CYBERDUDEBIVASH • THREATWIRE

How “DefenderWrite” Hijacks Microsoft Defender to Run Malicious DLLs

October 20, 2025 • Author: CyberDudeBivash • Read time: 10–12 min

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TL;DR: “DefenderWrite” (defensive codename) is an abuse-of-trust pattern: adversaries coerce legitimate Microsoft Defender processes or service paths into loading attacker-controlled DLLs. The outcome is signed-process masquerade, AV bypass, and reliable persistence. Defend with Tamper Protection, ASR rules, strict service permissions, WDAC/AppLocker allow-lists, and targeted EDR/SIEM hunts for DLL sideload chains, abnormal Defender child processes, and suspicious module loads.Contents

1. What is “DefenderWrite” (High Level)
2. Attack Flow (Conceptual)
3. Why It Works & Business Impact
4. How to Detect (SIEM/EDR Hunts)
5. How to Mitigate & Hardening Checklist
6. Artifacts & What to Collect
7. FAQs for SecOps & IT

Education-first, defender-safe analysis. We omit exploit details and provide practical detection/mitigation guidance for Blue Teams across US/EU/UK/AU/IN environments.

What is “DefenderWrite” (High Level)

“DefenderWrite” is an umbrella name for techniques that make Microsoft Defender (or its helper binaries/services) load attacker-controlled DLLs. It’s a variation of DLL sideloading / proxy loading under the trusted identity of a Microsoft-signed process. Because EDRs and allow-lists tend to trust Defender, the malicious code gains credibility, stealth, and persistence.

Key characteristics (conceptual):

• Abuse of trust: Signed Defender executables confer reputational trust to loaded modules.
• Service path & permissions: Misconfigurations or writable directories in search order are leveraged to place a malicious DLL.
• Living-off-the-AV: Execution looks like “Defender activity,” reducing detection by naive allow-lists.

Attack Flow (Conceptual)

1. Initial foothold: Phishing, drive-by, vulnerable driver/tool, or stolen admin token yields limited execution.
2. Placement: Attacker drops a DLL into a location that the Defender process will probe during normal load (e.g., search-order abuse or companion DLL).
3. Trigger: A legitimate Defender binary or service (signed) starts and resolves the malicious DLL first, loading it into a high-trust process.
4. Payload goals: Persistence (registry/service), credential access (via APIs/DPAPI), and quiet C2 using the Defender process as cover.

Why It Works & Business Impact

• Trust inheritance: Security tools and allow-lists often trust Microsoft-signed binaries, making alerts less obvious.
• Noise reduction: Activity appears as Defender telemetry; naive detections ignore it.
• Outcome: Silent persistence, policy tampering, and staged data theft. In enterprise/regulated sectors, this raises compliance exposure and prolongs dwell time.

How to Detect (SIEM/EDR Hunts)

Goal: Find trusted process → untrusted module mismatches and abnormal child-process trees related to Defender.Hunt 1 — Defender process loading unusual DLLs

Look for MsMpEng.exe (and helper binaries) with loaded modules from non-standard directories (user-writable, temp, programdata subfolders) or unsigned DLLs.
SIEM fields to pivot on: ProcessName, ImageLoaded, Signed, Company, OriginalFileName, Directory.Hunt 2 — Defender spawning script shells

Alert when Defender processes spawn cmd.exe, powershell.exe, wscript.exe, rundll32.exe, or mshta.exe. Under normal conditions, this should be rare.
Fields: ParentImage, ChildImage, CommandLine, IntegrityLevel.Hunt 3 — Module mismatch & reputation

Build a baseline of known-good Defender module hashes and locations; flag deviations. Cross-check code-signing and OriginalFileName anomalies.Hunt 4 — Persistence after Defender service restart

Watch for persistence artifacts that re-trigger the same suspicious module after WinDefend service restarts or OS reboot. Correlate with registry/service additions.

How to Mitigate & Hardening Checklist

• Enable Microsoft Defender Tamper Protection and lock Defender policies via MDM/Intune/GPO. Prevent local changes.
• ASR Rules (Attack Surface Reduction): Block process creation from Office apps; Block Win32 API calls from Office; Block abuse of signed binaries (tune for your estate).
• WDAC or AppLocker allow-listing: Constrain DLL load paths and only allow known-good, signed modules in security process contexts.
• Service & directory permissions: Remove “Everyone/Users: Write” from directories in DLL search paths for AV/EDR services; audit inherited ACLs.
• EDR policies: Alert/block when AV processes spawn scripting engines or when unsigned modules load into AV processes.
• Update hygiene: Keep Defender engine/platform, signatures, and Windows up to date; disable legacy components you don’t need.
• Telemetry retention: Retain module-load and service-change logs (≥ 30–90 days) for retro hunts.

Artifacts & What to Collect

Artifact	Why	Notes
Module load logs for MsMpEng.exe & helpers	Confirm sideload/proxy-load patterns	Look for unsigned or unusual paths
File ACLs of searched directories	Identify writable folders in search order	Harden permissions
Service config & startup events	Catch persistence via service hijack	Correlate restarts with module loads

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FAQs

Is “DefenderWrite” a single CVE?

No. It describes a class of misuse patterns (sideload/proxy load) against trusted Defender components. Specific CVEs may exist for individual bugs, but the risk persists as a configuration/allow-listing issue.Will enabling Tamper Protection stop it?

Tamper Protection raises the bar but doesn’t solve search-order abuse by itself. Combine with WDAC/AppLocker, ASR rules, and hardened directory/service permissions.Can we safely test detections?

Yes—use a lab and benign, signed modules to validate telemetry and alerts. Avoid weaponized PoCs in production.

 #Cybersecurity #BlueTeam #WindowsSecurity #MicrosoftDefender #DLLSideloading #EDR #SIEM #ThreatHunting #ZeroTrust #AppLocker #WDAC #ASR #IncidentResponse #EnterpriseSecurity #SOC

 Microsoft Defender tamper protection, DLL sideloading detection, MsMpEng.exe child process, Windows hardening WDAC, AppLocker allowlist, SIEM hunts Defender, EDR policy AV process protection, service path hijack, enterprise Windows security best practices