A use after free vulnerability exists in the NtGdiResetDC() function of Win32k which can be leveraged by an attacker to escalate privileges to those of NT AUTHORITY\SYSTEM. The flaw exists due to the fact that this function calls hdcOpenDCW(), which performs a user mode callback. During this callback, attackers can call the NtGdiResetDC() function again with the same handle as before, which will result in the PDC object that is referenced by this handle being freed. The attacker can then replace the memory referenced by the handle with their own object, before passing execution back to the original NtGdiResetDC() call, which will now use the attacker's object without appropriate validation. This can then allow the attacker to manipulate the state of the kernel and, together with additional exploitation techniques, gain code execution as NT AUTHORITY\SYSTEM. This Metasploit module has been tested to work on Windows 10 x64 RS1 (build 14393) and RS5 (build 17763), however previous versions of Windows 10 will likely also work.

CVE-2021-40449