If you manage Business Premium tenants for SMB clients, Attack Surface Reduction (ASR) rules are arguably the highest-leverage defensive control you’re not fully exploiting. They ship with Defender for Business (included in Business Premium), they cost nothing extra to enable, and they block the precise behaviours that commodity ransomware and living-off-the-land attacks depend on — Office spawning child processes, Win32 API calls from macros, credential theft from LSASS, and PSExec-style lateral movement. This post assumes you already know what ASR is. The goal here is to help L2/L3 engineers deploy it without breaking line-of-business apps.
Prerequisites the documentation underplays
ASR has hard dependencies that silently downgrade rules to “not configured” if missed. Microsoft Defender Antivirus must be the active AV (not passive, not disabled by a third-party product), real-time protection must be on, and cloud-delivered protection must be on — several rules won’t evaluate without cloud lookups. Verify these under Microsoft Defender portal → Endpoints → Configuration management → Device configuration, and confirm the device shows as “Active” in the device inventory. Tamper Protection must also be enabled tenant-wide; without it, local admins or malware can disable ASR in seconds.
Where to configure in Business Premium
Business Premium gives you two supported paths. The simplified experience lives at security.microsoft.com → Device configuration → Next-generation protection / Firewall, but ASR rules specifically are configured through Intune. In the Microsoft Intune admin center, go to Endpoint security → Attack surface reduction and create an “Attack surface reduction rules” profile targeting Windows 10/11. Avoid mixing this with legacy Endpoint Protection templates or Group Policy — conflicting sources result in the classic “last writer wins” behaviour, and you will spend hours chasing phantom rules. If a device is co-managed or has stale GPOs, check Event Viewer → Applications and Services Logs → Microsoft → Windows → Windows Defender → Operational (event IDs 1121, 1122, 5007) to confirm which source applied.
The rollout pattern that works
Do not flip rules straight to Block. The proven pattern is: enable every rule in Audit mode, leave it for two to four weeks, then review the ASR report at security.microsoft.com → Reports → Attack surface reduction rules. Filter by rule and by device, identify the noisy ones, and build per-rule exclusions before switching to Block. Where available, use Warn mode as a staged step — the user gets a toast and can unblock once, which is invaluable for finance teams running macro-heavy spreadsheets. Warn mode is Windows 10 1809+ only; older builds silently treat it as Block.
Start with the “standard protection” set Microsoft recommends: Block credential stealing from LSASS, Block abuse of exploited vulnerable signed drivers, and Block persistence through WMI event subscription. These have the lowest false-positive rate and the highest return. The two rules that most commonly break things are “Block Office applications from creating child processes” (breaks legitimate mail-merge and older add-ins) and “Block executable content from email client and webmail” (breaks zipped installers).
Pitfalls to pre-empt
Exclusions are path-based and case-sensitive on the file name portion — wildcards work on folders, not extensions. Per-rule exclusions (added in later releases) are preferred over global Defender exclusions; use them so you don’t inadvertently hole-punch the whole AV. If a rule appears to do nothing, check whether the device is in EDR block mode and whether Defender is the active AV — ASR is silently disabled under third-party AV. Finally, ASR events do not always surface as alerts; plumb them into your SIEM via the Defender API or Advanced Hunting (DeviceEvents where ActionType startswith "Asr").
CIAOPS 