Reduce attack surfaces with Windows Defender Exploit Guard

Applies to:

  • Windows 10, version 1709 (and later)
  • Microsoft Office 365
  • Microsoft Office 2016
  • Microsoft Office 2013
  • Microsoft Office 2010

Audience

  • Enterprise security administrators

Manageability available with

  • Group Policy
  • PowerShell
  • Configuration service providers for mobile device management

Attack surface reduction helps prevent actions and apps that are typically used by exploit-seeking malware to infect machines.

It is part of Windows Defender Exploit Guard.

Tip

You can also visit the Windows Defender Testground website at demo.wd.microsoft.com to confirm the feature is working and see how it works.

Attack surface reduction works best with Windows Defender Advanced Threat Protection - which gives you detailed reporting into Windows Defender EG events and blocks as part of the usual alert investigation scenarios.

The feature is comprised of a number of rules, each of which target specific behaviors that are typically used by malware and malicious apps to infect machines, such as:

  • Executable files and scripts used in Office apps or web mail that attempt to download or run files
  • Scripts that are obfuscated or otherwise suspicious
  • Behaviors that apps undertake that are not usually initiated during normal day-to-day work

See the Attack surface reduction rules section in this topic for more information on each rule.

When a rule is triggered, a notification will be displayed from the Action Center. You can customize the notification with your company details and contact information. You can also enable the rules individually to customize what techniques the feature monitors.

You can also use audit mode to evaluate how Attack surface reduction would impact your organization if it were enabled.

Attack surface reduction rules

The following sections describe what each rule does. Each rule is identified by a rule GUID, as in the following table:

Rule name GUID
Block executable content from email client and webmail BE9BA2D9-53EA-4CDC-84E5-9B1EEEE46550
Block Office applications from creating child processes D4F940AB-401B-4EFC-AADC-AD5F3C50688A
Block Office applications from creating executable content 3B576869-A4EC-4529-8536-B80A7769E899
Block Office applications from injecting code into other processes 75668C1F-73B5-4CF0-BB93-3ECF5CB7CC84
Block JavaScript or VBScript from launching downloaded executable content D3E037E1-3EB8-44C8-A917-57927947596D
Block execution of potentially obfuscated scripts 5BEB7EFE-FD9A-4556-801D-275E5FFC04CC
Block Win32 API calls from Office macro 92E97FA1-2EDF-4476-BDD6-9DD0B4DDDC7B

The rules apply to the following Office apps running on Windows 10, version 1709. See the Applies to section at the start of this topic for a list of supported Office version.

Supported Office apps:

  • Microsoft Word
  • Microsoft Excel
  • Microsoft PowerPoint
  • Microsoft OneNote

The rules do not apply to any other Office apps.

Rule: Block executable content from email client and webmail

This rule blocks the following file types from being run or launched from an email seen in either Microsoft Outlook or webmail (such as Gmail.com or Outlook.com):

  • Executable files (such as .exe, .dll, or .scr)
  • Script files (such as a PowerShell .ps, VisualBasic .vbs, or JavaScript .js file)
  • Script archive files

Rule: Block Office applications from creating child processes

Office apps, such as Word or Excel, will not be allowed to create child processes.

This is a typical malware behavior, especially for macro-based attacks that attempt to use Office apps to launch or download malicious executables.

Rule: Block Office applications from creating executable content

This rule targets typical behaviors used by suspicious and malicious add-ons and scripts (extensions) that create or launch executable files. This is a typical malware technique.

Extensions will be blocked from being used by Office apps. Typically these extensions use the Windows Scripting Host (.wsh files) to run scripts that automate certain tasks or provide user-created add-on features.

Rule: Block Office applications from injecting code into other processes

Office apps, such as Word, Excel, or PowerPoint, will not be able to inject code into other processes.

This is typically used by malware to run malicious code in an attempt to hide the activity from antivirus scanning engines.

Rule: Block JavaScript or VBScript From launching downloaded executable content

JavaScript and VBScript scripts can be used by malware to launch other malicious apps.

This rule prevents these scripts from being allowed to launch apps, thus preventing malicious use of the scripts to spread malware and infect machines.

Rule: Block execution of potentially obfuscated scripts

Malware and other threats can attempt to obfuscate or hide their malicious code in some script files.

This rule prevents scripts that appear to be obfuscated from running.

It uses the AntiMalwareScanInterface (AMSI) to determine if a script is potentially obfuscated, and then blocks such a script, or blocks scripts when an attempt is made to access them.

Rule: Block Win32 API calls from Office macro

Malware can use macro code in Office files to import and load Win32 DLLs, which can then be used to make API calls to allow further infection throughout the system.

This rule attempts to block Office files that contain macro code that is capable of importing Win32 DLLs.

Requirements

The following requirements must be met before Attack surface reduction will work:

Windows 10 version Windows Defender Antivirus
Insider Preview build 16232 or later (dated July 1, 2017 or later) Windows Defender AV real-time protection must be enabled

Review Attack surface reduction events in Windows Event Viewer

You can review the Windows event log to see events that are created when an Attack surface reduction rule is triggered (or audited):

  1. Download the Exploit Guard Evaluation Package and extract the file asr-events.xml to an easily accessible location on the machine.

  2. Type Event viewer in the Start menu to open the Windows Event Viewer.

  3. On the left panel, under Actions, click Import custom view...

    Animation showing the import custom view on the Event viewer window

  4. Navigate to the Exploit Guard Evaluation Package, and select the file asr-events.xml. Alternatively, copy the XML directly.

  5. Click OK.

  6. This will create a custom view that filters to only show the following events related to Attack surface reduction:

    Event ID Description
    5007 Event when settings are changed
    1122 Event when rule fires in Audit-mode
    1121 Event when rule fires in Block-mode

Event fields

  • ID: matches with the Rule-ID that triggered the block/audit.
  • Detection time: Time of detection
  • Process Name: The process that performed the "operation" that was blocked/audited
  • Description: Additional details about the event or audit, including the signature, engine, and product version of Windows Defender Antivirus

In this section

Topic Description
Evaluate Attack surface reduction Use a tool to see a number of scenarios that demonstrate how the feature works, and what events would typically be created.
Enable Attack surface reduction Use Group Policy, PowerShell, or MDM CSPs to enable and manage Attack surface reduction in your network.
Customize Attack surface reduction Exclude specified files and folders from being evaluated by Attack surface reduction and customize the notification that appears on a user's machine when a rule blocks an app or file.