Introduction
A critical remote code execution (RCE) vulnerability in VMware vCenter Server (CVE-2021-21985) is under active exploitation, with cybercriminals mass scanning the internet for unpatched systems. This flaw, rated 9.8 on the CVSS scale, can allow attackers to gain full control of vCenter servers, posing a severe risk to enterprises, cloud environments, and data centers.
Given VMware's widespread use in enterprise virtualization, this vulnerability presents a major security threat, making immediate patching crucial.
What is CVE-2021-21985?
CVE-2021-21985 is a remote code execution vulnerability in VMware vCenter Server’s vSAN Health Check plug-in. The flaw arises due to insufficient input validation, allowing attackers to execute commands with full administrative privileges on the underlying operating system hosting vCenter Server.
Key Technical Details
- Affected Product: VMware vCenter Server (unpatched versions)
- Vulnerability Type: Remote Code Execution (RCE)
- Severity: Critical (CVSS 9.8)
- Attack Vector: Network-based attack (no authentication required)
- Exploitation Status: Actively exploited in the wild
- Patch Released: May 25, 2021
How the Exploit Works
- Attackers identify vulnerable vCenter servers by scanning the internet.
- They exploit the vSAN Health Check plug-in to bypass authentication.
- The flaw allows them to execute arbitrary commands with high-level system privileges.
- Once compromised, attackers can:
- Deploy ransomware
- Steal sensitive data
- Move laterally within the network
- Create persistent backdoors
Active Exploitation: What We Know So Far
Mass Scanning in Progress
Security researchers from Bad Packets and Kevin Beaumont detected mass scanning activity targeting vulnerable vCenter Servers on June 3, 2021.
- Troy Mursch, Chief Research Officer at Bad Packets, confirmed scanning activity from 104.40.252.159.
- Following the publication of a proof-of-concept (PoC) exploit, attacks against VMware vCenter servers surged.
VMware’s Warning to Users
Despite VMware releasing a patch on May 25, thousands of vCenter servers remain exposed. VMware strongly recommends treating this as an emergency update, warning:
“In this era of ransomware, it is safest to assume that an attacker is already inside the network somewhere, on a desktop, and perhaps even in control of a user account. This is why we strongly recommend declaring an emergency change and patching as soon as possible.”
14,858 vCenter Servers Exposed
Security researchers from Bad Packets and Binary Edge identified at least 14,858 vCenter servers accessible over the internet. Each of these systems is a potential target for attackers.
Previous Exploits Against VMware vCenter
This is not the first critical RCE vulnerability in VMware vCenter Server. In February 2021, VMware patched CVE-2021-21972, another severe RCE flaw that was also massively exploited by cybercriminals.
Necro Bot Malware Leveraging vCenter Exploit
New research from Cisco Talos found that the Python-based Necro bot was recently modified to exploit CVE-2021-21985. The malware uses this vulnerability to self-propagate, infecting more systems without user intervention.
Necro bot's impact includes:
- Expanding botnet infections
- Deploying cryptominers for financial gain
- Exfiltrating sensitive data
- Injecting additional malware payloads
This demonstrates how automated malware threats are adapting to exploit new vulnerabilities in real-time.
How to Protect Your VMware vCenter Server
1. Apply Security Patches Immediately
The best defense against this exploit is to install VMware’s security update without delay. If patching is not possible, organizations should implement temporary mitigations.
2. Restrict Access to vCenter Servers
- Disable external access to vCenter Server interfaces.
- Place vCenter management interfaces behind a VPN or firewall.
- Use zero-trust network principles to limit attack surfaces.
3. Monitor for Signs of Exploitation
- Check logs for unauthorized login attempts or suspicious system activity.
- Use intrusion detection systems (IDS) to spot scanning activity.
- Employ endpoint detection and response (EDR) tools to detect malicious behavior.
4. Disable Unused vSphere Plug-ins
- If not needed, disable the vSAN Health Check plug-in to reduce risk exposure.
- Regularly audit plug-ins for vulnerabilities.
5. Implement Network Segmentation
- Restrict vCenter access to authorized administrators.
- Separate critical infrastructure from internet-exposed environments.
Long-Term Security Considerations
The VMware vCenter RCE vulnerability highlights the ongoing threat of unpatched enterprise software. To prevent similar risks in the future:
- Regularly update software – Patch management should be a top security priority.
- Use vulnerability scanners – Detect and remediate weaknesses before attackers do.
- Deploy a zero-trust model – Assume all networks and devices are compromised by default.
- Train IT staff – Educate employees on cyber hygiene and incident response.
Conclusion
The CVE-2021-21985 RCE vulnerability in VMware vCenter Server is actively exploited by cybercriminals, ransomware groups, and botnets. With over 14,000 exposed servers, organizations must immediately patch their systems and implement strong security measures.
At Ackerworx, we specialize in penetration testing, vulnerability management, and cybersecurity consulting. If your business requires security assessments or patch management assistance, contact us today.
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