Pierluigi Paganini January 28, 2020

Citrix has released security patches for the recently disclosed CVE-2019-19781 flaw, but the number of attacks on vulnerable systems is increasing.

Last week, Citrix addressed the actively exploited CVE-2019-19781 flaw in Citrix Application Delivery Controller (ADC), Citrix Gateway, and Citrix SD-WAN WANOP appliances.

While security researchers were warning of ongoing scans for Citrix Application Delivery Controller (NetScaler ADC) and Citrix Gateway (NetScaler Gateway) servers affected by the CVE-2019-19781 vulnerability, many experts were announcing the availability online of proof-of-concept exploit code ([1, 2]).

Researchers at MDSsec published technical details of the vulnerability along with a video that shows the exploit they have developed, but they decided to not release it to avoid miscreants use it in the wild.

It has been estimated that 80,000 companies in 158 countries are potentially at risk, most of them in the U.S. (38%), followed by the UK, Germany, the Netherlands, and Australia. 

The CVE-2019-19781 vulnerability was discovered by Mikhail Klyuchnikov from Positive Technologies.

The flaw affects ADC and Gateway versions 10.5, 11.1, 12.0, 12.1, and 13.0, as well as SD-WAN WANOP versions 10.2.6 and 11.0.3.

Citrix released on Friday, the final set of permanent fixes for the vulnerability, for ADC and Gateway version 10.5.

“As with the permanent fixes made available for Citrix ADC and Citrix Gateway versions 11.1, 12.0, 12.1, 13.0, and Citrix SD-WAN 4000-WO, 5000-WO, 4100-WO, and 5100-WO earlier this week, these fixes are available to all customers regardless of whether they have an active maintenance contract with Citrix,” reads the post published by Fermin J. Serna, Chief Information Security Officer at Citrix.

Security experts are monitoring a spike in the number of attacks against Citrix servers after that researchers announced the availability online of proof-of-concept exploits for the CVE-2019-19781 flaw in Citrix NetScaler ADC and Citrix NetScaler Gateway servers.

Researchers from FireEye noticed that one of the threat actors involved in the attacks is patching the vulnerable Citrix servers, installing their own backdoor, tracked as NOTROBIN, to clean up other malware infections and to lock out any other threat from exploiting the CVE-2019-19781 Citrix flaw.

The NOTROBIN backdoor was designed to prevent subsequent exploitation of the flaw on Citrix servers and also to establish backdoor access, a circumstance that suggests that attackers are preparing future attacks. 

It is easy to predict that threat actors will continue to target organizations that will delay in applying the permanent patches or the mitigations. Organizations using Citrix appliances could use the tool released by the company to check for any signs of compromise.

Pierluigi Paganini

(SecurityAffairs – Citrix, hacking)

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