Microsoft released a patch on Wednesday to address a zero‑day vulnerability, identified as CVE‑2026‑50656, in its Windows Defender security engine that could allow remote attackers to consume all available disk space on Windows 10 and Windows 11 systems. The flaw, dubbed RoguePlanet, was disclosed in June by researcher NightmareEclipse, who also published exploit code. The vulnerability permits arbitrary file writes regardless of real‑time protection status, bypassing Defender’s usual file‑size limits. Microsoft’s update to the Malware Protection Engine (mpengine.dll) will be delivered automatically through Windows Update and includes additional defense‑in‑depth measures. NightmareEclipse noted that the new mitigations introduce a bug that may cause the driver to mishandle data, potentially leading to unlimited file creation. The issue stems from how the SpyNet cloud reporting component interacts with the engine, and Defender normally restricts file size during quarantine, but the researcher found an exception that allows caching of Zone.Identifier alternate data streams of any size. Microsoft has not indicated any immediate widespread exploitation, and the patch is expected to mitigate the risk once deployed, though the researcher warned that the underlying flaw remains until the bug in mpengine.dll is resolved.