CVE-2021-40449: 
vulnerability analysis and mitigation

CVE-2021-40449 is a Win32k Elevation of Privilege Vulnerability discovered in late August and early September 2021. The vulnerability affects multiple Microsoft Windows versions, including Windows Vista through Windows 10, and their corresponding server versions. This zero-day vulnerability was found being actively exploited in attacks targeting Windows servers (Securelist).

The vulnerability is a use-after-free issue in Win32k's NtGdiResetDC function. The exploitation relies on user-mode callbacks and unexpected API function execution during these callbacks. The vulnerability is triggered when the ResetDC function is executed a second time for the same handle during its own callback execution. The exploit achieves the desired memory state using GDI palette objects and leverages a kernel function call to build primitives for reading and writing kernel memory. The vulnerability received a CVSS v3.1 base score of 7.8 (HIGH) with vector: CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H (NVD).

When successfully exploited, this vulnerability allows an attacker to gain elevated system privileges on affected systems. The vulnerability enables attackers to escape from low-privilege environments and execute arbitrary code with kernel privileges (Securelist).

Microsoft released security patches for this vulnerability on October 12, 2021, as part of the October Patch Tuesday updates. Users are strongly advised to apply the security updates to affected systems (NVD).