CVE-2026-42906: Windows 10 21h2 Information Disclosure Flaw

CVE-2026-42906 Overview

CVE-2026-42906 is an information disclosure vulnerability in Windows Shell that allows an authorized local attacker to access sensitive information. The flaw is tracked under CWE-200: Exposure of Sensitive Information to an Unauthorized Actor. Microsoft published the advisory on June 9, 2026, covering Windows 10, Windows 11, and Windows Server platforms.

The vulnerability requires local access and low privileges to exploit. Successful exploitation impacts confidentiality but does not affect integrity or availability of the host.

Critical Impact

An authenticated local attacker can read sensitive data exposed by Windows Shell, potentially aiding lateral movement or follow-on privilege escalation attacks.

Affected Products

• Microsoft Windows 10 21H2 and 22H2 (x86, x64, ARM64)
• Microsoft Windows 11 23H2, 24H2, 25H2, and 26H1 (x64, ARM64)
• Microsoft Windows Server 2022 and Windows Server 2025

Discovery Timeline

• 2026-06-09 - CVE-2026-42906 published to NVD
• 2026-06-11 - Last updated in NVD database

Technical Details for CVE-2026-42906

Vulnerability Analysis

The vulnerability resides in Windows Shell, the component responsible for the user interface, file management, and desktop services in Windows. Windows Shell handles sensitive data including user profiles, recently accessed files, environment variables, and shell extension state.

The flaw is classified as [CWE-200], indicating that a component exposes sensitive information to an actor not explicitly authorized to access it. A locally authenticated attacker with standard user privileges can interact with Shell functionality to read data that should remain isolated from their security context.

Root Cause

The root cause is improper restriction of information returned by Windows Shell operations. Shell components fail to enforce sufficient access control on data returned to the calling process. This allows a low-privileged process to observe data belonging to higher-privileged contexts or other users on the same host.

Attack Vector

Exploitation requires local access to an affected Windows system with valid credentials. The attacker runs a process that interacts with Windows Shell APIs or interfaces. No user interaction is required from another user. The disclosed information can be leveraged for reconnaissance, credential harvesting, or chained with other vulnerabilities to escalate privileges.

Microsoft has not published technical exploitation details. Refer to the Microsoft CVE-2026-42906 Advisory for the official description.

Detection Methods for CVE-2026-42906

Indicators of Compromise

• Unusual processes spawned by standard user accounts that enumerate Windows Shell namespaces or recent items
• Unexpected reads of HKCU and HKLM shell-related registry keys by non-administrative processes
• Repeated Shell COM object instantiations from non-interactive sessions

Detection Strategies

• Monitor process command lines and parent-child relationships for unusual access to explorer.exe, shell32.dll, or Shell COM interfaces
• Audit file and registry access patterns on Shell-related paths such as %APPDATA%\Microsoft\Windows\Recent and Shell\Bags registry hives
• Correlate local logon events with subsequent enumeration of user profile directories belonging to other accounts

Monitoring Recommendations

• Enable Windows Security event logging for object access (Event IDs 4656, 4663) on Shell-related artifacts
• Forward endpoint telemetry to a centralized SIEM for behavioral analytics on local information-gathering activity
• Alert on standard user accounts performing reconnaissance commands such as whoami /all, net user, or PowerShell shell-namespace enumeration following login

How to Mitigate CVE-2026-42906

Immediate Actions Required

• Apply the Microsoft security update referenced in the MSRC advisory for CVE-2026-42906 to all affected Windows 10, Windows 11, and Windows Server systems
• Inventory endpoints to confirm patch deployment across all supported builds, including 21H2, 22H2, 23H2, 24H2, 25H2, and 26H1
• Restrict interactive and remote logon rights on multi-user systems to limit who can execute local code

Patch Information

Microsoft has released a security update addressing CVE-2026-42906. Administrators should deploy the update via Windows Update, Windows Server Update Services (WSUS), or Microsoft Update Catalog. Consult the Microsoft CVE-2026-42906 Advisory for the specific KB article and applicable build numbers.

Workarounds

• Enforce least-privilege policies so standard users cannot execute arbitrary binaries on shared or sensitive hosts
• Apply Windows Defender Application Control or AppLocker policies to restrict unsigned or untrusted executables that could invoke Shell APIs
• Monitor and audit local logon activity, prioritizing servers and workstations that host privileged user sessions

# Verify patch installation via PowerShell
Get-HotFix | Sort-Object -Property InstalledOn -Descending | Select-Object -First 10

# Confirm Windows build for patch eligibility
[System.Environment]::OSVersion.Version