Attack Surface Reduction Rules to Enable First

Introduction

In today's complex cyber threat landscape, proactive defense mechanisms are paramount. Attack Surface Reduction (ASR) rules, a core component of Microsoft Defender for Endpoint, offer a powerful suite of capabilities designed to prevent common and emerging attack techniques by restricting specific behaviors often exploited by malware and malicious actors. These rules operate at the endpoint level, monitoring and blocking suspicious activities that deviate from expected norms, without requiring file-based signatures.

This article targets security architects, IT administrators, and security operations center (SOC) analysts vested in bolstering their organization's endpoint security posture. We will delve into the strategic implementation of ASR rules, specifically identifying the foundational rules that should be prioritized for immediate enablement. By focusing on these high-impact rules first, organizations can achieve a significant reduction in their attack surface with minimal operational overhead, laying a solid foundation for a more comprehensive security strategy.

Why this matters

Implementing ASR rules delivers tangible benefits across several critical organizational domains. From a compliance perspective, actively reducing the attack surface aligns with mandates from regulatory bodies and industry standards that require robust endpoint protection against known threats. Technically, ASR rules directly mitigate exploit efforts, credential theft, and ransomware propagation, which are common vectors for costly breaches. Minimizing successful incursions directly translates to reduced incident response costs, decreased system downtime, and avoidance of reputational damage.

Furthermore, by proactively blocking malicious behaviors, ASR rules contribute to a more secure and productive environment. Employees are less likely to encounter infected files or suspicious activities that disrupt their work, while IT staff can dedicate fewer resources to remediation and more to strategic initiatives. This layered defense, integral to a Zero Trust architecture, hardens endpoints against sophisticated attack chains, thereby improving the overall resilience of the organization's digital assets.

Key concepts

• Attack Surface Reduction (ASR) Rules: A feature of Microsoft Defender for Endpoint that helps prevent common attack techniques by blocking suspicious or malicious actions at the endpoint. They are behavior-based and don't rely on signature files.
• Audit Mode: A deployment mode for ASR rules where detected activities are logged to the event log but not blocked. This allows organizations to assess potential impact before enforcing rules.
• Block Mode: A deployment mode where ASR rules actively block detected malicious activities and log the event.
• Exclusions: Specific files, folders, or processes that can be exempted from ASR rule evaluation to prevent false positives or accommodate legitimate applications.
• Defender for Endpoint: Microsoft's enterprise endpoint security platform that provides preventative protection, post-breach detection, automated investigation, and response capabilities. ASR rules are managed within this platform.
• Microsoft Intune (Endpoint Manager): A cloud-based service that focuses on mobile device management (MDM) and mobile application management (MAM). It's a primary tool for deploying and managing ASR rules across organizational endpoints.
• Group Policy: On-premises management tool used to configure user and computer settings, including ASR rules, within an Active Directory environment.

Step-by-step implementation

The most common and effective way to deploy ASR rules at scale is through Microsoft Intune.

1. Access the Microsoft Defender portal: Navigate to https://security.microsoft.com.
2. Navigate to Attack Surface Reduction rules: In the left navigation pane, select Endpoint security > Attack surface reduction.
3. Create a new policy: Click Create new policy.
4. Select Platform and Profile:

Platform: Windows 10, Windows 11, and Windows Server Profile: Attack Surface Reduction Rules Click Create*.

1. Configuration Settings: In the "Configuration settings" step, you will define the ASR rules. Identify the initial rules to set to Audit mode for assessment, then transition to Block mode.

Prioritize the following rules for initial audit/block: Block Adobe Reader from creating child processes: Prevents exploits targeting Adobe products from launching malicious executables. Block all Office applications from creating child processes: A crucial defense against macro-enabled malware and exploit kits. Block Office applications from injecting code into other processes: Prevents techniques used by adversaries to gain persistence or elevate privileges. Block credential stealing from the Windows local security authority subsystem (lsass.exe): Protects against a paramount method of credential theft. Block execution of potentially obfuscated scripts: Targets scripting attacks often used for initial access and lateral movement. Block executable content from email client and webmail:* Prevents common delivery mechanisms for malware.

For each selected rule, set its status to Audit initially. After a period of monitoring (e.g., 2-4 weeks), transition compliant rules to Block.

1. Assign the Policy: On the "Assignments" step, select the user or device groups to which this policy will apply. Start with a pilot group (e.g., IT staff) before expanding to a broader audience.
2. Review and Create: Review your settings and click Create.

To gather audit event data and plan for exclusions, you can utilize PowerShell.

# Connect to Microsoft Defender for Endpoint via PowerShell
# Ensure you have the Defender for Endpoint PowerShell module installed
# Install-Module -Name WindowsDefenderATP -Force

# Get ASR rule audit events from the last 7 days
# Replace 'YourOrgId' with your actual Microsoft Defender for Endpoint Tenant ID.
# To find your Tenant ID: MDE Portal -> Settings -> Endpoints -> Permissions -> API secrets
Get-WinEvent -FilterHashTable @{LogName='Microsoft-Windows-Windows Defender/Operational'; ID=1121} -MaxEvents 500 |
    Where-Object { $_.Message -like '*Attack Surface Reduction Rule*' -and $_.TimeCreated -ge (Get-Date).AddDays(-7) } |
    Select-Object TimeCreated, Message |
    Format-List

This PowerShell command helps identify applications that might be legitimately blocked when ASR rules are in audit mode, informing any necessary exclusions.

Example configuration

Here's an example of a JSON snippet representing a portion of an ASR policy, focusing on the recommended initial rules configured for audit mode. This is illustrative; actual Intune policy exports might be more verbose.

{
  "displayName": "ASR Rules - Initial Audit Pilot",
  "templateType": "endpointSecurity-attackSurfaceReduction",
  "settings": [
    {
      "settingId": "c1db55ab-c21a-4637-bb3f-a1256810a975",
      "settingInstance": {
        "definitionUrl": "https://graph.microsoft.com/beta/deviceManagement/groupPolicyDefinitions/c1db55ab-c21a-4637-bb3f-a1256810a975",
        "value": {
          "@odata.type": "#microsoft.graph.defenderAttackSurfaceReductionRule",
          "id": "c1db55ab-c21a-4637-bb3f-a1256810a975",
          "name": "Block Adobe Reader from creating child processes",
          "state": "auditMode"
        }
      }
    },
    {
      "settingId": "d4f940ab-401b-4efc-aadc-ad5f3c50688a",
      "settingInstance": {
        "definitionUrl": "https://graph.microsoft.com/beta/deviceManagement/groupPolicyDefinitions/d4f940ab-401b-4efc-aadc-ad5f3c50688a",
        "value": {
          "@odata.type": "#microsoft.graph.defenderAttackSurfaceReductionRule",
          "id": "d4f940ab-401b-4efc-aadc-ad5f3c50688a",
          "name": "Block all Office applications from creating child processes",
          "state": "auditMode"
        }
      }
    },
    {
      "settingId": "7674ba52-3d2a-4596-ab47-f4b01e0b0a2d",
      "settingInstance": {
        "definitionUrl": "https://graph.microsoft.com/beta/deviceManagement/groupPolicyDefinitions/7674ba52-3d2a-4596-ab47-f4b01e0b0a2d",
        "value": {
          "@odata.type": "#microsoft.graph.defenderAttackSurfaceReductionRule",
          "id": "7674ba52-3d2a-4596-ab47-f4b01e0b0a2d",
          "name": "Block Office applications from injecting code into other processes",
          "state": "auditMode"
        }
      }
    },
    {
      "settingId": "9e6c4e71-79d1-4b95-a226-5b878d0faf12",
      "settingInstance": {
        "definitionUrl": "https://graph.microsoft.com/beta/deviceManagement/groupPolicyDefinitions/9e6c4e71-79d1-4b95-a226-5b878d0faf12",
        "value": {
          "@odata.type": "#microsoft.graph.defenderAttackSurfaceReductionRule",
          "id": "9e6c4e71-79d1-4b95-a226-5b878d0faf12",
          "name": "Block credential stealing from the Windows local security authority subsystem (lsass.exe)",
          "state": "auditMode"
        }
      }
    },
    {
      "settingId": "5beb7efe-fd9a-4556-801d-275e5ffc04cc",
      "settingInstance": {
        "definitionUrl": "https://graph.microsoft.com/beta/deviceManagement/groupPolicyDefinitions/5beb7efe-fd9a-4556-801d-275e5ffc04cc",
        "value": {
          "@odata.type": "#microsoft.graph.defenderAttackSurfaceReductionRule",
          "id": "5beb7efe-fd9a-4556-801d-275e5ffc04cc",
          "name": "Block execution of potentially obfuscated scripts",
          "state": "auditMode"
        }
      }
    },
    {
      "settingId": "be9ba2d9-53ea-4d9a-8586-4cecb0372dfd",
      "settingInstance": {
        "definitionUrl": "https://graph.microsoft.com/beta/deviceManagement/groupPolicyDefinitions/be9ba2d9-53ea-4d9a-8586-4cecb0372dfd",
        "value": {
          "@odata.type": "#microsoft.graph.defenderAttackSurfaceReductionRule",
          "id": "be9ba2d9-53ea-4d9a-8586-4cecb0372dfd",
          "name": "Block executable content from email client and webmail",
          "state": "auditMode"
        }
      }
    }
  ]
}

Common pitfalls

• Deploying directly to Block mode without auditing: This leads to widespread false positives, user frustration, and potential business disruption as legitimate applications are blocked.
• Neglecting a staggered rollout: Rolling out to all users at once prevents controlled testing, making it difficult to pinpoint issues and assess impact.
• Insufficient monitoring of audit events: Without regularly reviewing audit logs, organizations miss critical information needed to identify necessary exclusions and confidently transition rules to block mode.
• Overly broad exclusions: Adding too many or incorrectly scoped exclusions can inadvertently weaken the security posture that ASR rules are designed to enhance.
• Lack of communication with end-users and helpdesk: Users encountering blocked legitimate actions without prior notification or clear guidance can overwhelm IT support and foster resistance to security initiatives.
• Ignoring conflicting policies: Multiple deployment methods (e.g., Intune and Group Policy) or overlapping policies can lead to unpredictable behavior and reduce ASR effectiveness.

Best practices

• Adopt a phased approach (Audit -> Evaluate -> Exclude -> Block): Aligning with the Microsoft Cloud Adoption Framework's governance and security principles, this iterative process minimizes disruption and maximizes security effectiveness.
• Start with high-impact, low-false-positive rules: Prioritize rules that address common attack vectors with minimal expected impact on legitimate business operations. This builds confidence in the ASR program.
• Leverage Microsoft Defender for Endpoint telemetry: Utilize the Defender portal's "Reports" and "Automated investigations" to monitor ASR audit events, identify legitimate applications, and fine-tune exclusions.
• Principle of Least Privilege for Exclusions: When creating exclusions, be as specific as possible (e.g., specific file hash, signed process) rather than broad exclusions (e.g., entire folders or unsigned applications).
• Document and communicate changes: Maintain clear documentation of ASR rule configurations, exclusions, and the rationale behind them. Inform users and helpdesk staff about new security capabilities and how to report issues.
• Regularly review and update ASR policies: As per Zero Trust principles, continuously verify and adapt your security controls. New attack techniques emerge, and business applications change, necessitating periodic review of ASR rules and exclusions.

Further reading

• Use Attack Surface Reduction rules to prevent malware infection
• Test Attack Surface Reduction rules
• Attack Surface Reduction rules reference
• Configure Attack Surface Reduction rules for Microsoft Defender for Endpoint