Citrix warns admins to patch NetScaler CVE-2023-4966 bug immediately

Citrix warned of attacks actively exploiting the vulnerability CVE-2023-4966 in NetScaler ADC and Gateway appliances.

Citrix is urging administrators to secure all NetScaler ADC and Gateway appliances against the CVE-2023-4966 vulnerability, which is actively exploited in attacks.

On October 10, Citrix published a security bulletin related to a critical vulnerability, tracked as CVE-2023-4966, in Citrix NetScaler ADC/Gateway devices.

“Exploits of CVE-2023-4966 on unmitigated appliances have been observed.” reported Citrix. “Cloud Software Group strongly urges customers of NetScaler ADC and NetScaler Gateway to install the relevant updated versions of NetScaler ADC and NetScaler Gateway as soon as possible: 

• NetScaler ADC and NetScaler Gateway 14.1-8.50  and later releases
• NetScaler ADC and NetScaler Gateway  13.1-49.15  and later releases of 13.1
• NetScaler ADC and NetScaler Gateway 13.0-92.19 and later releases of 13.0  
• NetScaler ADC 13.1-FIPS 13.1-37.164 and later releases of 13.1-FIPS  
• NetScaler ADC 12.1-FIPS 12.1-55.300 and later releases of 12.1-FIPS  
• NetScaler ADC 12.1-NDcPP 12.1-55.300 and later releases of 12.1-NDcPP 

Note: NetScaler ADC and NetScaler Gateway version 12.1 is now End-of-Life (EOL). Customers are recommended to upgrade their appliances to one of the supported versions that address the vulnerabilities.”

Researchers from Mandiant observed the exploitation of this vulnerability as a zero-day since late August.

Threat actors exploited this vulnerability to hijack existing authenticated sessions and bypass multifactor authentication or other strong authentication requirements. The researchers warn that these sessions may persist after the update to mitigate CVE-2023-4966 has been deployed. 

Mandiant also observed threat actors hijacking sessions where session data was stolen prior to the patch deployment and subsequently used by the threat actor. 

“The authenticated session hijacking could then result in further downstream access based upon the permissions and scope of access that the identity or session was permitted. A threat actor could utilize this method to harvest additional credentials, laterally pivot, and gain access to additional resources within an environment.” states Mandiant.

The attacks observed by Mandiant aimed at professional services, technology, and government organizations. 

The security firm published the CVE-2023-4966 guidance document for remediating and reducing risks related to this flaw.

Citrix has now confirmed that it is aware of attacks in the wild exploiting this vulnerability.

“We now have reports of incidents consistent with session hijacking, and have received credible reports of targeted attacks exploiting this vulnerability.” reads the update published by Citrix this week.

“If you are using affected builds and have configured NetScaler ADC as a gateway (VPN virtual server, ICA proxy, CVPN, RDP proxy) or as an AAA virtual server, we strongly recommend that you immediately install the recommended builds because this vulnerability has been identified as critical. No workarounds are available for this vulnerability. In both this communication and the related security bulletin, we are sharing limited technical details to protect our customers from exploits leveraging this vulnerability within NetScaler to conduct session hijacking of other systems.”

The company urges to install the updated builds and also recommends killing all active and persistent sessions using the following commands:

kill icaconnection -all

kill rdp connection -all

kill pcoipConnection -all

kill aaa session -all

clear lb persistentSessions

US CISA already added CVE-2023-4966 to its Known Exploited and Vulnerabilities Catalog, ordering federal agencies to address the issue by November 8.

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Pierluigi Paganini

(SecurityAffairs – hacking, CVE-2023-4966)