality to those provided by ASR. For older versions of Microsoft Windows, alternative measures will need to be implemented to mitigate certain threats addressed by ASR, such as the likes of Dynamic Data Exchange (DDE) attacks [3].

For organisations using Windows Defender Antivirus, the following Group Policy setting can be implemented to enforce the above ASR rules.

Group Policy Setting

Recommended Option

Computer Configuration\Policies\Administrative Templates\Windows Components\Windows Defender Antivirus\Windows Defender Exploit Guard\Attack Surface Reduction

Configure Attack Surface Reduction rules

Enabled

Set the state for each ASR rule:

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Latest version

Newer versions of Microsoft Office offer significant improvements in security features, functionality and stability. It is often the lack of improved security features that allows an adversary to easily compromise older versions of Microsoft Office. To reduce this risk, the latest supported version of Microsoft Office (Microsoft Office 365 ProPlus or Office 2019) should be used.

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 to compromise workstations in order to exfiltrate or deny access to sensitive information. To reduce this risk, organisations should either disable or secure their use of Microsoft Office macros.

For information on securing the use of Microsoft Office macros see the Microsoft Office Macro Security publication [4].

Patching

To address security vulnerabilities identified in Microsoft Office, Microsoft regularly releases patches. If patches are not applied in an appropriate timeframe it can allow an adversary to easily compromise workstations. To reduce this risk, patches should be applied in an appropriate timeframe as determined by the severity of security vulnerabilities they address and any mitigating measures already in place.

For more information on determining the severity of security vulnerabilities and appropriate timeframes for applying patches see the Assessing Security Vulnerabilities and Applying Patches publication [5].

Medium priorities

The following security controls, listed in alphabetical order, are considered to have a very good effectiveness and should be treated as medium priorities when hardening Microsoft Office deployments.

ActiveX

While ActiveX controls can be used for legitimate business purposes to provide additional functionality for Microsoft Office, they can also be used by an adversary to gain unauthorised access to sensitive information or to execute malicious code. To reduce this risk, ActiveX controls should be disabled for Microsoft Office.

The following Group Policy setting can be implemented to disable the use of ActiveX controls in Microsoft Office.

Group Policy Setting

Recommended Option

User Configuration\Policies\Administrative Templates\Microsoft Office 2013\Security Settings

Disable All ActiveX

Enabled

Add-ins

While add-ins can be used for legitimate business purposes to provide additional functionality for Microsoft Office, they can also be used by an adversary to gain unauthorised access to sensitive information or to execute malicious code. To reduce this risk, add-in use should be managed.

The following Group Policy settings can be implemented to manage add-ins in Microsoft Excel, Microsoft PowerPoint and Microsoft Word.

Group Policy Setting

Recommended Option

User Configuration\Policies\Administrative Templates\Microsoft Excel 2013\Miscellaneous

Block all unmanaged add-ins

Enabled

List of managed add-ins

Enabled

List of managed add-ins: <organisation defined>

User Configuration\Policies\Administrative Templates\Microsoft PowerPoint 2013\Miscellaneous

Block all unmanaged add-ins

Enabled

List of managed add-ins

Enabled

List of managed add-ins: <organisation defined>

User Configuration\Policies\Administrative Templates\Microsoft Word 2013\Miscellaneous

Block all unmanaged add-ins

Enabled

List of managed add-ins

Enabled

List of managed add-ins: <organisation defined>

Extension Hardening

Extension Hardening mitigates a number of scenarios whereby an adversary would deceive users into opening malicious Microsoft Excel files. By default, users will be warned when file content or MIME type doesn’t match the file extension; however, users can still allow such files to open. As such, it is important that only Microsoft Excel files that pass integrity checks are allowed to be opened. To reduce this risk, Extension Hardening functionality should be enabled for Microsoft Excel.

The following Group Policy setting can be implemented to enable Extension Hardening functionality in Microsoft Excel.

Group Policy Setting

Recommended Option

User Configuration\Policies\Administrative Templates\Microsoft Excel 2013\Excel Options\Security

Force file extension to match file type

Enabled

Always match file type

File Type Blocking

File Type Blocking can be used to block insecure file types such as legacy, binary and beta file types from opening in Microsoft Office. By failing to block such file types, an adversary can exploit vulnerabilities in these file types to execute malicious code on workstations. To reduce this risk, insecure file types should be prevented from opening in Microsoft Office.

The following Group Policy settings can be implemented to block specified file types in Microsoft Excel, Microsoft PowerPoint and Microsoft Word.

Group Policy Settings

Recommended Option

User Configuration\Policies\Administrative Templates\Microsoft Excel 2013\Excel Options\Security\Trust Center\File Block Settings

dBase III / IV files

Enabled

File block setting: Open/Save blocked, use open policy

Dif and Sylk files

Enabled

File block setting: Open/Save blocked, use open policy

Excel 2 macrosheets and add-in files

Enabled

File block setting: Open/Save blocked, use open policy

Excel 2 worksheets

Enabled

File block setting: Open/Save blocked, use open policy

Excel 3 macrosheets and add-in files

Enabled

File block setting: Open/Save blocked, use open policy

Excel 3 worksheets

Enabled

File block setting: Open/Save blocked, use open policy

Excel 4 macrosheets and add-in files

Enabled

File block setting: Open/Save blocked, use open policy

Excel 4 workbooks

Enabled

File block setting: Open/Save blocked, use open policy

Excel 4 worksheets

Enabled

File block setting: Open/Save blocked, use open policy

Excel 95 workbooks

Enabled

File block setting: Open/Save blocked, use open policy

Excel 95-97 workbooks and templates

Enabled

File block setting: Open/Save blocked, use open policy

Set default file block behavior

Enabled

Blocked files are not opened

Web pages and Excel 2003 XML spreadsheets

Enabled

File block setting: Open/Save blocked, use open policy

User Configuration\Policies\Administrative Templates\Microsoft PowerPoint 2013\PowerPoint Options\Security\Trust Center\File Block Settings

PowerPoint beta convertors

Enabled

File block setting: Open/Save blocked, use open policy

Set default file block behavior

Enabled

Blocked files are not opened

User Configuration\Policies\Administrative Templates\Microsoft Word 2013\Word Options\Security\Trust Center\File Block Settings

Set default file block behavior

Enabled

Blocked files are not opened

Word 2 and earlier binary documents and templates

Enabled

File block setting: Open/Save blocked, use open policy

Word 6.0 binary documents and templates

Enabled

File block setting: Open/Save blocked, use open policy

Word 95 binary documents and templates

Enabled

File block setting: Open/Save blocked, use open policy

Word 97 binary documents and templates

Enabled

File block setting: Open/Save blocked, use open policy

Hidden markup

To assist users in collaborating on the development of Microsoft Office files, Microsoft Office allows users to track changes relating to insertions, deletions and formatting of content, as well as providing the ability to make comments. Users may choose to either view or hide these markups. If markup content is hidden, users may be unaware that sensitive changes or comments may still be included when Microsoft Office files are distributed to external parties or released into the public domain. To reduce this risk, users should be made aware of hidden markup in Microsoft Office files.

The following Group Policy settings can be implemented to make users aware of hidden markup in Microsoft PowerPoint and Microsoft Word files.

Group Policy Settings

Recommended Option

User Configuration\Policies\Administrative Templates\Microsoft PowerPoint 2013\PowerPoint Options\Security

Make hidden markup visible

Enabled

User Configuration\Policies\Administrative Templates\Microsoft Word 2013\Word Options\Security

Make hidden markup visible

Enabled

Office File Validation

Office File Validation (OFV) checks that the format of a Microsoft Office file conforms to an expected standard. By default, Microsoft Office files that fail OFV checking will be opened in Protected View, with users given the option to enable editing. Alternatively, OFV can be configured to open Microsoft Office files in Protected View in an enforced read-only state or simply block them from opening. If Microsoft Office is configured to disable OFV, users may be unaware that they are opening a Microsoft Office file that may be malicious in nature. To reduce this risk, OFV functionality should be enabled for Microsoft Office.

The following Group Policy settings can be implemented to enable OFV functionality in Microsoft Excel, Microsoft PowerPoint and Microsoft Word.

Group Policy Settings

Recommended Option

User Configuration\Policies\Administrative Templates\Microsoft Office 2013\Security Settings

Turn off error reporting for files that fail file validation

Enabled

User Configuration\Policies\Administrative Templates\Microsoft Excel 2013\Excel Options\Security

Turn off file validation

Disabled

User Configuration\Policies\Administrative Templates\Microsoft PowerPoint 2013\PowerPoint Options\Security

Turn off file validation

Disabled

User Configuration\Policies\Administrative Templates\Microsoft Word 2013\Word Options\Security

Turn off file validation

Disabled

Protected View

Protected View can be used to open Microsoft Office files from untrusted locations in a sandboxed environment. By default, Protected View is enabled for Microsoft Office files that have been downloaded from the Internet, opened from a defined unsafe location or opened as an attachment from Microsoft Outlook. However, organisations can choose to disable Protected View for any or all of these scenarios. If so, an adversary could exploit any of these avenues to deliver a malicious Microsoft Office file to a user’s workstation. To reduce this risk, Protected View should be enabled for Microsoft Office.

The following Group Policy settings can be implemented to enable Protected View functionality in Microsoft Excel, Microsoft PowerPoint and Microsoft Word.

Group Policy Settings

Recommended Option

User Configuration\Policies\Administrative Templates\Microsoft Excel 2013\Excel Options\Security\Trust Center\Protected View

Do not open files from the Internet zone in Protected View

Disabled

Do not open files in unsafe locations in Protected View

Disabled

Set document behaviour if file validation fails

Enabled

Block files

Turn off Protected View for attachments opened from Outlook

Disabled

User Configuration\Policies\Administrative Templates\Microsoft PowerPoint 2013\PowerPoint Options\Security\Trust Center\Protected View

Do not open files from the Internet zone in Protected View

Disabled

Do not open files in unsafe locations in Protected View

Disabled

Set document behaviour if file validation fails

Enabled

Block files

Turn off Protected View for attachments opened from Outlook

Disabled

User Configuration\Policies\Administrative Templates\Microsoft Word 2013\Word Options\Security\Trust Center\Protected View

Do not open files from the Internet zone in Protected View

Disabled

Do not open files in unsafe locations in Protected View

Disabled

Set document behaviour if file validation fails

Enabled

Block files

Turn off Protected View for attachments opened from Outlook

Disabled

Trusted documents

Macros, ActiveX controls and other active content in trusted documents are assumed to be safe by Microsoft Office. An adversary can exploit this trust by modifying trusted documents to contain malicious code. To reduce this risk, trusted documents should be disabled for Microsoft Office.

The following Group Policy settings can be implemented to disable the use of trusted documents in Microsoft Excel, Microsoft PowerPoint and Microsoft Word.

Group Policy Settings

Recommended Option

User Configuration\Policies\Administrative Templates\Microsoft Excel 2013\Excel Options\Security\Trust Center

Turn off trusted documents

Enabled

Turn off Trusted Documents on the network

Enabled

User Configuration\Policies\Administrative Templates\Microsoft PowerPoint 2013\PowerPoint Options\Security\Trust Center

Turn off trusted documents

Enabled

Turn off Trusted Documents on the network

Enabled

User Configuration\Policies\Administrative Templates\Microsoft Word 2013\Word Options\Security\Trust Center

Turn off trusted documents

Enabled

Turn off Trusted Documents on the network

Enabled

Low priorities

The following security controls, listed in alphabetical order, are recommended for consideration when hardening Microsoft Office deployments.

Reporting information

Microsoft Office contains in-built functionality, namely the Office Feedback Tool, which allows users to provide feedback, including screenshots, to Microsoft. This information if captured by an adversary could expose sensitive information on workstations such as file names, directory names, versions of installed applications or content open in other applications. This information could subsequently be used by an adversary to tailor malicious code to target specific workstations or users. To reduce this risk, functionality in Microsoft Office that allows reporting of information to Microsoft should be disabled.

The following Group Policy settings can be implemented to prevent users reporting information to Microsoft.

Group Policy Setting

Recommended Option

User Configuration\Policies\Administrative Templates\Microsoft Office 2013\Privacy\Trust Center

Allow including screenshot with Office Feedback

Disabled

Automatically receive small updates to improve reliability

Disabled

Disable Opt-in Wizard on first run

Enabled

Enable Customer Experience Improvement Program

Disabled

Send Office Feedback

Disabled

Further information

The Australian Government Information Security Manual (ISM) assists in the protection of information that is processed, stored or communicated by organisations’ systems. It can be found at https://www.cyber.gov.au/ism.

The Strategies to Mitigate Cyber Security Incidents complements the advice in the ISM. The complete list of strategies can be found at https://www.cyber.gov.au/publications/strategies-to-mitigate-cyber-security-incidents.

Contact details

Organisations or individuals with questions regarding this advice can email asd.assist@defence.gov.au or call 1300 CYBER1 (1300 292 371).

Date
April 30th, 2019