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On January 19, 2020, Citrix released firmware updates for Citrix Application Delivery Controller (ADC) and Citrix Gateway versions 11.1 and 12.0 to address CVE-2019-19781. Citrix expects to release updates for other vulnerable versions of Citrix ADC, Gateway, and SD-WAN WANOP appliances through January 24, 2020. (See Mitigations for update schedule).[1]

A remote, unauthenticated attacker could exploit CVE-2019-19781 to perform arbitrary code execution.[2] This vulnerability has been detected in exploits in the wild.[3]

The Cybersecurity and Infrastructure Agency (CISA) strongly recommends that all users and administrators upgrade their vulnerable appliances as soon as possible once the appropriate firmware update becomes available.

Timeline of Specific Events

• December 17, 2019 – Citrix releases Security Bulletin CTX267027 with mitigations steps.
• January 8, 2020 – The CERT Coordination Center (CERT/CC) releases Vulnerability Note VU#619785: Citrix Application Delivery Controller and Citrix Gateway Web Server Vulnerability, [4] and CISA releases a Current Activity entry.[5]
• January 10, 2020 – The National Security Agency (NSA) releases a Cybersecurity Advisory on CVE-2019-19781.[6]
• January 11, 2020 – Citrix releases blog post on CVE-2019-19781 with timeline for fixes.[7]
• January 13, 2020 – CISA releases a Current Activity entry describing their utility that enables users and administrators to test whether their Citrix ADC and Citrix Gateway firmware is susceptible to the CVE-2019-19781 vulnerability.[8] 
• January 16, 2020 – Citrix announces that Citrix SD-WAN WANOP appliance is also vulnerable to CVE-2019-19781.
• January 19, 2020 – Citrix releases firmware updates for Citrix ADC and Citrix Gateway versions 11.1 and 12.0 and blog post on accelerated schedule for fixes.[9]
• January 24, 2020 – Citrix expects to release firmware updates for Citrix ADC and Citrix Gateway versions 10.5, 12.1, and 13.0 and Citrix SD-WAN WANOP release 10.2.6 and 11.0.3.

Impact

On December 17, 2019, Citrix reported vulnerability CVE-2019-19781. A remote, unauthenticated attacker could exploit this vulnerability to perform arbitrary code execution. This vulnerability has been detected in exploits in the wild.

The vulnerability affects the following appliances:

• Citrix NetScaler ADC and NetScaler Gateway version 10.5 – all supported builds
• Citrix ADC and NetScaler Gateway version 11.1 – all supported builds before 11.1.63.15
• Citrix ADC and NetScaler Gateway version 12.0 – all supported builds before 12.0.63.13
• Citrix ADC and NetScaler Gateway version 12.1 – all supported builds
• Citrix ADC and Citrix Gateway version 13.0 – all supported builds
• Citrix SD-WAN WANOP firmware and appliance models 4000, 4100, 5000, and 5100 – all supported builds. (Citrix SD-WAN WANOP is vulnerable because it packages Citrix ADC as a load balancer).

Detection Measures

CISA has released a utility that enables users and administrators to detect whether their Citrix ADC and Citrix Gateway firmware is susceptible to CVE-2019-19781.[10] CISA encourages administrators to visit CISA’s GitHub page to download and run the tool.

See the National Security Agency’s Cybersecurity Advisory on CVE-2020-19781 for other detection measures.[11]

CISA strongly recommends users and administrators update Citrix ADC, Citrix Gateway, and Citrix SD-WAN WANOP once the appropriate firmware updates become available.

The fixed builds can be downloaded from Citrix Downloads pages for Citrix ADC and Citrix Gateway.

Until the appropriate update is accessible, users and administrators should apply Citrix’s interim mitigation steps for CVE-2019-19781.[12] Verify the successful application of the above mitigations by using the tool in CTX269180 – CVE-2019-19781 – Verification ToolTest. Note: these mitigation steps apply to Citrix ADC and SD-WAN WANOP deployments.[13]

Refer to table 1 for Citrix’s planned fix schedule.[14]

Table 1. Fix schedule for Citrix appliances vulnerable to CVE-2019-19781

Administrators should review NSA’s Citrix Advisory for other mitigations, such as applying the following defense-in-depth strategy:

“Consider deploying a VPN capability using standardized protocols, preferably ones listed on the National Information Assurance Partnership (NIAP) Product Compliant List (PCL), in front of publicly accessible Citrix ADC and Citrix Gateway appliances to require user authentication for the VPN before being able to reach these appliances. Use of a proprietary SSLVPN/TLSVPN is discouraged.”