How to Apply Microsoft 365 Security Hardening: 15 Technical Controls for Desktop Deployments

Securing Microsoft Office desktop deployments requires disabling macros, enforcing Attack Surface Reduction (ASR) rules, restricting Dynamic Data Exchange (DDE) and Object Linking and Embedding (OLE), blocking malicious file types at the gateway, and keeping Office fully patched. The 15 controls below address every major attack vector used against Office in real-world campaigns.

Why Are Microsoft 365 Desktop Deployments a High-Value Attack Target?

Microsoft has been pushing browser-based Microsoft 365 cloud for all office functions. However, their desktop office applications are just as popular as ever - especially Word, Excel and PowerPoint. This creates an exploitable attack surface: the end user workstation is a prime target for attack and often an overlooked link in any cyber security strategy.

With ransomware infections stealing terabytes of personal and corporate data, the numbers quickly become eye-watering. According to Coveware's Q4 2023 Ransomware Report, the average ransom payment reached $568,705, with a median payment of $200,000. The Sophos State of Ransomware 2024 report found that the mean time to recover from a ransomware attack is over one month, with recovery costs averaging $2.73 million excluding the ransom payment itself. Couple that with lost revenue, recovery costs, lost productivity, reputational damage and fines for loss of data, and unfortunately it can be the end for a lot of businesses. For UK organisations subject to Cyber Essentials or ISO 27001 controls, misconfigured Office deployments represent a documented audit failure point and a direct pathway to ransomware.

For attackers looking to compromise an organisation and manipulate an end user, using familiar software is naturally preferred. Microsoft Office is used by virtually every IT-enabled organisation on the planet. According to the Microsoft Digital Defense Report 2024, Microsoft processes over 78 trillion security signals per day and tracks more than 1,500 unique threat groups - and Office documents remain a primary delivery mechanism for initial access. While attackers have pivoted to XLL add-ins, OneNote files, and archive containers following Microsoft's 2022 macro-blocking changes, the Office attack surface remains active and well-documented. Cofense's 2026 annual threat intelligence report found that threat actors now launch a malicious email every 19 seconds - more than double the pace of 2024 - and 82% of malicious files now carry unique hashes that defeat pattern-matching defences, making application-level hardening more important than ever.

There is a need to protect assets or risk irrecoverable loss.

15 Technical Controls for Microsoft 365 Security Hardening

The majority of the protections outlined below should be implemented using Group Policy and will therefore require the Administrative Template XML (ADMX) files provided by Microsoft here: https://www.microsoft.com/en-au/download/details.aspx?id=49030. The CIS Microsoft Office Enterprise Benchmark (v1.2.0) also provides consensus-based hardening guidelines freely available from https://www.cisecurity.org/benchmark/microsoft_office and covers all 15 controls in this article.

Microsoft Office Security Controls at a Glance

1. Stop Malicious Files Reaching the User

It's obvious that to be opened in Office a 'bad' file must arrive at a user workstation in some way. A favourite delivery method for attackers looking to distribute these nefarious documents is the classic email - Phishing. Although not a protection applied to the Office installation directly, it's still such an important step it's worth mentioning here. The best way to stop a user falling victim is to stop the document ever reaching them. Configure Exchange Online Protection (EOP) transport rules to block .docm, .xlsm, .pptm, .xlsb, and .xll file types at the mail gateway - by blocking these known extensions used by attackers you can greatly reduce the employee's exposure to these files. Organisations using on-premises Exchange or a dedicated secure email gateway (SEG) can apply equivalent transport rules at the same layer.

2. Update Office When Available

Security is a moving target; attackers constantly find ways to directly attack software or subvert existing controls for their own and often illicit ends. Updates not only apply functional fixes but often contain security improvements. By applying security updates in a timely manner, it is not only possible to protect an organisation from known attacks against the software itself but to implement the latest security controls released by Microsoft. Ensuring you are always running the latest version of Office is a crucial first step in protecting an organisation and reducing risk.

3. Microsoft Office Macro Security

Office macros are code embedded within office documents that can be used by attackers to gain access to a user's workstation. Written in the Visual Basic for Applications (VBA) programming language, the only thing stopping attackers from doing virtually anything they wish on a user's machine is often a single click. Threat actors including Emotet (TA542) and QakBot relied heavily on macro-enabled Office documents as their primary delivery mechanism - Emotet's campaigns typically used invoice-lure .docm attachments that prompted users to "Enable Content", executing a PowerShell downloader that retrieved the Emotet loader and, in many cases, subsequent ransomware payloads. To disable macros across the organisation, navigate in Group Policy to: User Configuration > Administrative Templates > Microsoft Office [version] > Security Settings > Block macros from running in Office files from the Internet - set this to Enabled. For the highest-security environments, "Disable VBA for Office applications" is also available as a policy option.

Real-world example: A UK manufacturing company received a phishing email purporting to be an overdue invoice from a known supplier. The attachment - a .docm Word file - instructed the recipient to "Enable Content" to view the invoice. Once macros were enabled, embedded VBA ran a PowerShell command that downloaded and executed the Emotet loader, which subsequently retrieved a QakBot payload and, 72 hours later, deployed Conti ransomware across the network. Approximately 60% of workstations and three file servers were encrypted. Recovery took 23 days and cost an estimated £340,000 in IT response, lost revenue, and third-party forensics. A single Group Policy setting - User Configuration > Administrative Templates > Microsoft Word > Word Options > Security > Block macros from running in Office files from the Internet, set to Enabled - would have prevented macro execution entirely. (Composite scenario based on the Emotet/Conti delivery chain, consistent with NCSC-documented UK campaigns.)

4. Attack Surface Reduction (ASR) Rules

In Windows 10, Microsoft introduced Attack Surface Reduction (ASR). ASR provides rule-based protection against risky behaviour at a software level; for example: - Launching executable files and scripts that attempt to download or run files - Running obfuscated or otherwise suspicious scripts - Performing behaviours that apps don't usually initiate during normal day-to-day work

ASR rules are configured via Microsoft Defender for Endpoint policy in Intune or via Group Policy at: Computer Configuration > Administrative Templates > Windows Components > Microsoft Defender Antivirus > Microsoft Defender Exploit Guard > Attack Surface Reduction > Configure Attack Surface Reduction rules. The Microsoft Defender ASR documentation lists all available rules with their GUIDs and recommended audit/block modes.

5. Dynamic Data Exchange (DDE) and External Content

The Dynamic Data Exchange (DDE) protocol is a method of inter-application data transfer provided by Windows. This means it allows applications such as Outlook and Excel to load and embed data from an external source into a document. Attackers can exploit this functionality to run external code - for example, an attacker can use custom DDE fields to interact directly with the command line and execute their own commands.

In October-November 2017, APT28 (Fancy Bear) - the Russian GRU-attributed threat group - exploited the DDE protocol in Word documents to execute PowerShell payloads against US government and defence sector targets, as documented by McAfee Advanced Threat Research. The attack embedded DDEAUTO fields that called cmd.exe and PowerShell to download subsequent payloads including Seduploader, X-Agent, and X-Tunnel - all without requiring macros to be enabled. Separately, Microsoft patched CVE-2017-11826 during the same period, a memory corruption remote code execution vulnerability in the Office document parser that similarly required no macro interaction. Microsoft issued Security Advisory 4053440 to guide organisations on disabling DDE in Office applications.

In 2017, due to widespread use in malware campaigns, Microsoft officially disabled support for DDE in Word, however support for DDE in Excel and Outlook persists. To disable DDE in Word: set DontUpdateLinks = 1 in the registry at HKCU\Software\Microsoft\Office\[version]\Word\Options\WordMail, or via Group Policy: User Configuration > Administrative Templates > Microsoft Word [version] > Word Options > Advanced - disable automatic link updates. The equivalent registry path for Excel is HKCU\Software\Microsoft\Office\[version]\Excel\Options.

6. Flash Content

Thankfully Flash reached end-of-life (EOL) on December 31st, 2020, when Adobe terminated Flash Player. Unfortunately, this doesn't mean it isn't still used across a large number of organisations. Office documents allow Flash content to be embedded meaning that attackers use Flash code to target unsuspecting users. To protect against this attack, Flash can be blocked in Office documents by setting a Kill Bit via registry for the {ShockwaveFlash.ShockwaveFlash} ActiveX CLSID, or through Group Policy using the ActiveX Control Initialization setting in Microsoft Office security settings.

7. Object Linking and Embedding (OLE)

Content from other applications can be embedded into Excel Worksheets, Word Documents and PowerPoint Presentations through Object Linking and Embedding (OLE). Much like office macros, users can use embedded objects such as Visual Basic (VB) and JavaScript (JS) scripts to execute their code on a user's workstation. To protect users, organisations should use Group Policy to make registry changes that disable OLE across the aforementioned Office products. The relevant registry key is HKCU\Software\Microsoft\Office\[version]\Word\Security with PackagerPrompt set to 2 (block), which controls whether OLE package activation is permitted.

8. Restrict Office DCOM

Abusing Microsoft Office Distributed Component Object Model (DCOM) is a technique used by attackers for stealthy lateral movement within a target network. DCOM works by extending the benefits of Component Object Model (COM) objects to remote computers and allowing, for example, macros to be run on remote machines. To restrict Office DCOM, use dcomcnfg.exe (Component Services) to set the Default Authentication Level to "Packet Integrity" or higher. Group Policy also provides controls via Computer Configuration > Windows Settings > Security Settings > Local Policies > Security Options - specifically "DCOM: Machine Launch Restrictions" and "DCOM: Machine Access Restrictions".

9. ActiveX

ActiveX can be used to provide users with a rich Office experience - for example, a document author can retrieve data from an internet source. However, ActiveX can also be used by attackers to execute code and access sensitive data. CVE-2021-40444, a critical vulnerability in the MSHTML (Internet Explorer) rendering engine, was exploited in the wild via malicious Office documents that used ActiveX controls to trigger remote code execution - without the target needing to enable macros. To restrict ActiveX in Office, use Group Policy: User Configuration > Administrative Templates > Microsoft Office [version] > Security Settings > ActiveX - set "ActiveX Control Initialization" to 6 (disable all). The equivalent registry key is HKCU\Software\Policies\Microsoft\Office\[version]\[application]\Security with DisableAllActiveX = 1.

10. Custom Add-Ins

Office supports Custom Add-Ins - third-party or internally developed extensions that load automatically when Office applications start. Attackers use malicious COM add-ins (Component Object Model add-ins registered in the Windows registry) to execute arbitrary code persistently, survive reboots, and evade detection by running within trusted Office processes - this technique is documented under MITRE ATT&CK T1137.002 (Office Application Startup: Office Add-ins). To restrict add-ins, use Group Policy: navigate to User Configuration > Administrative Templates > Microsoft Office [version] > Security Settings > Disable all Trust Bar notifications for unsigned application add-ins, and set "Require that application add-ins are signed by Trusted Publisher" to Enabled. The corresponding registry key is HKCU\Software\Policies\Microsoft\Office\[version]\[application]\Security\RequireAddinSig.

Adjust Office version references as appropriate for the target environment.

11. Use Protected View

By default, Office files originating from untrusted locations such as the internet or email attachments are opened within a sandbox environment. In uncommon situations organisations have disabled Protected View, sometimes without knowing the consequences. If this is the case an attacker can deliver malicious Office files to users via the internet or email. To enforce Protected View via Group Policy, navigate to User Configuration > Administrative Templates > Microsoft [App] [version] > [App] Options > Security > Protected View - ensure "Turn off Protected View for files originating from the Internet" is set to Disabled (i.e., keep Protected View enabled for internet-sourced files). Apply the equivalent policy for "Turn off Protected View for files in unsafe locations" as well.

12. Disable Running External Programs

PowerPoint allows users to run external programs through action buttons, allowing attackers to execute programs or use existing programs to compromise a user's workstation. To disable this, apply Group Policy at: User Configuration > Administrative Templates > Microsoft PowerPoint [version] > PowerPoint Options > Security > Disable Run Programs - set to Enabled. The equivalent registry key is HKCU\Software\Policies\Microsoft\Office\[version]\PowerPoint\Security with DisableRunPrograms = 1. Action buttons are a PowerPoint-specific interactive feature; disabling them via policy has no impact on Word or Excel.

13. Use Office File Validation (OFV)

Office File Validation (OFV) ensures that documents opened in Office conform to an expected standard and performs security checks on files. According to Microsoft, "Office File Validation helps detect and prevent a kind of exploit known as a file format attack or file fuzzing attack". OFV applies specifically to Office 97-2003 binary formats (.doc, .xls, .ppt) and can be enabled via Group Policy at: User Configuration > Administrative Templates > Microsoft Office [version] > Security Settings - "Turn on file validation" set to Enabled. The registry equivalent is HKCU\Software\Policies\Microsoft\Office\[version]\[application]\Security\FileValidation with EnableOnLoad = 1.

14. Disable Legacy and Insecure File Types

Office has been part of our lives for over 30 years and understandably it has evolved many times since its inception in 1990, but is still required to support legacy file formats. Older binary formats such as .xla, .xlm, .xlc, .xlt, .wk* (Lotus), and .dif carry significantly higher risk because their parsers are less actively maintained and more prone to exploitation. Block these at the gateway alongside the macro-enabled formats, and use Group Policy Trust Center file block settings to prevent users opening them locally: User Configuration > Administrative Templates > Microsoft Excel [version] > Excel Options > Security - "Block opening of pre-release format files". For Word, block .doc, .dot, and .rtf downloads via gateway transport rules.

15. Test and Verify

Securing Microsoft Office deployments can be a challenging task, with so many policies and registries to modify, it can be easy to misconfigure. Changes made over time to accommodate edge-case and bespoke situations can inadvertently affect the organisation's ability to protect itself from common methods of attack.

Use Atomic Red Team's open-source test library to validate your Office hardening controls: T1559.002 tests DDE execution, T1137.001 tests macro persistence via Office templates, T1204.002 tests malicious file execution, and T1203 covers Office-based exploit execution. Run these tests in an isolated environment and confirm that your ASR rules, macro blocking, and Group Policy settings generate the expected alerts in Microsoft Defender or your SIEM. The MITRE ATT&CK for Enterprise navigator provides a coverage map to confirm which attack techniques each control addresses. For cloud-managed environments, Microsoft Secure Score in the Microsoft 365 Defender portal provides ongoing visibility into your Office security posture. Microsoft's free Attack Surface Analyzer tool also allows you to compare system states before and after policy changes to confirm controls are applied as intended.

A misconfigured control typically manifests as a policy that is applied at the wrong scope (user vs. computer), a registry key overridden by a competing GPO, or a setting applied to only one Office application when it should apply to all. Scheduled testing - at minimum after any Office update cycle - is the only reliable way to confirm your hardening baseline remains intact.

Where Should You Start When Hardening Microsoft Office?

Not every control in this guide carries equal weight. Start with gateway filtering and Office patching - these stop the majority of attacks before they reach the desktop. Layer in macro controls and ASR rules next, as these address the highest-volume active threats. DDE, OLE, and Protected View settings follow as the second tier. The remaining controls - add-ins, ActiveX, Flash, DCOM, OFV, and legacy file types - close off less common but still documented attack paths.

Recommended implementation sequence: gateway filtering → patches → macros → ASR rules → DDE/OLE → Protected View → remaining controls.

Microsoft Office hardening is one component of a complete Windows endpoint hardening strategy. For UK organisations, these controls directly support Cyber Essentials Plus and ISO 27001 compliance requirements - in particular the patch management, malware protection, and access control domains.

If you want help prioritising these controls for your environment or identifying where gaps already exist, call us on 0113 467 8855 or email info@precursorsecurity.com. Our managed endpoint security service includes configuration review and ongoing monitoring for UK mid-market and enterprise organisations.

Frequently Asked Questions

What are the most important Microsoft 365 security hardening settings for desktop Office?

The three highest-priority controls are: blocking macro-enabled file types at the email gateway (stops the majority of malware delivery attempts before they reach the user), disabling VBA macros via Group Policy (neutralises the most common code execution technique), and enabling Attack Surface Reduction (ASR) rules (blocks process-level behaviours that macros and exploits rely on). These three alone address the highest-volume attack vectors documented in real-world campaigns.

What are Office 365 security best practices for organisations with remote workers?

For remote and hybrid environments where Group Policy may not reach all devices, deploy Office security settings via Microsoft Intune MDM policies. Prioritise: enforcing macro blocking through Intune configuration profiles, enabling ASR rules through Microsoft Defender for Endpoint, and ensuring Protected View is enforced for all internet-sourced files. Supplement with Exchange Online Protection (EOP) transport rules to block dangerous attachment types at the mail gateway before files reach the endpoint.

How do I disable macros in Office 365 using Group Policy?

Navigate to User Configuration > Administrative Templates > Microsoft Office [version] > Security Settings > Block macros from running in Office files from the Internet and set this to Enabled. For the equivalent Intune MDM policy, configure the Office CSP under the Security Settings node. To verify the setting is applied, open Word, go to File > Options > Trust Center > Trust Center Settings > Macro Settings - if Group Policy has taken effect, all options will be greyed out and the policy-enforced setting will be displayed.

Which Microsoft Office vulnerabilities are most actively exploited?

The most significant recent examples are CVE-2021-40444 (MSHTML/ActiveX remote code execution triggered via Office documents, exploited in the wild prior to patch), and the 2017 DDE exploitation campaigns associated with APT28 (Fancy Bear), which executed PowerShell payloads without requiring macros. CVE-2017-11826 (memory corruption in the Office XML parser) demonstrated that even the document parsing layer can be weaponised. Keeping Office fully patched and enabling Office File Validation (OFV) are the primary mitigations for parser-based vulnerabilities.

How can I verify that my Microsoft Office hardening controls are working?

Use Atomic Red Team's open-source test library (github.com/redcanaryco/atomic-red-team) to simulate specific attack techniques: T1559.002 for DDE execution, T1137.001 for macro persistence, and T1204.002 for malicious file execution. Run tests in an isolated environment and confirm alerts fire in Microsoft Defender or your SIEM. For cloud environments, review Microsoft Secure Score in the Microsoft 365 Defender portal. The CIS Microsoft Office Enterprise Benchmark (v1.2.0) at cisecurity.org/benchmark/microsoft_office provides a checklist you can audit your configuration against.