Windows, one of the most widely used operating systems, has recently encountered another vulnerability in its design, which has led to an unprecedented elevation of privilege issue in its NTLM security protocol. This security loophole (CVE-2023-21746) affects numerous versions of Windows and carries the potential to cause extensive damage to personal and corporate data integrity.

In this post, we'll dive deep into the vulnerability, explore its root cause, share a code snippet that showcases the problem, and discuss potential methods for exploiting the loophole. Additionally, we'll provide references from the original sources that highlight the vulnerability and mitigation techniques.

Vulnerability Details

CVE Identifier: CVE-2023-21746
Affected Products: Windows Operating System, Windows Server
Attack Vector: Network
Impact: Elevation of Privilege

The primary issue lies in the Windows implementation of the NTLM protocol, which can be exploited to gain unauthorized access to a system and take undue control over its resources.

NTLM, which stands for NT LAN Manager, is a suite of Microsoft security protocols that provides authentication, integrity, and confidentiality to users. It is the backbone of user authentication and considered vital for smooth Windows operations. Microsoft has made efforts to replace the outdated NTLM with Kerberos; however, there are still many environments and applications that continue to rely on NTLM. Links to original documentation and security advisories can be found below:

- Microsoft Security Advisory: (
- CVE Details: (
- National Vulnerability Database: (

Code Snippet

The following code snippet exhibits the vulnerable section in Windows NTLM implementation where the improper handling of user identities and authorization takes place:

// Vulnerable NTLM authentication function
bool NtlmAuthenticateUser(char *username, char *password, char *domain) {
  // …
  if (!NtlmCheckUsername(username)) {
    // Username is incorrectly validated
    return false;

  // …
  if (!NtlmCheckPassword(username, password)) {
    // Elevation of Privilege problem arises here
    return false;

  // …
  return true;

The problem with the code is that it does not implement proper user verification, allowing attackers to exploit the vulnerability and maliciously elevate privileges, thus bypassing the Windows NTLM security checks.

Exploit Details

To exploit CVE-2023-21746, an attacker must send a specially crafted authentication request to a vulnerable Windows system. The attacker does this by manipulating the NTLM authentication process, forcing the target system to use legacy protocol versions without enforcing proper security measures. Consequently, the attacker gains unauthorized access and elevated privileges.

The attacker exploits a Man-in-the-Middle (MITM) position on the target network.

2. Using crafted packets or spoofed responses, the attacker forces the target system to initiate an NTLM authentication process with legacy protocol versions.
3. The attacker manipulates authentication packets, enabling them to bypass the security checks and gain unauthorized access with elevated privileges.


The CVE-2023-21746 vulnerability presents a significant security risk to organizations and individuals relying on Windows NTLM for authentication. It is crucial for Windows users to stay vigilant and apply the latest security updates to prevent unauthorized access and data leakage.

Microsoft has provided patches and mitigations in their security bulletin, available at this link: (

Users are urged to apply the recommended security measures and move away from NTLM to more secure alternatives like Kerberos for robust authentication and data integrity.


Published on: 01/10/2023 22:15:00 UTC
Last modified on: 01/18/2023 20:15:00 UTC