Reduce attack surfaces with attack surface reduction rules

Applies to:

  • Windows Defender Advanced Threat Protection (Windows Defender ATP)

Attack surface reduction rules help prevent actions and apps that are typically used by exploit-seeking malware to infect machines. Attack surface reduction rules work best with Windows Defender Advanced Threat Protection, which gives you detailed reporting into events and blocks as part of the usual alert investigation scenarios.

Attack surface reduction rules each 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

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 rules would impact your organization if they were enabled.

Requirements

Attack surface reduction rules require Windows 10 Enterprise E5 and Windows Defender AV real-time protection.

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 all 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
Block executable files from running unless they meet a prevalence, age, or trusted list criteria 01443614-cd74-433a-b99e-2ecdc07bfc25
Use advanced protection against ransomware c1db55ab-c21a-4637-bb3f-a12568109d35
Block credential stealing from the Windows local security authority subsystem (lsass.exe) 9e6c4e1f-7d60-472f-ba1a-a39ef669e4b2
Block process creations originating from PSExec and WMI commands d1e49aac-8f56-4280-b9ba-993a6d77406c
Block untrusted and unsigned processes that run from USB b2b3f03d-6a65-4f7b-a9c7-1c7ef74a9ba4
Block only Office communication applications from creating child processes 26190899-1602-49e8-8b27-eb1d0a1ce869
Block Adobe Reader from creating child processes 7674ba52-37eb-4a4f-a9a1-f0f9a1619a2c

The rules apply to the following 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 all Office applications from creating child processes

Office apps will not be allowed to create child processes. This includes Word, Excel, PowerPoint, OneNote, and Access.

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.

Rule: Block executable files from running unless they meet a prevalence, age, or trusted list criteria

This rule blocks the following file types from being run or launched unless they meet prevalence or age criteria set by admins, or they are in a trusted list or exclusion list:

  • Executable files (such as .exe, .dll, or .scr)

Note

You must enable cloud-delivered protection to use this rule.

Rule: Use advanced protection against ransomware

This rule provides an extra layer of protection against ransomware. Executable files that enter the system will be scanned to determine whether they are trustworthy. If the files exhibit characteristics that closely resemble ransomware, they are blocked from being run or launched, provided they are not already in the trusted list or exception list.

Note

You must enable cloud-delivered protection to use this rule.

Rule: Block credential stealing from the Windows local security authority subsystem (lsass.exe)

Local Security Authority Subsystem Service (LSASS) authenticates users who log in to a Windows computer. Windows Defender Credential Guard in Windows 10 normally prevents attempts to extract credentials from LSASS. However, some organizations can't enable Credential Guard on all of their computers because of compatibility issues with custom smartcard drivers or other programs that load into the Local Security Authority (LSA). In these cases, attackers can use tools like Mimikatz to scrape cleartext passwords and NTLM hashes from LSASS. This rule helps mitigate that risk by locking down LSASS.

Note

Some apps are coded to enumerate all running processes and to attempt opening them with exhaustive permissions. This results in the app accessing LSASS even when it's not necessary. ASR will deny the app's process open action and log the details to the security event log. Entry in the event log for access denial by itself is not an indication of the presence of a malicious threat.

Rule: Block process creations originating from PSExec and WMI commands

This rule blocks processes through PsExec and WMI commands from running, to prevent remote code execution that can spread malware attacks.

Warning

[Only use this rule if you are managing your devices with Intune or another MDM solution. This rule is incompatible with management through System Center Configuration Manager because this rule blocks WMI commands that the Configuration Manager client uses to function correctly.]

Rule: Block untrusted and unsigned processes that run from USB

With this rule, admins can prevent unsigned or untrusted executable files from running from USB removable drives, including SD cards. Blocked file types include:

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

Rule: Block only Office communication applications from creating child processes (available for beta testing)

Office communication apps will not be allowed to create child processes. This includes Outlook.

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 Adobe Reader from creating child processes (available for beta testing)

This rule blocks Adobe Reader from creating child processes.

Review attack surface reduction rule 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 rules:

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