Windows 11 January Security Updates Trigger UNMOUNTABLE_BOOT_VOLUME Boot Failures
Microsoft is investigating and has acknowledged a limited issue where some Windows 11 devices fail to boot after installing the January 2026 Patch Tuesday security updates, presenting a BSOD/black crash screen with stop code UNMOUNTABLE_BOOT_VOLUME. Impacted systems can become stuck in a restart loop and are unable to start Windows without manual recovery efforts, with Microsoft collecting reports from users and enterprise administrators to determine scope and root cause.
Reporting indicates the problem affects physical devices (with no virtual machines reported as impacted so far) and is tied to specific Windows 11 builds and cumulative updates, including Windows 11 25H2 and Windows 11 24H2 after installing KB5074109. Microsoft has not yet confirmed the underlying cause or provided a universal remediation beyond recovery steps, and is requesting affected customers submit diagnostics via the Feedback Hub while it determines whether the behavior is a regression introduced by the update.
Sources
Related Stories
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.
4 weeks ago
Microsoft Windows 11 Updates Trigger Boot Failures and Security-Driven Driver/Privilege Changes
Microsoft attributed **Windows 11 no-boot failures** seen after installing the January 2026 cumulative update `KB5074109` (Windows 11 **24H2/25H2**) to devices that had previously **failed to install the December 2025 security update** and were left in an “**improper state**” after rollback. Affected systems can crash on startup with a BSOD `UNMOUNTABLE_BOOT_VOLUME`; Microsoft said the issue appears limited to **physical devices** (no confirmed VM impact) and is working on a **partial mitigation** to prevent additional systems from entering a no-boot scenario, while continuing to investigate why some devices fail updates or end up unstable after rollback. Separately, Microsoft’s recent Windows 11 servicing and security work included **deliberately disabling legacy dial-up modem drivers** (e.g., `AGRSM64.SYS`/`AGRSM.SYS`, `SMSERL64.SYS`/`SMSERIAL.SYS`) due to reported vulnerabilities including **CVE-2023-31096** (EoP) and **CVE-2025-24052** (stack-based buffer overflow), which can present risk even if the modem hardware is unused—at the cost of breaking connectivity for niche systems relying on those drivers. Microsoft also patched **nine bypasses** reported by Google Project Zero that could undermine the new **Windows Administrator Protection** feature by enabling silent admin privilege gains via legacy Windows/UAC behaviors (including a token/Logon Sessions-related technique involving `NtQueryInformationToken` and DOS device object directory creation), ahead of broader availability beyond Insider builds.
1 months ago
Windows 11 January Security Update KB5074109 Triggers Widespread Stability and Remote Desktop Issues
Microsoft’s January 2026 Patch Tuesday release for Windows 11 (notably **KB5074109** for 24H2/25H2) delivered roughly 100+ security fixes (including reported **zero-days**) but also introduced significant reliability regressions. Reported impacts include **system lockups**, **black screens** on systems with Nvidia/AMD GPUs, graphics-related hangs in GPU/DirectX-intensive applications, and application failures such as **Outlook Classic** freezing or failing to launch; additional issues were reported with File Explorer behavior and cloud-backed storage workflows (e.g., OneDrive/Dropbox) causing apps to become unresponsive when saving or syncing data.
1 months ago