Windows Ancillary Function Driver (afd.sys) Elevation of Privilege Vulnerability (CVE-2025-21418) Technical Analysis and Mitigation Guide

1. Vulnerability Overview & Root Cause

This vulnerability (CVE-2025-21418) is a heap-based buffer overflow (CWE-122) residing in afd.sys (Ancillary Function Driver), the core kernel-mode driver responsible for handling Windows Sockets (WinSock). The root cause lies in the insufficient validation of input buffer sizes passed from user-mode applications via specific Input/Output Control (IOCTL) requests. This lack of validation allows an attacker to overwrite adjacent memory allocations within the Kernel Pool.

2. Exploitation Mechanics & Attack Scenario

To exploit this vulnerability, an attacker must first establish local execution capabilities on the target system with low-privileged access. The attacker then crafts a malicious payload and transmits a specially designed IOCTL request to the \Device\Afd device object. By leveraging 'Pool Feng Shui' techniques to groom the kernel pool layout, the attacker corrupts adjacent kernel structures (such as process tokens or object headers). This corruption is manipulated to replace the current process's security token with the NT AUTHORITY\SYSTEM token, achieving full local privilege escalation (LPE).

3. Infrastructure & Business Impact

As CVE-2025-21418 is actively exploited in the wild and listed in CISA's Known Exploited Vulnerabilities (KEV) catalog, it poses an immediate threat to enterprise environments. If successfully exploited, it allows threat actors who have gained initial entry to elevate their privileges to SYSTEM, bypass endpoint security controls, perform lateral movement, and deploy ransomware or establish persistent backdoors across the infrastructure.