Why the Citrix Zero-Day Exploit Calls for More Than Just Patching

Report: Critical Vulnerability in Citrix NetScaler Under Active Attack Since August

Introduction

A critical security vulnerability in Citrix NetScaler, which was patched last week, has been under active attack since at least August. The bug, known as CVE-2023-4966 and rated with a CVSS score of 9.4, poses a significant threat to organizations. What makes the situation worse is that simply applying the patch does not fully remediate the vulnerability. This report explores the implications of the bug, the ongoing attacks, and provides advice for organizations to mitigate the risk.

The Flaw and its Consequences

CVE-2023-4966 is an information-disclosure vulnerability that allows cyber attackers to hijack existing authenticated sessions and potentially bypass multifactor authentication (MFA). This gives them full control over NetScaler environments, which are responsible for controlling and managing application delivery within enterprises.

Mandiant, a leading cybersecurity firm, has traced attacks exploiting this bug back to late summer. The ongoing exploitation has targeted professional services, technology, and government organizations, suggesting a focus on cyberespionage. However, Mandiant warns that other threat actors with financial motivations may exploit this vulnerability in the future.

Ineffectiveness of Patching

One of the key challenges with this vulnerability is that even after applying the patch, organizations need to manually terminate all active sessions. This is because authenticated sessions persist after the patch is deployed, and threat actors can use stolen session data to authenticate and gain access to resources until the sessions are terminated. Therefore, simply relying on patching is not sufficient to mitigate the risk posed by this flaw.

Poor Mitigation History

The history of organizations failing to adequately mitigate known threats against Citrix gear raises concerns regarding the effectiveness of addressing this vulnerability. For example, CVE-2023-3519, a critical pre-authentication remote code-execution (RCE) vulnerability in Citrix NetScaler gateways addressed in July, was heavily targeted even after the disclosure. Thousands of credential theft attacks followed, and as of early October, more than 1,300 compromised NetScaler instances were still being detected.

Recommendations for Organizations

To protect against attacks exploiting CVE-2023-4966, organizations are advised to:

• Apply the patch provided by Citrix to address the vulnerability.
• Manually terminate all active sessions to prevent threat actors from leveraging stolen session data to authenticate and gain access to resources.
• Consider implementing additional security measures such as monitoring for suspicious activity, conducting regular vulnerability assessments, and strengthening access controls.
• Stay informed about the latest cybersecurity threats and vulnerabilities by regularly monitoring advisories issued by Citrix and reputable cybersecurity firms.
• Ensure timely and comprehensive patching of all vulnerable systems and applications.

Conclusion

The ongoing exploitation of the Citrix NetScaler vulnerability highlights the persistent and evolving threats faced by organizations in the digital age. It also emphasizes the importance of proactive cybersecurity measures and the need for organizations to prioritize the timely application of patches and the termination of active sessions. By following best practices and staying vigilant, organizations can significantly reduce the risk of falling victim to cyber attacks.