Microsoft Defender Patch Bypass: High Severity Zero-Day Privilege Escalation (CVE-2026-50656/RoguePlanet, ShieldBreak)

**Executive Summary:** A critical zero-day (CVE-2026-50656, "RoguePlanet") in Microsoft Defender's mpengine.dll enables local privilege escalation to NT AUTHORITY\SYSTEM, and a subsequent public exploit chain called "ShieldBreak" has bypassed Microsoft's patch, leaving up-to-date Defender deployments at risk; the report details impacted Windows platforms, exploitation timeline, recommended layered mitigations (host inventory, blocking vulnerable binaries, ASR rules, tamper protection), and notes no official fix for the bypass at time of publication.