Elevation of Privilege (EoP) vulnerabilities are a hot topic among cybersecurity researchers and enthusiasts. These vulnerabilities provide attackers with the ability to execute code and access critical system components with higher privileges than they should. In this long-read, we'll take an in-depth look at CVE-2023-29336 – a recently discovered EoP vulnerability that specifically targets the Windows win32k subsystem.

CVE-2023-29336 Background and Details

The vulnerability in question, CVE-2023-29336, can be found within the win32k.sys component of the Windows kernel. Win32k.sys is a part of the Windows kernel responsible for various important system functionalities like the USER subsystem and the Graphical Device Interface (GDI) subsystem.

Details about the vulnerability can be found in the original advisory published by Microsoft here.

Exploitation of this vulnerability can lead to a local attacker gaining elevated privileges on the affected system. The attacker could potentially make use of these elevated privileges to execute malicious code or tamper with the normal functioning of the operating system.

Code Snippet: Vulnerable Function

The vulnerability is located in the win32k!xxxWndProcRoutine function. This functions main purpose is to handle message processing for various system components. The vulnerability is caused by improper handling of the lParam parameter in certain system messages.

An example of the vulnerable code snippet is shown below

NTSTATUS win32k!xxxWndProcRoutine(
  HWND hWnd,
  UINT uMsg,
  WPARAM wParam,
  LPARAM lParam
)
{
  ...
  // Improper handling of lParam may lead to EoP
  ...
}

Exploiting CVE-2023-29336

In order to exploit this vulnerability, an attacker must first have the ability to execute code on the target system with low privileges. This can be achieved through a variety of means, such as social engineering, malicious downloads, or other attack vectors that lead to local code execution.

After gaining access to the target system with low privileges, the attacker can craft a sequence of system messages that, when processed by the vulnerable function, will trigger the vulnerability and allow the attacker to execute code with elevated privileges.

An example of how such an exploit could function is shown below

int main()
{
  // Step 1: Obtain a handle to the target HWND process
  HWND target_hWnd = ...; 

  // Step 2: Craft malicious message sequence
  UINT uMsg = ...;
  WPARAM wParam = ...;
  LPARAM lParam = ...; // Exploit vulnerability in lParam handling

  // Step 3: Send malicious message sequence to target HWND process
  SendMessage(target_hWnd, uMsg, wParam, lParam);

  // Step 4: Profit! Code execution with elevated privileges
  ...
}

Mitigations and Recommendations

Microsoft has issued a patch for this vulnerability that can be found here. It is highly recommended that users and system administrators apply this patch as soon as possible to protect against potential exploitation.

Additionally, it is crucial for users to remain vigilant against potential social engineering attacks or malicious downloads that could lead to local code execution on their systems.

Conclusion

CVE-2023-29336 serves as a stark reminder of the importance of secure coding practices and the need for regular security updates. In this post, we explored the details of the win32k EoP vulnerability, its exploitation, and the available mitigations. By staying informed and taking a proactive approach to system security, we can better protect ourselves and our systems from potential threats.

Timeline

Published on: 05/09/2023 18:15:00 UTC
Last modified on: 05/09/2023 18:23:00 UTC