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Attacks on ADC Ramp Up as Citrix Releases Remaining Patches

Citrix has released the full set of patches for the recently disclosed security flaw tracked as CVE-2019-19781, but attacks on vulnerable systems are ramping up.

Citrix has released the full set of patches for the recently disclosed security flaw tracked as CVE-2019-19781, but attacks on vulnerable systems are ramping up.

Impacting Citrix Application Delivery Controller (ADC) and Gateway (previously known as NetScaler ADC and NetScaler Gateway), the vulnerability was disclosed in December 2019, and the first attacks targeting it followed only weeks later, shortly after PoC exploits were released.

The bug impacts 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, and Citrix started rolling out patches on January 19. Several days later, the company released a second wave of patches, along with a tool to detect potential compromise.

On Friday, Citrix published the final set of permanent fixes for the vulnerability, for ADC and Gateway version 10.5, encouraging all customers to apply them as soon as possible.

“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,” noted Fermin J. Serna, Chief Information Security Officer at Citrix.

Although fixes are now available for all vulnerable products, the risk for organizations that do not apply them is, in fact, increasing.

According to FireEye, most of the activity targeting CVE-2019-19781 has been focused on deploying cryptocurrency miners, and some of the more recent incidents reveal attempts to deploy ransomware on vulnerable systems.

As part of these attacks, a shell script is used to check whether the python2 binary is present on the target, which is an indication of Citrix Gateway 12.x and 13.x systems. The same shell script is also used to download two additional files.

One of the files, FireEye reveals, included code to perform additional reconnaissance and to install a piece of ransomware dubbed Ragnarok. The malware was initially detected on January 17.

Throughout the attack, the threat actor employs numerous exploits in their attempt to take advantage of the vulnerable systems and move laterally within the compromised organization’s network, using the Gateway as a central pivot point.

“FireEye continues to observe multiple actors who are currently seeking to take advantage of CVE-2019-19781,” the security company concludes.

In addition to applying the permanent patches or the previously published mitigations, organizations using Citrix appliances are advised to use the recently released tool to check for any signs of compromise.

Related: Citrix Releases More Patches for Exploited Flaw, Tool to Detect Compromise

Related: Attacker Installs Backdoor, Blocks Others From Exploiting Citrix ADC Vulnerability

Written By

Ionut Arghire is an international correspondent for SecurityWeek.

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