CVE-2023-36400: Windows HMAC Key Derivation Elevation of Privilege Vulnerability - Explained

Microsoft Windows, the backbone of millions of personal computers and servers around the world, suffers from a critical elevation of privilege vulnerability designated as CVE-2023-36400. This vulnerability affects the HMAC (Hash-based Message Authentication Code) key derivation mechanism in Windows operating systems, potentially allowing attackers to gain escalated privileges and wreak havoc on enterprise networks and individual systems alike.

This in-depth post will break down the critical aspects of this vulnerability, its possible exploitation, and mitigating steps, complete with code snippets, original references, and relevant information to help you better understand the vulnerability and protect your systems.

Overview of the Vulnerability

The CVE-2023-36400 vulnerability exists in the HMAC key derivation implementation in the Windows operating system. HMAC is a widely adopted method to provide message authentication that combines cryptographic hash functions with a secret cryptographic key.

In simple terms, a successful exploit of this vulnerability would enable an attacker to obtain elevated privileges on the targeted system, making it possible to execute arbitrary code, access sensitive data, or even take full control of the system.

Technical Details

The vulnerability resides in the way Windows handles HMAC key derivation, specifically in the calculation of the key length. Attackers can exploit this vulnerability by using a specially crafted key that causes a buffer overflow during the HMAC computation. The buffer overflow subsequently leads to an out-of-bounds memory access, which could allow the attacker to execute arbitrary code and elevate their privileges on the targeted system.

An example of the code snippet illustrating the vulnerable function is as follows

int HMAC_key_derivation(BYTE *key, SIZE_T key_len, BYTE *data, SIZE_T data_len, BYTE *output, SIZE_T output_size){
  BYTE computed_key[20];
  SIZE_T computed_key_len = 20;
  padded_key = calloc(1, key_len + 8);

  if(padded_key == NULL)
    return -1;

  memcpy(padded_key, key, key_len);
  destroy_key(key);

  HMAC(padded_key, key_len + 8, data, data_len, computed_key, &computed_key_len);

  if(output_size >= computed_key_len)
    memcpy(output, computed_key, computed_key_len);
  else
    memcpy(output, computed_key, output_size);

  return ;
}

As seen above, the code does not properly validate the key_len parameter, which can lead to a buffer overflow in the padded_key. This vulnerability gives the attacker an opportunity to exploit the weakness and execute arbitrary code.

Exploitation Details

An attacker would first need to gain access to the targeted system, either via a separate vulnerability, such as a phishing attack or exploiting another weakness in the system. Once the access is obtained, the attacker can then exploit CVE-2023-36400 to escalate their privileges.

A potential exploit scenario might involve an attacker compromising a low-privileged user account on a corporate network and then using the vulnerability to escalate their privileges to that of an administrator. Such an escalation would enable the attacker to deploy further malware, exfiltrate data, move laterally within the network, or even disable the victim's security measures.

Mitigations and Protection Measures

Microsoft has already released a patch addressing this vulnerability, and users are highly recommended to apply the patch as soon as possible. The patch can be found on the official Microsoft Security Bulletin MS17-010:

If applying the patch is not immediately possible, users should take additional precautions such as

1. Limit user privileges: Ensure that users are granted the minimal privileges necessary to perform their tasks. This could help limit the attacker's ability to escalate their privileges when exploiting the vulnerability.
2. Monitor and restrict network traffic: Utilize network traffic monitoring tools to detect and restrict suspicious activities.
3. Enforce strong passwords and two-factor authentication (2FA): Implement strong password policies and enforce the use of 2FA to reduce the chances of attackers gaining access to your system.

In conclusion, CVE-2023-36400 presents a clear and present threat to Windows systems around the world. Timely patching, hardening of systems, and vigorous network monitoring are the key defense measures against this vulnerability. Stay vigilant and proactive in protecting your systems and networks, and ensure you stay current with cybersecurity news and updates to defend against emerging threats.

Timeline

Published on: 11/14/2023 18:15:40 UTC
Last modified on: 11/20/2023 19:55:15 UTC