Over 640 Citrix Servers are Compromised & Implanted with Web Shells

A series of targeted attacks have resulted in the compromise and infection of multiple Citrix Netscaler ADC and Gateway servers. These attacks exploited a significant remote code execution vulnerability, specifically identified as CVE-2023-3519. Notably, this vulnerability was previously exploited as a zero-day exploit to breach the network of a critical infrastructure organization within the United States.

The Shadowserver Foundation, a non-profit organization dedicated to enhancing internet security, has recently disclosed that attackers have successfully planted web shells on a minimum of 640 Citrix servers as part of these orchestrated attacks.

Shadowserver CEO Piotr Kijewski says the recent attack resembles the China Chopper pattern. They haven't shared more details due to the circumstances. There are fewer detected instances than believed. Shadowserver found 640 compromised appliances with web shells as of July 30, 2023. They know about widespread exploitation on July 20th. If your system isn't patched, it's likely compromised. Shadowserver thinks there are more web shells linked to CVE-2023-3519 than the reported 640 instances.

Around two weeks ago, an estimated 15,000 vulnerable Citrix appliances were found susceptible to CVE-2023-3519 attacks. However, this number has since decreased to fewer than 10,000, indicating some positive progress in addressing the vulnerability.

Citrix responded to the situation on July 18th by issuing security updates aimed at mitigating the Remote Code Execution (RCE) vulnerability. The company acknowledged the presence of exploits targeting vulnerable appliances and urgently advised customers to swiftly apply the provided patches.

The vulnerability primarily impacts unpatched Netscaler appliances configured as gateways, including VPN virtual servers, ICA Proxy, CVPN, and RDP Proxy, as well as authentication virtual servers (AAA server).

On the same day, Citrix not only addressed CVE-2023-3519 but also tackled two other high-severity vulnerabilities: CVE-2023-3466 and CVE-2023-3467. These vulnerabilities have the potential to be exploited for reflected cross-site scripting (XSS) attacks and privilege escalation to gain root access.

In response to the ongoing attacks, the U.S. Cybersecurity and Infrastructure Security Agency (CISA) issued a directive to U.S. federal agencies, mandating the safeguarding of Citrix servers on their networks by August 9th.

The advisory from CISA also underscored that the vulnerability had already been leveraged to breach the systems of a critical infrastructure entity within the United States.

According to CISA, threat actors capitalized on the vulnerability as a zero-day exploit in June 2023. They deployed a web shell on a NetScaler ADC appliance owned by a critical infrastructure organization. This allowed the attackers to explore the victim's active directory (AD) and obtain AD data, which they subsequently exfiltrated. Fortunately, their attempt to move laterally to a domain controller was thwarted by network-segmentation controls in place on the appliance.

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