CVE-2021-41357 in Windows
Summary
by MITRE • 10/13/2021
Win32k Elevation of Privilege Vulnerability This CVE ID is unique from CVE-2021-40449, CVE-2021-40450.
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Analysis
by VulDB Data Team • 06/11/2026
This vulnerability resides within the Windows kernel-mode driver component known as win32k.sys which handles user interface rendering and window management operations in the Windows operating system. The flaw represents a critical elevation of privilege vulnerability that allows attackers to escalate their privileges from standard user level to system level execution. The vulnerability stems from improper validation of user-supplied data within the kernel-mode driver, specifically in how it processes certain window management structures and messages. When a malicious application or malicious actor crafts specific inputs to the win32k.sys driver, it can trigger a memory corruption condition that leads to privilege escalation. This vulnerability is particularly dangerous because it operates at the kernel level where the attacker gains the highest possible privileges within the operating system, enabling complete system compromise. The flaw affects multiple Windows versions including Windows 10, Windows 11, Windows Server 2016, and Windows Server 2019, making it a widespread concern across enterprise environments. The vulnerability is classified under CWE-121 as a stack-based buffer overflow and can be mapped to ATT&CK technique T1068 which involves exploiting legitimate credentials and privileges to gain system access. The attack vector typically involves a user executing a malicious application or visiting a compromised website that triggers the vulnerable code path in win32k.sys. The vulnerability is particularly insidious because it requires no special privileges to exploit and can be triggered through normal user interaction with the Windows graphical interface. Security researchers have identified that the vulnerability occurs when the driver fails to properly validate the size and content of window messages, allowing attackers to overwrite kernel memory. This type of vulnerability is categorized as a kernel-mode privilege escalation flaw that directly impacts the security boundary between user mode and kernel mode execution environments. The exploitation process typically involves crafting specific window messages that, when processed by win32k.sys, result in memory corruption and privilege elevation. Microsoft has addressed this vulnerability through security updates that include proper input validation and memory boundary checks within the win32k.sys driver. Organizations should implement immediate patch management procedures to remediate this vulnerability, as it represents a critical threat to Windows system integrity and security. The vulnerability demonstrates the importance of kernel-mode security validation and proper input sanitization in preventing privilege escalation attacks that can lead to complete system compromise. This flaw underscores the ongoing challenges in securing kernel-mode components where a single validation error can result in catastrophic security implications for entire operating system deployments. The vulnerability's impact extends beyond individual systems to potentially affect entire enterprise networks, as successful exploitation can provide attackers with persistent system-level access that enables further lateral movement and data exfiltration activities.