CVE-2021-21980
📋 TL;DR
CVE-2021-21980 is an unauthorized arbitrary file read vulnerability in the vSphere Web Client (FLEX/Flash) that allows attackers with network access to port 443 on vCenter Server to access sensitive information. It affects VMware vCenter Server deployments using the deprecated Flash-based web client, potentially exposing configuration files, logs, or credentials.
💻 Affected Systems
- VMware vCenter Server
📦 What is this software?
⚠️ Risk & Real-World Impact
Worst Case
Attackers could read critical system files, such as configuration or credential files, leading to full compromise of the vCenter Server and associated virtual infrastructure.
Likely Case
Sensitive information disclosure, including configuration details or logs, which could facilitate further attacks like privilege escalation or lateral movement.
If Mitigated
Limited impact if network access is restricted and the vulnerable component is disabled or patched, reducing exposure to internal threats.
🎯 Exploit Status
Exploitation is straightforward via network requests to the vulnerable endpoint, with proof-of-concept code available publicly.
🛠️ Fix & Mitigation
✅ Official Fix
Patch Version: vCenter Server 6.5 Update 3p, 6.7 Update 3l, and 7.0 Update 2c or later (refer to vendor advisory for exact versions).
Vendor Advisory: https://www.vmware.com/security/advisories/VMSA-2021-0027.html
Restart Required: Yes
Instructions:
1. Download the patch from VMware's official site. 2. Apply the patch according to VMware's documentation. 3. Restart the vCenter Server services or the entire system as required.
🔧 Temporary Workarounds
Disable Flash-based vSphere Web Client
allDisable the vulnerable Flash client to prevent exploitation, as it is deprecated and not required for most operations.
Refer to VMware KB article for specific steps to disable the Flash client via configuration changes.
Restrict Network Access
allLimit access to port 443 on vCenter Server to trusted IP addresses only using firewalls or network segmentation.
Use firewall rules (e.g., iptables on Linux or Windows Firewall) to block unauthorized access to TCP port 443.
🧯 If You Can't Patch
- Disable the Flash-based vSphere Web Client immediately to mitigate the vulnerability.
- Implement strict network controls to limit access to vCenter Server to only necessary users and systems.
🔍 How to Verify
Check if Vulnerable:
Check if the Flash-based vSphere Web Client is enabled and if the vCenter Server version is within the affected range by reviewing system logs or configuration files.Check Version:
On vCenter Server, run: 'vmware -v' or check the version in the vSphere Client interface.Verify Fix Applied:
Verify the patch is applied by checking the vCenter Server version against the patched versions listed in the vendor advisory.📡 Detection & Monitoring
Log Indicators:
- Unusual file access attempts in vCenter Server logs, especially to sensitive paths via the web client.
Network Indicators:
- Suspicious HTTP requests to the vSphere Web Client endpoint on port 443 from unauthorized sources.
SIEM Query:
Example: 'source="vcenter.log" AND (event="file read" OR uri="/ui/vsphere-client/*") AND src_ip NOT IN [trusted_ips]'