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.
Timeline
Feb 15, 2026
Microsoft still lists no known issues for KB5077181
As of February 15, 2026, Microsoft reportedly had not acknowledged any known issues with KB5077181 on its release page or Windows health dashboard despite user reports of boot problems. This contrasted with external reporting about affected devices.
Feb 15, 2026
Users report new boot loops and startup failures after KB5077181
By February 15, 2026, widespread user reports linked KB5077181 itself to critical startup problems, including infinite restart loops that prevented systems from reaching the desktop. Suggested workarounds included uninstalling the update, pausing reinstallation, and using Windows Recovery Environment tools.
Feb 10, 2026
KB5077181 fully resolves prior Windows 11 boot failure bug
Microsoft said the February 10, 2026 KB5077181 update fully resolved the boot failure issue tied to failed December 2025 updates, though already-unbootable systems might still need extra remediation. Enterprises were advised to contact Microsoft Support for Business for affected devices.
Feb 10, 2026
Microsoft releases February Patch Tuesday update KB5077181
On February 10, 2026, Microsoft released cumulative security update KB5077181 for Windows 11 24H2 and 25H2. The update included security fixes, prior quality improvements, and deployment of new Secure Boot certificates ahead of 2011 certificate expirations in June 2026.
Jan 29, 2026
Microsoft ships preview update to mitigate further boot failures
On January 29, 2026, Microsoft released the optional preview update KB5074105 to prevent additional devices from being affected by the Windows 11 boot failure issue. This mitigation was aimed at limiting further impact before a full fix was available.
Jan 13, 2026
January security update triggers unbootable Windows 11 systems
After installing the January 13, 2026 security update KB5074109 or later updates, some affected Windows 11 devices began failing to boot, showing black screens, restart prompts, or "UNMOUNTABLE_BOOT_VOLUME" errors. The issue impacted a limited number of physical commercial systems and could require manual recovery.
Dec 1, 2025
December 2025 Windows 11 security update leaves some systems in improper state
A failed installation or rollback of the December 2025 Windows 11 security update left some commercial devices running versions 24H2 and 25H2 in an improper state. This latent condition later contributed to boot failures on affected systems.
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