Netizen: Technical Analysis and Advisory on CVE-2023-4966

The highly exploitable CVE-2023-4966 vulnerability in Citrix NetScaler at first glance proves incredibly dangerous to NetScaler environments. While initial analyses have highlighted the potential risk and exploitation scenarios, a deeper technical examination is essential to fully comprehend its intricacies and the subsequent steps for mitigation.

Affected Products and Versions:

Affected ProductAffected VersionFixed Version
NetScaler ADC and NetScaler GatewayPrior to 13.0-92.1913.0-92.19 and later releases of 13.0
NetScaler ADC and NetScaler GatewayPrior to 13.1-49.1513.1-49.15 and later releases of 13.1
NetScaler ADC and NetScaler GatewayPrior to 14.1-8.5014.1-8.50 and later releases
NetScaler ADC 12.1-NDcPPPrior to 12.1-55.30012.1-55.300 and later releases of 12.1-NDcPP
NetScaler ADC 12.1-FIPSPrior to 12.1-55.30012.1-55.300 and later releases of 12.1-FIPS
NetScaler ADC 13.1-FIPSPrior to 13.1-37.16413.1-37.164 and later releases of 13.1-FIPS
Note: Citrix has emphasized that NetScaler ADC and NetScaler Gateway versions 12.1 are End of Life (EOL). Users are strongly advised to upgrade to a supported version immediately.

CVE Details:

CVE-2023-4966Sensitive information disclosure9.4Critical
CVE-2023-4967Denial of service (DoS)8.2High
Note: Apart from CVE-2023-4966, Citrix addressed one additional vulnerability in security bulletin CTX579459.

Technical Analysis of CVE-2023-4966

CVE-2023-4966 is rooted in an information disclosure vulnerability that has far-reaching implications. Although it’s categorized as an information disclosure type, the flaw’s potential to allow session hijacking amplifies its severity. When an attacker exploits this vulnerability, they gain the ability to hijack authenticated sessions, an action that could potentially bypass MFA. This means unauthorized actors could gain full control over NetScaler environments, pivotal in managing application delivery within corporate settings.

Cybersecurity firm Mandiant’s discovery that the flaw has been under active exploitation since August only emphasizes the ongoing issue. The attacks have primarily targeted professional services, technology, and government organizations. Charles Carmakal, Mandiant’s CTO, emphasized the persistence of authenticated sessions even after the application of patches, leading to a scenario where attackers could still utilize stolen session data for unauthorized access. On LinkedIn, he advised all organizations to “terminate all active sessions,” explaining that “these authenticated sessions will persist after the update to mitigate CVE-2023-4966 has been deployed. Therefore, even after the patch is applied, a threat actor could use stolen session data to authenticate to resources until the sessions are terminated.”

The exploitation of CVE-2023-4966 isn’t straightforward, as it leverages the persistence of authenticated sessions post-patch. This means the attackers could potentially maintain control over the sessions until they are manually terminated. The information disclosure mechanism is potent, giving attackers insights into session IDs and other sensitive data pivotal for maintaining unauthorized access.

The Importance of Proactive Security

The patches introduced by Citrix for the CVE-2023-4966 vulnerability are targeted specifically at rectifying issues in certain versions of the NetScaler ADC and NetScaler Gateway. A notable point of concern in these patch notes is the recommendation for users of the 12.1 version to transition to a more recent version, given its designation as End of Life (EOL). This situation accentuates the broader principle that cybersecurity is not just about reactive measures like patching, but also proactive strategies. It’s crucial for organizations to not only apply timely patches but also to be forward-thinking by upgrading to supported and more secure software versions, to avoid issues like this vulnerability. In addition, maintaining a robust security posture necessitates an ongoing and thorough review of security practices and infrastructures to ensure vulnerabilities are identified and addressed promptly, and potential risks are mitigated effectively.


The critical nature of this vulnerability, combined with the active exploitation in the wild, calls for an immediate and comprehensive response. Beyond patch application, organizations must follow Carmakal’s advice and terminate all active sessions to nullify the persistence of potential unauthorized access. Citrix has issued patches for the affected versions, but the termination of sessions is a manual step that organizations must undertake to ensure the complete eradication of the risk.

How Can Netizen Help?

Netizen ensures that security gets built-in and not bolted-on. Providing advanced solutions to protect critical IT infrastructure such as the popular “CISO-as-a-Service” wherein companies can leverage the expertise of executive-level cybersecurity professionals without having to bear the cost of employing them full time. 

We also offer compliance support, vulnerability assessments, penetration testing, and more security-related services for businesses of any size and type. 

Additionally, Netizen offers an automated and affordable assessment tool that continuously scans systems, websites, applications, and networks to uncover issues. Vulnerability data is then securely analyzed and presented through an easy-to-interpret dashboard to yield actionable risk and compliance information for audiences ranging from IT professionals to executive managers.

Netizen is an ISO 27001:2013 (Information Security Management), ISO 9001:2015, and CMMI V 2.0 Level 3 certified company. We are a proud Service-Disabled Veteran-Owned Small Business that is recognized by the U.S. Department of Labor for hiring and retention of military veterans. 

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