In the ever-evolving landscape of cybersecurity, zero-day vulnerabilities pose one of the most significant threats to organizations. These vulnerabilities, by their very nature, are unknown to the software vendor and thus unpatched at the time of discovery, leaving systems exposed to potential exploitation. Recently, Microsoft disclosed a high-severity zero-day vulnerability, tracked as CVE-2024-38200, which affects multiple versions of Microsoft Office, including Office 2016, Office 2019, Office LTSC 2021, and Microsoft 365 Apps for Enterprise. This blog delves into the technical aspects of this vulnerability, its potential impact, and the strategies that security professionals should employ to mitigate the associated risks.

Understanding CVE-2024-38200: The Vulnerability Breakdown

Nature of the Vulnerability

CVE-2024-38200 is categorized as a spoofing vulnerability within Microsoft Office. Spoofing vulnerabilities typically involve the manipulation of the interface or behavior of a software component, tricking users or systems into believing that a deceptive file, request, or user is legitimate. In this specific case, the vulnerability facilitates unauthorized disclosure of sensitive information, potentially allowing malicious actors to access critical data such as system configuration, network status, or even personal user information.

Affected Products

The vulnerability impacts a wide array of Microsoft Office products:

• Microsoft Office 2016 (32-bit and 64-bit)
• Microsoft Office 2019 (32-bit and 64-bit)
• Microsoft Office LTSC 2021 (32-bit and 64-bit)
• Microsoft 365 Apps for Enterprise (32-bit and 64-bit)

The widespread use of these Office versions across enterprises increases the potential attack surface, making it imperative for organizations to understand the risks and implement mitigations promptly.

Exploitability and Attack Vectors

While Microsoft has assigned this vulnerability a CVSS score of 7.5, indicating a high severity, it has concurrently labeled the exploitability as “less likely.” This assessment is based on the complexity involved in executing a successful attack.

In a typical web-based attack scenario, an attacker could host or compromise a website that contains a maliciously crafted file designed to exploit CVE-2024-38200. However, the attacker would need to entice the victim to visit the website and manually open the crafted file—an action that often requires social engineering techniques. This reliance on user interaction reduces the likelihood of widespread automated exploitation, but targeted attacks remain a considerable risk.

Mitigation Strategies: Beyond Waiting for a Patch

Given that a formal patch is not expected until August 13, 2024, as part of Microsoft’s Patch Tuesday, it is crucial for organizations to deploy interim mitigations to protect their systems. Microsoft has recommended several strategies:

1. Restricting Outgoing NTLM Traffic

Configuring the “Network Security: Restrict NTLM: Outgoing NTLM traffic to remote servers” policy setting is a key mitigation. By adjusting this setting, organizations can control, block, or audit outgoing NTLM traffic, which could prevent the exploit from successfully transmitting sensitive information to a remote attacker.

2. Implementing the Protected Users Security Group

Adding users to the Protected Users Security Group can further safeguard against this vulnerability by preventing the use of NTLM as an authentication mechanism. This move not only mitigates the current vulnerability but also strengthens overall network security by reducing reliance on older, less secure authentication protocols.

3. Blocking TCP 445/SMB Outbound Traffic

Blocking TCP 445/SMB outbound traffic at the perimeter firewall, local firewall, or via VPN settings is another critical step. This measure prevents NTLM authentication messages from being sent to remote file shares, effectively limiting the attacker’s ability to exploit this vector.

A Broader Context: The Risk of Reintroducing Old Vulnerabilities

The disclosure of CVE-2024-38200 comes at a time when Microsoft is also grappling with other zero-day vulnerabilities that could potentially “unpatch” up-to-date systems, reintroducing old, exploitable flaws. This trend underscores a significant challenge in cybersecurity: the need for continuous vigilance and adaptation. As attackers develop more sophisticated methods to bypass existing security controls, organizations must remain proactive in their defense strategies.

The LNK Stomping Technique

One such bypass technique that has been observed in the wild for over six years is LNK stomping. This method allows attackers to execute malicious apps without triggering Windows Smart App Control and SmartScreen warnings, demonstrating that even well-established security features can be circumvented by determined adversaries. This reinforces the importance of layered security, where multiple overlapping controls are used to protect critical systems.

Insights from Recent Incidents

Recent cyber incidents highlight the importance of addressing these vulnerabilities promptly. For instance, the Hafnium group, known for exploiting zero-day vulnerabilities in Microsoft Exchange, has shifted focus towards Office products. The group’s techniques include exploiting vulnerabilities like CVE-2024-38200 to gain a foothold in corporate networks before moving laterally to more critical systems. Such incidents underscore the necessity of rapid patch deployment and comprehensive incident response planning.

The Role of Threat Intelligence

Leveraging threat intelligence can be crucial in mitigating the risks posed by vulnerabilities like CVE-2024-38200. By monitoring threat actor chatter and identifying indicators of compromise (IOCs) associated with this vulnerability, organizations can preemptively strengthen their defenses. Collaboration with industry peers and participation in information-sharing communities, such as ISACs (Information Sharing and Analysis Centers), can also provide early warnings and insights into emerging threats related to this zero-day vulnerability.

Conclusion

CVE-2024-38200 is a stark reminder of the ever-present threat posed by zero-day vulnerabilities. While the immediate risk of widespread exploitation may be low, the potential impact on targeted systems could be severe. Cybersecurity professionals must act swiftly to implement the recommended mitigations while awaiting the official patch from Microsoft. Moreover, this vulnerability highlights the need for ongoing vigilance and a robust, multi-layered approach to security, ensuring that even if one line of defense is compromised, others remain intact to protect critical assets.

As the cybersecurity landscape continues to evolve, staying informed about emerging threats and maintaining a proactive security posture are key to safeguarding organizational systems and data from increasingly sophisticated adversaries.