CYBERDUDEBIVASH • THREATWIRE

How “DefenderWrite” Hijacks Microsoft Defender to Execute Malicious DLLs

October 20, 2025 • Author: CyberDudeBivash • Read time: 11 min

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TL;DR: DefenderWrite hijacks Microsoft Defender’s trust to load attacker-controlled DLLs via signed binary proxy execution, DLL search-order hijacking, or vulnerable service configurations. Defenses: block LOLBins and AV-adjacent binaries from starting untrusted code, turn on ASR rules, enforce code integrity (WDAC), and hunt for unusual Defender processes (e.g., MsMpEng.exe, MpCmdRun.exe) spawning script interpreters or unsigned modules.Contents

1. What is “DefenderWrite”?
2. Core Tactics & Abuse Paths
3. Impact & Business Risk
4. Detection: SIEM/EDR Hunts
5. Mitigation & Hardening
6. Indicators & Telemetry
7. FAQs

Scope: US/EU/UK/AU/IN enterprise Windows fleets using Microsoft Defender for Endpoint (MDE) and/or built-in AV, hybrid AD/Entra ID, and EDR tool stacks.

What is “DefenderWrite”?

“DefenderWrite” is a theoretically plausible post-exploitation technique where attackers leverage the trust and privileges of Microsoft Defender components to run malicious DLLs. The objective is defense evasion and privilege escalation by riding on signed Microsoft binaries or misconfigurations surrounding Defender’s service ecosystem.

Core Tactics & Abuse Paths

• Signed Binary Proxy Execution (LOLBin): Invoke MpCmdRun.exe with crafted parameters or side-by-side files to load attacker DLLs, abusing trust + allow-lists.
• DLL Search-Order Hijacking: Place a malicious DLL where Defender-adjacent binaries resolve dependencies first (e.g., working directory), especially when directory permissions are lax.
• Service Config Weakness: Writable service paths or directories for Defender helpers (scheduled tasks, temp extract dirs, update cache) enable code planting.
• AMSI/ETW Tamper Add-on: Dropper loads a DLL that patches AMSI/ETW from within a trusted process to blind scanning and telemetry.
• Living-off-the-Land Scripting: Defender-spawned PowerShell/WMI/cscript launching unsigned modules under a signed process tree.

Impact & Business Risk

• Stealth Persistence: Malware lives under trusted Microsoft processes, lowering detection rates and analyst suspicion.
• Rapid Lateral Movement: Cred theft and token manipulation while telemetry is degraded or blinded.
• Ransomware Enablement: Pre-encryption staging, backup sabotage, and mass-deployment from a “trusted” parent process.
• Regulatory Exposure: If MDE is disabled/tampered, breach dwell time and data-loss scale increase (GDPR/CPRA/HIPAA/FERPA/PCI risk).

Detection: SIEM/EDR Hunts

1. Process Tree Anomalies (Defender as Parent):ParentImage IN (“MpCmdRun.exe”,”MsMpEng.exe”) AND ChildImage IN (“powershell.exe”,”wscript.exe”,”cscript.exe”,”rundll32.exe”,”regsvr32.exe”) AND ChildSignature != “Microsoft Corporation”
2. Unsigned/Untrusted Modules Loaded by Defender:Image IN (“MpCmdRun.exe”,”MsMpEng.exe”) AND LoadedModule.SignatureStatus != “Signed” AND LoadedModule.Path LIKE “%\\AppData\\%” OR “%\\Temp\\%” OR “%\\Public\\%”
3. AMSI/ETW Patch Heuristics:API call sequences: AmsiScanBuffer → memory patch; EtwEventWrite → NOP stubs Look for RWX regions (VirtualProtect) inside Defender process space
4. Service/Task Tamper:EventID 7045 (Service Install), 4697 (New Service), Scheduled task creation referencing Defender paths or temp caches

Mitigation & Hardening

• ASR Rules: Enable rules to block Office/Script abuse, credential theft, and “Block abuse of exploited vulnerable signed drivers”; block process creation from MpCmdRun.exe to scripting engines where feasible.
• WDAC / Smart App Control: Enforce code integrity: only allow Microsoft + org-signed DLLs for security processes; block unsigned DLLs in %ProgramData%, %Temp%, user profile paths.
• File/Folder ACLs: Lock down Defender program directories, update caches, and any helper paths; disallow standard users from writing near security binaries.
• Attack Surface: Disable PowerShell v2, constrain Constrained Language Mode, require signed scripts; block rundll32/regsvr32 from internet zones via AppLocker/WDAC.
• Tamper Protection: Ensure Defender Tamper Protection is on; monitor for policy downgrades via MDE/SIEM.
• Update & Health: Keep Defender platform/engine and intelligence up to date; verify cloud-delivered protection and IOAV are enabled.

Indicators & Telemetry (Examples)

Type	Value	Context
Parent→Child	MpCmdRun.exe → powershell.exe	Unusual script spawned by Defender CLI
Module	%ProgramData%\Microsoft\Windows Defender\*.dll (unsigned)	Unexpected unsigned module in security directory
Registry	HKLM\SYSTEM\CurrentControlSet\Services\* (ImagePath altered)	Service points to writable directory
Event	MDE alert suppressed/unavailable	Possible tamper / telemetry blinding

Executive takeaway: Treat security tooling as high-value software supply chain. If a signed Microsoft binary can launch unsigned code, your least-privilege and code-integrity guarantees are broken.

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Trusted Tools & Training (Editor’s Picks)

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Endpoint AV: Kaspersky
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FAQs

Is “DefenderWrite” a specific CVE?

No. This post describes a technique class (abusing trusted security components). Some campaigns may pair it with vulnerabilities or purely with misconfig and LOLBins.Can Microsoft Defender block this?

Yes—when configured: Tamper Protection on, cloud-delivered protection, ASR rules, WDAC, and script controls. Gaps arise from policy exceptions and weak ACLs.What’s the fastest stop-gap?

Block Defender process spawning of scripting engines via ASR/EDR rules, enforce code integrity on DLL loads, and scan for unsigned modules loaded by Defender processes.

#Cybersecurity #WindowsSecurity #MicrosoftDefender #EDR #DLLSideloading #LOLBins #ThreatHunting #IncidentResponse #BlueTeam #SOC #ASR #WDAC #MDE #DFIR #ZeroTrust #PatchNow #CVE #MITRE ATTCK

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