All these attacks are taking place, hackers are scanning the internet for Citrix appliances which were unpatched for the CVE-2019-19781 [1] vulnerability. Vulnerable devices include the Citrix Application Delivery Controller (ADC), Citrix Gateway, and two older versions of Citrix SD-WAN WANOP. The vulnerability was disclosed in mid-December; however, internet-wide attacks began after January 11, when proof-of-concept exploit code was published online and became broadly available to anyone.
Citrix released the final permanent fix for the actively exploited CVE-2019-19781 vulnerability, needed to secure all vulnerable Citrix Application Delivery Controller (ADC), Citrix Gateway, and Citrix SD-WAN WANOP appliances.

The CyberSecOp team has identified attacks scanning multiple client Citrix gateway to take advantage of vulnerabilities in Citrix gateway applications.

Timeline
On January 19, 2020, Citrix released firmware updates for Citrix Application Delivery Controller (ADC) and Citrix Gateway versions 11.1 and 12.0.
On January 22, 2020, Citrix released security updates for vulnerable SD-WAN WANOP appliances.
On January 23, 2020, Citrix released firmware updates for Citrix ADC and Gateway versions 12.1 and 13.0.
On January 24, 2020, Citrix released firmware updates for Citrix ADC and Gateway version 10.5.
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.
Timeline of Specific Events

• December 17, 2019 – Citrix released Security Bulletin CTX267027 with mitigations steps.

• January 8, 2020 – The CERT Coordination Center (CERT/CC) released Vulnerability Note VU#619785: Citrix Application Delivery Controller and Citrix Gateway Web Server Vulnerability, and CISA releases a Current Activity entry.

• January 10, 2020 – The National Security Agency (NSA) released a Cybersecurity Advisory on CVE-2019-19781.

• January 11, 2020 – Citrix released blog post on CVE-2019-19781 with timeline for fixes.

• January 13, 2020 – CISA released 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. 

• January 16, 2020 – Citrix announced that Citrix SD-WAN WANOP appliance is also vulnerable to CVE-2019-19781.

• January 19, 2020 – Citrix released firmware updates for Citrix ADC and Citrix Gateway versions 11.1 and 12.0 and blog post on accelerated schedule for fixes.

• January 22, 2020 – Citrix released security updates for Citrix SD-WAN WANOP release 10.2.6 and 11.0.3.]

• January 22, 2020 – Citrix and FireEye Mandiant released an indicator of compromise (IOC) scanning tool for CVE-2019-19781.

• January 23, 2020 – Citrix released firmware updates for Citrix ADC and Citrix Gateway versions 12.1 and 13.0.

• January 24, 2020 – Citrix released firmware updates for Citrix ADC and Citrix Gateway version 10.5.

Technical Details

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 before 12.1.55.18

• Citrix ADC and Citrix Gateway version 13.0 – all supported builds before 13.0.47.24

• Citrix SD-WAN WANOP appliance models 4000-WO, 4100-WO, 5000-WO, and 5100-WO – all supported software release builds before 10.2.6b and 11.0.3b. (Citrix SD-WAN WANOP is vulnerable because it packages Citrix ADC as a load balancer).

What Customers Should Do
Exploits of this issue on unmitigated appliances have been observed in the wild. Citrix strongly urges affected customers to immediately upgrade to a fixed build OR apply the provided mitigation which applies equally to Citrix ADC, Citrix Gateway and Citrix SD-WAN WANOP deployments. Customers who have chosen to immediately apply the mitigation should then upgrade all of their vulnerable appliances to a fixed build of the appliance at their earliest schedule. Subscribe to bulletin alerts at https://support.citrix.com/user/alerts to be notified when the new fixes are available.
The following knowledge base article contains the steps to deploy a responder policy to mitigate the issue in the interim until the system has been updated to a fixed build: CTX267679 - Mitigation steps for CVE-2019-19781
Upon application of the mitigation steps, customers may then verify correctness using the tool published here: CTX269180 - CVE-2019-19781 – Verification Tool
Fixed builds have been released across all supported versions of Citrix ADC and Citrix Gateway. Fixed builds have also been released for Citrix SD-WAN WANOP for the applicable appliance models. Citrix strongly recommends that customers install these updates at their earliest schedule. The fixed builds can be downloaded from https://www.citrix.com/downloads/citrix-adc/ and https://www.citrix.com/downloads/citrix-gateway/ and https://www.citrix.com/downloads/citrix-sd-wan/
 
 If you would like to learn more about CVE-2019-19781 vulnerability & risk mitigation, please contact CyberSecOp at the following support@cybersecop.com