KB5077243

Microsoft has released cumulative update KB5077243 for Windows 11 version 25H2 for Insiders on the Dev Channel — Windows 11 Insider Preview Quality Update (26300.7939). This update includes several significant changes, including a new protected mode for batch files, improved shared audio management, new features for Narrator, and a number of system reliability and performance improvements.

• Protected mode for batch files (.bat and CMD). Microsoft added a file locking mechanism during execution, preventing modification while running and reducing the risk of code substitution. To activate the mode, create the LockBatchFilesInUse (DWORD) registry parameter at HKEY_LOCAL_MACHINE\Software\Microsoft\Command Processor and set its value to 1 (0 disables the mode). Additionally, Application Control policies can use the LockBatchFilesWhenInUse element in the application manifest.
• Code integrity check performance improvement. Previously, digital signature verification was performed for each line of a batch file, slowing down large scripts. Now verification occurs once for the entire file, significantly speeding up operation, especially in corporate environments with strict security requirements.
• Shared audio (preview) — separate volume sliders. The feature introduced in the fall allows two users to simultaneously use compatible Bluetooth LE Audio devices (headphones, speakers, hearing aids). Windows now provides separate volume sliders for each listener, allowing independent level adjustment without affecting the other user. The master volume (via quick settings, hardware buttons, or keyboard) still changes sound for both simultaneously.
• Shared audio — taskbar indicator. When sharing audio, an indicator appears on the taskbar to remind that the audio stream is active. Clicking the indicator quickly opens sharing settings, adjusts volume, or stops the stream.
• LE Audio ecosystem expansion. The shared audio feature is now supported on Samsung Galaxy Buds 4, Samsung Galaxy Buds 4 Pro, Sony WF-1000XM6, and Xbox Wireless Headset.
• Narrator — new command for status bar. Narrator users can use the keyboard shortcut “Narrator key + \” to read the contents of the status bar in applications such as Microsoft Word, Excel, and PowerPoint. For example, in Word you can hear the number of pages or words in the document, and in Excel the sum or average of selected cells without changing the current focus.
• Taskbar — auto-hide animation pause and reliability improvements. The deployment of enhanced animations when using automatic taskbar hiding has been temporarily paused. Also, the reliability of loading taskbar components in Safe Mode has been improved.
• Storage — update file deletion reliability. Reliability when deleting Windows update files and the windows.old folder through Settings > System > Storage has been improved.
• Input — ADLaM keyboard reliability. Text input reliability when using the ADLaM keyboard has been improved.

To install the update, go to Settings > Windows Update and click Check for updates. A computer restart is required to complete installation.

Insiders on the Dev Channel who previously used build 26220 will be automatically migrated to build 26300 via the enabling package KB5073032.

How the new features work:

Protected mode for batch files (.bat and CMD): Kernel-level file locking — when launching cmd.exe for a .bat/.cmd file, CreateFile is called with FILE_SHARE_READ without FILE_SHARE_WRITE. This prevents writing to the file until the process exits. The mode is enabled by LockBatchFilesInUse=1 in HKEY_LOCAL_MACHINE\Software\Microsoft\Command Processor. Application Control additionally checks the element in the manifest.

Code integrity check performance improvement: Previously, digital signature verification via WinVerifyTrust ran for each batch line, causing O(n) I/O. Now verification occurs once on file open, and the result is cached in the process context. For large scripts, speedup reaches 10–20x, especially on network drives or with strict Device Guard policies.

Shared audio (preview) — separate volume sliders: Uses Bluetooth LE Audio with multipoint — two users' audio sessions route through one adapter but are handled by separate AudioEndpointVolume API instances. Each gets its own virtual audio stream inside audiodg.exe, allowing sndvol.exe sliders to independently control IAudioEndpointVolume per listener. Master volume (hardware keys or ISimpleAudioVolume) affects both streams simultaneously.

Shared audio — taskbar indicator: When sharing starts, a background COM server subscribes to AudioSharingStarted event from Audiosrv. The indicator is a native ITaskbarList3 overlay icon. Clicking calls ShellExecute on sound control panel with ms-settings:sound-devices?shared=1. Stopping uses IMMDevice::Activate to disconnect the virtual endpoint.

LE Audio ecosystem expansion: In bthleenum.sys, the list of supported PACS/ASCS profiles is updated. For each model, PnP identifiers (VID/PID) are added to the registry (e.g., Samsung_Galaxy_Buds4_Pro), verifying LC3 codec version. Sony and Xbox‑specific CCID adaptations are implemented for proper multi‑user mode.

Narrator — new command for status bar: Hotkey (Narrator key + ) handler in Narrator.exe uses low‑level input hook (SetWindowsHookEx). It retrieves IUIAutomationElement for the status bar via UIA, calls GetCurrentPropertyValue(UIA_NamePropertyId), and speaks via ISpVoice without focus change. In Excel, it additionally sums selected cells via COM Range.Sum.

Taskbar — auto-hide animation pause and reliability improvements: Auto‑hide transitions disabled via DWMWA_TRANSITIONS_FORCEDISABLED in dwmapi.dll due to compatibility issues with some drivers. Safe Mode loading improved — Taskbar.dll checks SafeBoot via GetSystemMetrics(SM_CLEANBOOT) and skips problematic animations.

Storage — update file deletion reliability: In cleanmgr.exe and StartComponentsCleanupWork, locked file handling improved — MoveFileEx with MOVEFILE_DELAY_UNTIL_REBOOT is used for non‑removable files. For windows.old, GetFileSecurity checks permissions, and RemoveDirectory retries after stopping services holding the folder.

Input — ADLaM keyboard reliability: Fixed kbdadam.dll driver — previously, fast layout switching (ActivateKeyboardLayout) caused virtual‑key translation buffer leak in user32!SendInput. Now WM_INPUTLANGCHANGE is properly handled, and character mapping is synchronized via ImmActivateLayout, eliminating freezes in input editors.

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