CVE-2025-24983

What is Windows Win32k?

Win32k (win32k.sys) is the Windows kernel subsystem that implements the graphical user interface — window management, the GDI (Graphics Device Interface), and the USER subsystem for keyboard/mouse input. It runs in kernel mode and provides the core drawing, window creation, and message-passing services that every Windows GUI application depends on. Because Win32k accepts inputs from all user-mode applications (including sandboxed processes like Chrome's renderer), vulnerabilities in Win32k are accessible from a wide range of attack contexts, making it a persistently targeted component for local privilege escalation.

Win32k has one of the longest histories of exploited kernel vulnerabilities in Windows: North Korean Lazarus Group, Chinese APT41, Russian APT groups, and financially motivated ransomware operators have all used Win32k zero-days. CVE-2025-24983 is the latest in a long series.

Overview

CVE-2025-24983 is a use-after-free vulnerability (CWE-416) in the Windows Win32k kernel subsystem that allows a locally authenticated low-privilege attacker to escalate to SYSTEM. Disclosed as a zero-day in the March 2025 Patch Tuesday, CISA added it to the KEV catalog on patch day alongside three other Windows zero-days. The High attack complexity (AC:H) indicates exploitation required specific timing or conditions — but these were met in active exploitation before the patch was released.

Affected Versions

Technical Details

The use-after-free (CWE-416) occurs in the Win32k kernel subsystem during processing of window management or GDI operations. Win32k allocates kernel objects (window objects, device context handles, region objects) to track UI state. An attacker triggers a sequence of window operations that causes a Win32k object to be freed while a stale reference to it remains in a related kernel data structure. By controlling when and how the freed memory is reallocated (heap grooming), the attacker places controlled data at the freed address. When the stale pointer is subsequently dereferenced during Win32k's processing of a subsequent window message or GDI operation, the kernel operates on attacker-controlled data.

Key characteristics:

• Low-privilege local account required (PR:L) — any standard user account
• High attack complexity (AC:H) — requires precise timing of concurrent window operations to win the race or trigger the UAF window; more difficult than most LPE exploits but demonstrated in the wild
• No user interaction required (UI:N) — exploitable silently from any logged-on user process
• Accessible from browser sandboxes — Win32k is reachable from sandboxed renderer processes, making it a sandbox escape component in multi-stage exploit chains

Discovery

Microsoft Threat Intelligence identified active exploitation before March 2025 Patch Tuesday. The specific reporter was not publicly disclosed. March 2025 Patch Tuesday included five simultaneously exploited zero-days across Windows filesystem drivers and Win32k, reflecting concurrent sophisticated exploitation of multiple Windows subsystems.

Exploitation Context

Confirmed zero-day exploitation before March 11, 2025. Win32k has been exploited by a wide range of threat actors in its history. The High complexity rating did not prevent active exploitation — sophisticated actors (nation-state and advanced ransomware operators) routinely develop reliable Win32k exploits. Win32k UAFs are commonly used as the second stage in multi-stage attack chains: initial access (phishing, web exploit) → Win32k UAF for SYSTEM → ransomware deployment or persistent implant.

Remediation

1. Apply the March 2025 cumulative update for your Windows version. The CISA deadline was April 1, 2025.
2. Apply all March 2025 zero-day patches simultaneously from the same cumulative update: CVE-2025-24983 (Win32k), CVE-2025-24985 (Fast FAT), CVE-2025-24993 (NTFS), CVE-2025-26633 (MMC).
3. Enable Windows Defender Exploit Guard — win32k-specific mitigations, Control Flow Guard, and arbitrary code guard reduce exploitation reliability.
4. Restrict RDP and local console access — Win32k UAFs require a Windows desktop session context; limiting logon access reduces the available attack surface.
5. Enable virtualization-based security (VBS) and Credential Guard — kernel integrity protections reduce the impact of kernel-level compromises on credential theft.
6. Monitor for signs of exploitation: unexpected SYSTEM-privileged processes spawned from standard-user parent processes (Windows Event Log Event ID 4688) before the patch was applied.