Windows NTFS Kernel Integer Underflow (CWE-191) Local Privilege Escalation Vulnerability (CVE-2021-31956) Technical Analysis and Mitigation Guide

1. Vulnerability Overview & Root Cause

This vulnerability (CVE-2021-31956) is an integer underflow vulnerability (CWE-191) residing within the Windows NTFS file system driver (ntfs.sys). When the NTFS driver processes specially crafted Extended Attributes (EA) or malformed file system metadata structures, a lack of proper validation during buffer size calculations leads to an integer underflow. This underflow results in a heap-based buffer overflow in the Kernel Pool memory space when copying data into an undersized buffer.

2. Exploitation Mechanics & Attack Scenario

An attacker must first obtain local execution capabilities with low privileges on the target system. 1) The attacker creates a file or directory containing maliciously structured Extended Attributes (EA). 2) The attacker invokes system calls such as NtSetEaFile or NtQueryEaFile to force the kernel to parse the crafted file. 3) An integer underflow occurs inside ntfs.sys during size calculation, triggering an out-of-bounds write operation in the kernel pool. 4) By corrupting adjacent kernel pool chunks, the attacker manipulates kernel structures (such as process tokens) to elevate their privileges to NT AUTHORITY\SYSTEM.

3. Infrastructure & Business Impact

This vulnerability is cataloged in CISA's Known Exploited Vulnerabilities (KEV), indicating active exploitation in the wild by advanced threat actors. Local Privilege Escalation (EoP) vulnerabilities are critical components of multi-stage attack chains. Once initial access is gained via phishing or web exploits, attackers leverage this vulnerability to compromise domain controllers and critical servers, leading to full infrastructure takeover and potential ransomware deployment.