Microsoft Security Essentials

CVE-2026-50656, Microsoft Defender engine 'RoguePlanet' local privilege escalation now patched; NCSC-CH tracks the ongoing 'Nightmare Eclipse' zero-day series

NCSC-CH's running tracker on the "Nightmare Eclipse" (aka Chaotic Eclipse) researcher's 2026 zero-day PoC series was updated on 2026-07-09 to record that a CVE has been assigned to RoguePlanet: CVE-2026-50656, a local privilege-escalation vulnerability (CWE-59, improper link resolution before file access / "link following") in the Microsoft Malware Protection Engine that underpins Microsoft Defender, System Center Endpoint Protection and Microsoft Security Essentials (NCSC-CH, 2026-07-09). The researcher first disclosed RoguePlanet as an unpatched zero-day on 2026-06-10, describing a race condition in Defender that lets a local attacker "execute arbitrary code or spawn a command shell with SYSTEM-level privileges" (T1068), at which point NCSC-CH logged its status as "Proof of Concept Available, no patch available" (NCSC-CH, 2026-07-09). Microsoft's own record shows the CVE was published 2026-06-16 (CVSS 3.1 7.8, AV:L/AC:L/PR:L/UI:N, rated "Exploitation More Likely", exploitation status "No") and remained without a fix for over three weeks; a revision dated 2026-07-08 confirms Microsoft has now shipped an engine update that closes it, last vulnerable Malware Protection Engine build 1.1.26050.11, first fixed build 1.1.26060.3008 (Microsoft MSRC, 2026-07-08).

Because the Malware Protection Engine (mpengine.dll) auto-updates multiple times a day by default, most estates will already carry the fixed build; Microsoft's guidance is that no manual action is normally required. The operational nuance for this constituency is the exception set: any environment where engine updates are pinned, WSUS-gated, air-gapped, or centrally deferred (System Center Endpoint Protection deployments, offline or OT-adjacent Windows hosts) should explicitly verify the installed engine version rather than assume auto-remediation occurred (Microsoft MSRC, 2026-07-08). Microsoft also notes that hosts with Defender disabled are not in an exploitable state even though vulnerability scanners flag the on-disk binaries. Triage: the exploit abuses a symlink/junction race against files Defender is actively scanning, so the telemetry class is symlink/junction creation targeting Defender scan paths and, on success, MsMpEng.exe (the engine's scan host) spawning an unexpected child process with a SYSTEM token outside the normal signature/engine-update cadence; the update-cadence anchoring is the discriminator from routine engine activity. This closes RoguePlanet specifically; NCSC-CH continues to track the wider Nightmare Eclipse PoC series as a home-region authority, which is the reason a single-host LPE closure like this one is worth surfacing to this constituency at all.