Malicious Hackers Override Critical Patch in Citrix Netscaler, Exploiting Vulnerability
In a recent warning, cybersecurity firm Mandiant has revealed that exploitation of the high-risk vulnerability, CVE-2023-4966, in Citrix NetScaler has been taking place at professional services and technology firms, government agencies, and other organizations. Even after the application of the patch, issued by Citrix on October 10, attackers could potentially continue unauthorized access using compromised session tokens or credentials.
To ensure the security of your environment, it is essential to terminate all active sessions established before the patch application. Key steps include:
- Apply the official Citrix patch for CVE-2023-4966 as soon as possible. This vulnerability allows sensitive information disclosure and remote session hijacking.
- Force termination of all existing user sessions on the NetScaler appliance immediately after patching. This invalidates session tokens in memory that could have been leaked or stolen before patching.
- Restart relevant services or the entire NetScaler appliance if recommended by Citrix patch documentation to ensure complete session state reset.
- Review and monitor logs for suspicious session activity indicating attempts to reuse old session tokens or hijack active sessions.
- Implement additional detection and response controls, such as monitoring for characteristic CitrixBleed exploit patterns via your SIEM (e.g., Splunk), as attackers continue to attempt exploitation.
While the exact CLI or GUI commands for terminating sessions may vary, best practice for NetScaler after patching a memory disclosure flaw typically involves logging into the NetScaler management console, navigating to the user sessions or active connections area, and manually disconnecting or killing sessions or using CLI commands such as or similar commands per Citrix official documentation. Consult Citrix official security advisories or support for exact steps and commands customized for your NetScaler version and configuration.
Mandiant CTO Charles Carmakal is urging organizations to terminate all active sessions, emphasizing that a threat actor could use stolen session data to authenticate to resources until the sessions are terminated. Mandiant officials also confirm that successful exploitation of the vulnerability can allow hackers to hijack existing authenticated sessions.
Despite the patch, the exploitation of the flaw continues, with Mandiant observing cases where session data was stolen prior to the patch deployment and later used by hackers. The identity of the threat actor is unknown, but they are currently focused on cyber espionage. Mandiant expects hackers with financial motivations to eventually get involved.
The Cybersecurity and Infrastructure Security Agency has referred back to Mandiant's guidance for comment. It is crucial to note that there was no indication from Citrix's customers or industry partners that an exploit existed in the wild when the vulnerability was made public with a patch. Mandiant's warning came on Tuesday, and this article has been updated to include a statement from Citrix with further details about when the patch works.
- The incident of cybersecurity vulnerability, CVE-2023-4966, in Citrix NetScaler, which was exposed by cybersecurity firm Mandiant, has been detected in various sectors, including professional services, technology firms, government agencies, and general-news organizations.
- Mandiant's CTO, Charles Carmakal, is urging organizations to terminate all active sessions to prevent potential cybercriminals from using stolen session data to authenticate to resources.
- Despite the application of the Citrix patch for the CVE-2023-4966 vulnerability, cybercrime-and-justice incidents continue, with threats from both cyber espionage and financially motivated hackers.
