Attack Surface Reduction rules don't just detect malware — they block the behaviors malware uses (like Office applications launching executable child processes). This is prevention at the behavior layer, not the signature layer.
Most organizations leave ASR rules disabled because they fear breaking legacy applications or interrupting business workflows. The fear is justified — incorrectly tuned ASR rules halt production lines and flood helpdesks.
Confidence-Based deployment: Start in Audit Mode to visualize impact, move to Warn Mode to educate users, and enforce Block Mode only after exclusions are tuned. This eliminates the false positive risk that keeps ASR disabled.
You neutralize entire classes of attacks — including Ransomware and Zero-Days — by eliminating the vectors they rely on, without generating a single support ticket from a misconfigured rule.
Understanding Attack Surface Reduction
One of the most powerful yet frequently overlooked capabilities of Microsoft Defender for Endpoint is Attack Surface Reduction. Despite its ambitious name, ASR delivers substantial security value by targeting software behaviors that attackers commonly exploit.
ASR focuses on identifying and blocking suspicious activities that legitimate applications rarely perform during normal operations, but which are frequently leveraged by malware and threat actors. By implementing intelligent behavioral analysis, ASR can prevent attacks before they fully materialize — significantly reducing your organization's risk exposure.
"Attack Surface Reduction represents a proactive approach to endpoint security, targeting the behaviors and techniques commonly exploited by attackers before they can establish a foothold in your environment."
Primary ASR Target Behaviors
Executable Downloads
Blocking files and scripts that attempt to download or execute additional payloads
Obfuscated Scripts
Detecting and preventing execution of suspicious or obfuscated script content
Anomalous Application Behavior
Identifying activities that applications do not typically perform during legitimate operations
Process Injection Techniques
Preventing malicious code injection into legitimate processes
Credential Theft Attempts
Blocking unauthorized access to authentication subsystems including LSASS
While some legitimate applications may occasionally exhibit these behaviors, they represent significant security risks due to their frequent abuse by attackers. ASR rules provide intelligent constraints on these activities — helping organizations maintain security without disrupting productivity when properly tuned.
ASR Rules Reference: The 6 Attack Categories
Microsoft Defender for Endpoint provides 17 ASR rules organized by attack vector. The following covers the most consequential rules with NPS Advisories — the operational context that determines whether enabling each rule in Block mode is safe for your environment.
Block executable content from email client and webmail
BE9BA2D9-53EA-4CDC-84E5-9B1EEEE46550NPS Advisory
This is typically the safest rule to enable first — it blocks .exe, .dll, and script files arriving via Outlook or webmail. Most organizations can enable this in Block mode immediately.
Operational Risk
Some industries still receive legitimate executables via email (software vendors, development partners). Run Audit mode for two weeks and review the logs before blocking.
Block all Office applications from creating child processes
D4F940AB-401B-4EFC-AADC-AD5F3C50688ANPS Advisory
The most impactful — and most dangerous — rule. This prevents Word, Excel, and PowerPoint from spawning other processes, which is exactly how macro-based malware operates.
Operational Risk
This single rule will break more business processes than any other. We've seen it halt invoice processing, disable HR onboarding workflows, and crash ERP integrations that use Excel automation. Can you name every Office macro and add-in used across your organization? If not, enabling Block mode means your first Monday morning will start with a flood of helpdesk tickets.
+4 more rules in this category (Block Office macros from creating executable content, Block Office apps from injecting code into other processes, Block Win32 API calls from Office macros, Block Office communication app from creating child processes) — in the complete reference.
Block execution of potentially obfuscated scripts
5BEB7EFE-FD9A-4556-801D-275E5FFC04CCNPS Advisory
Uses the AMSI (Antimalware Scan Interface) to detect encoded or obfuscated PowerShell, JavaScript, and VBScript — the hallmark of "fileless" malware.
Operational Risk
Many legitimate admin scripts use encoding for handling special characters or passwords. We've seen this rule block SCCM task sequences, deployment scripts, and vendor-provided automation tools. Does your IT team have a standard for script development that avoids obfuscation patterns? If not, expect friction with your operations team.
+2 more rules in this category (Block JavaScript or VBScript from launching downloaded executable content, Block execution of files unless they meet a prevalence, age, or trusted list criterion) — in the complete reference.
Use advanced protection against ransomware
c1db55ab-c21a-4637-bb3f-a12568109d35NPS Advisory
Applies aggressive heuristics to identify ransomware-like behavior: rapid file modifications, suspicious encryption patterns, and shadow copy tampering.
Operational Risk
Legitimate encryption and backup tools can trigger this rule. We've seen it flag disk encryption software, archive utilities, and database compaction routines. Have you inventoried all applications that perform bulk file operations or encryption? Without that list, you'll be chasing false positives for weeks.
Block credential stealing from Windows local security authority subsystem (lsass.exe)
9e6c4e1f-7d60-472f-ba1a-a39ef669e4b2NPS Advisory
Critical for stopping lateral movement. Prevents tools like Mimikatz from dumping credentials from memory — a core technique in almost every ransomware attack chain.
Operational Risk
Some legitimate backup agents, PAM solutions, and security tools need to access LSASS. We've seen this rule break Veeam backup jobs and conflict with CyberArk integrations. Do you have a complete inventory of every application that touches credential stores? Audit mode data alone won't tell you which ones are business-critical until they break.
+1 more rule in this category (Block abuse of exploited vulnerable signed drivers) — in the complete reference.
Block process creations originating from PSExec and WMI commands
d1e49aac-8f56-4280-b9ba-993a6d77406cNPS Advisory
PSExec and WMI are the primary tools attackers use for lateral movement after initial compromise. This rule significantly limits their ability to spread across your network.
Operational Risk
This rule will impact your IT operations team more than attackers. SCCM, Intune, and most RMM tools rely heavily on WMI for remote management. Have you mapped every administrative workflow that uses remote execution? Blocking this without proper exclusions means your patching pipelines, software deployment, and remote troubleshooting capabilities go dark.
+1 more rule in this category (Block persistence through WMI event subscription) — in the complete reference.
2 rules covering USB device control (Block untrusted and unsigned processes from running from USB) and Adobe Reader hardening (Block Adobe Reader from creating child processes) — in the complete reference.
Get the Complete 17-Rule ASR Reference
The six categories above cover the most consequential rules with NPS Advisories. The complete reference documents all 17 ASR rules — including per-rule exclusion support, Warn mode availability, licensing requirements, and the specific business applications we've seen each rule conflict with.
Request the Full ASR Rules GuideThe Confidence-Based Deployment Framework
Turning on ASR rules without proper tuning creates more problems than it solves: alert floods, blocked business apps, frustrated users. The Confidence-Based framework moves you from disabled to enforced safely.
Audit Mode — Visualize Without Blocking
Enable all target rules in Audit mode. MDE logs every detection as an alert without taking blocking action. Review the Microsoft 365 Defender portal for two to four weeks. Identify which detections are legitimate business processes and build your exclusion list before any rule goes live.
Warn Mode — Educate While Enforcing
Move rules to Warn mode. Users see a notification when a blocked action is attempted and can override for 24 hours. This trains users, reduces support pressure, and reveals remaining false positives with real-world context. Note: three rules do not support Warn mode (JavaScript/VBScript downloader, WMI persistence, advanced ransomware protection).
Block Mode — Enforce with Confidence
Move to Block mode only after exclusions are tuned and override rates have declined to near zero. Document every exclusion with a business justification. False positives that reach Block mode require a Submission First workflow: validate the file in a cloud sandbox before creating a global allowance indicator — response time under 30 minutes.
Licensing and System Requirements
Understanding the technical and licensing requirements determines which ASR capabilities are available in your environment.
| License Tier | Capabilities |
|---|---|
| Windows 10 Pro | Core ASR rule enforcement capability. Event Viewer for log review. No centralized management or analytics portal. |
| Microsoft E3 | Full ASR rule feature set. Event Viewer log access. Centralized policy deployment via Group Policy or Intune. The baseline for enterprise ASR deployments. |
| Microsoft E5 / MDE Plan 2 | Full ASR management and reporting layer in Microsoft 365 Defender portal. Endpoint detection correlation, timeline views, advanced hunting via KQL, and workflow automation for false positive remediation. |
Microsoft Defender Antivirus must be active (not passive mode) for ASR rules to function. If a third-party antivirus is installed, MDE automatically enters passive mode and ASR is disabled.
Rule Exclusions: What You Need to Know
Exclusions are necessary — but they carry significant operational caveats that most deployment guides omit.
Exclusions apply globally across ALL ASR rules. You cannot exclude a path from one rule only — the exclusion affects every active ASR rule simultaneously.
Excluded files and folders run without any logging or alert. If a threat actor drops a payload in an excluded path, ASR provides zero protection and zero visibility.
ASR exclusions are managed separately from Microsoft Defender Antivirus exclusions. Do not assume that AV exclusions carry over.
Wildcards in exclusion paths are supported only via Microsoft Intune. SCCM-managed ASR policies do not support wildcards.
ASR rules run as NT AUTHORITY\SYSTEM. You cannot use user-profile environment variables (like %USERPROFILE%) in exclusion paths — they will not resolve correctly.
The Four ASR Rule States
Not Configured
Disabled. The rule takes no action and generates no events. This is the default for all rules.
Audit
Detects and logs matching activity in the Microsoft 365 Defender portal. Does not block. Use this for initial deployment and tuning.
Warn
Blocks the detected behavior but shows the user a notification with a 24-hour override option. Useful for user education before full enforcement.
Block
Enforces the rule and logs the blocked event. The target action is prevented immediately. Deploy only after Audit and Warn phases are complete.
You Do Not Have to Enable Everything at Once
Many business applications were written with limited security concerns, and they may perform tasks that resemble malicious activity. The recommended approach is to enable ASR rules in Audit mode first, monitor the data, build exclusions for necessary applications, and then deploy without impacting productivity.
Office macro rules deserve special attention. They represent an effortless attack vector, but many Dutch organizations still depend on Office macros for core business processes. Special planning is required before enforcing the related rules in Block mode — it cannot be a surprise for Finance, HR, or ERP teams.
New Paradigm Security is ready to help you deploy MDE's ASR capabilities using your tool of choice — Intune, SCCM, or Group Policy. Contact us to learn how we map, tune, and enforce these guardrails in complex environments.
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