Attack Surface Reduction (ASR) Policies in Intune

Beginning September 2025, Information Technology Services (ITS) will roll out Microsoft Defender Attack Surface Reduction (ASR) policies to all Windows devices managed in Intune.
ASR rules harden PCs by blocking or auditing behaviors that malware frequently abuses—such as Office macros spawning scripts or credential-stealing attacks—before traditional antivirus signatures even come into play.

Why we’re doing this

Goal	Benefit to the organization

Goal	Benefit to the organization
Reduce zero-day risk	ASR rules target entire classes of attack techniques, not single CVEs.
Meet Microsoft “Standard Protection” baseline	Defender’s security baseline now flags several ASR rules as required for basic protection.
Streamline incident response	Fewer successful exploits mean fewer tickets and faster containment for SOC/IRT teams.

Scope & Audience

Primary: Desktop Engineering, Security Operations, Intune administrators
Secondary: Power users & general staff who may notice new block or warning pop-ups

Roll-out Plan

All phases will be monitored through Defender Security Center → Incidents & alerts and Intune Endpoint Analytics dashboards. Feedback loops with local IT contacts are baked into each step.

Phase	Target devices	Mode	Notes

Phase	Target devices	Mode	Notes
0 – Pilot (complete)	50 ITS workstations	Audit	Validated logging & exclusions
1 – ITS & Shared Spaces	All ITS Employee Devices Classroom/lab PCs Conference-room PCs	Audit → Block	Three-week soak in Audit, then flip to Block
2 – Campus Group A	Random ⅓ of colleges/departments	Audit → Block	Composition chosen by Intune dynamic group (deviceId mod 3 = 0)
3 – Campus Group B	Next random ⅓ of units	Audit → Block	Same 21-day cadence
4 – Campus Group C	Final ⅓ of units	Audit → Block	Completes full population

*Dates may shift if telemetry reveals unexpected application impact.

How the groups are built

Randomized assignment: Devices are auto-tagged into Group A/B/C via an Intune dynamic rule that hashes the device ID; this prevents bias toward any single department.
Staggered 21-day windows: Each group spends three weeks in Audit so Desktop Engineering can review logs and add exclusions before the automatic switch to Block mode.
Communication cadence:
- T-7 days: Heads-up email to local IT contacts and unit leadership
- T-0: Post in Daily Digest
- T + 0–7 days (Audit): Daily telemetry review; high-noise rules adjusted or excluded
- T + 21 days: Mode change to Block followed by intensified monitoring for two weeks

What End Users Will Experience

Scenario	User prompt / action

Scenario	User prompt / action
Legitimate action blocked	A Windows Security toast: “This action was blocked by your IT administrator.” Clicking Details shows the rule name and a link to this KB.
Allowed via Warn mode (future enhancement)	A dialog offering Unblock (grants 24-hour exemption) or Cancel.
No noticeable change	Most users won’t see anything unless their workflow triggers a rule.

Administrator Checklist

Confirm device readiness
Windows 10 21H2 or later and Defender platform ≥ 4.18.2008.9 required.
Verify inclusion in correct group
Endpoint security → Attack surface reduction → “ITS-ASR-Baseline” and group tag.
Review Audit telemetry during the three-week soak period.
Add exclusions for business-critical apps if necessary.
Switch to Block mode on the scheduled date (automatic unless postponed).
Monitor alerts after Block activation; escalate sustained false-positives via Jira Service Desk.

Troubleshooting / FAQs

Question	Answer

Question	Answer
An approved app is now blocked. What do I do?	Open a Jira ticket with the ASR event ID, timestamp, and full file path. Device Management and Security can look into an exclusion
Can I temporarily disable a rule for testing?	Yes—Intune’s “Override” profile lets you set a rule to Audit for a device group. Use sparingly and time-bound the scope.
Will ASR replace AppLocker or Defender Application Control?	No; ASR complements allow-listing solutions by focusing on behavior, not file hash or path.