Windows 11 KB5077181 Patch Tuesday Update Triggers and Fixes Boot Failures

Microsoft’s February 2026 Windows 11 cumulative security update KB5077181 (for versions 24H2 and 25H2) was associated with significant boot reliability issues reported shortly after deployment, including systems entering infinite restart loops and failing to reach the desktop. Reports described login-time errors (including System Event Notification Service (SENS) procedure errors) and network symptoms such as DHCP failures, while Microsoft’s public release notes and health dashboard were reported as not listing known issues at the time. The update also shipped broad security remediation, with reporting citing 58 vulnerabilities addressed and six actively exploited zero-days referenced via CISA’s Known Exploited Vulnerabilities catalog, including fixes for issues such as SmartScreen bypass (CVE-2026-21510), Desktop Window Manager EoP (CVE-2026-21519), Remote Desktop Services EoP (CVE-2026-21533), and a Notepad RCE via crafted Markdown (CVE-2026-20841).

Separately, Microsoft stated that KB5077181 fully resolved a specific Windows 11 boot failure condition affecting a limited set of commercial physical devices on 24H2/25H2 that could become unbootable (e.g., "UNMOUNTABLE_BOOT_VOLUME") after installing KB5074109 or later updates when a December 2025 security update had previously failed and rolled back, leaving the OS in an “improper state.” Microsoft indicated an earlier mitigation shipped in the optional preview update KB5074105 (Jan 29, 2026) to prevent additional devices from being impacted, and that the February Patch Tuesday release delivered the complete fix; the issue was not reported as affecting home users or virtual machines.