CVE-2021-21986: VMware vCenter Server Auth Bypass Flaw

CVE-2021-21986 Overview

CVE-2021-21986 is an authentication bypass vulnerability affecting the vSphere Client (HTML5) interface in VMware vCenter Server. The vulnerability exists in the authentication mechanism used by several vSphere plug-ins, including Virtual SAN Health Check, Site Recovery, vSphere Lifecycle Manager, and VMware Cloud Director Availability. A malicious actor with network access to port 443 on a vulnerable vCenter Server can perform actions allowed by these impacted plug-ins without requiring any authentication.

Critical Impact

Unauthenticated remote attackers can execute privileged plug-in operations on vCenter Server by exploiting the flawed authentication mechanism, potentially compromising the entire virtualized infrastructure.

Affected Products

• VMware vCenter Server 6.5 (all versions through Update 3n)
• VMware vCenter Server 6.7 (all versions through Update 3m)
• VMware vCenter Server 7.0 (all versions through Update 2a)
• VMware Cloud Foundation 3.x and 4.x

Discovery Timeline

• May 26, 2021 - CVE-2021-21986 published to NVD
• November 21, 2024 - Last updated in NVD database

Technical Details for CVE-2021-21986

Vulnerability Analysis

This vulnerability is classified as CWE-306 (Missing Authentication for Critical Function). The vSphere Client HTML5 interface implements a plug-in architecture that extends the functionality of vCenter Server. The affected plug-ins—Virtual SAN Health Check, Site Recovery, vSphere Lifecycle Manager, and VMware Cloud Director Availability—contain a flaw in their authentication validation logic.

When requests are made to the endpoints exposed by these plug-ins, the authentication mechanism fails to properly verify that the requesting user has been authenticated. This allows an unauthenticated attacker with network access to port 443 to interact directly with these plug-in interfaces and execute operations that should require valid credentials.

The impact is severe because these plug-ins have access to critical infrastructure management functions. The Virtual SAN Health Check plug-in can access storage health data and diagnostics. The vSphere Lifecycle Manager can manage updates and patches across the infrastructure. Site Recovery and Cloud Director Availability plug-ins have access to disaster recovery and cloud migration capabilities.

Root Cause

The root cause of CVE-2021-21986 is a missing authentication check in the vSphere Client's authentication mechanism when processing requests to the affected plug-ins. The vSphere Client fails to enforce proper session validation for requests destined to these specific plug-in endpoints, allowing unauthenticated access to critical functionality that should be protected.

Attack Vector

The attack vector for this vulnerability is network-based. An attacker requires only network connectivity to port 443 (HTTPS) on the target vCenter Server. No prior authentication, user interaction, or special privileges are required to exploit this vulnerability.

The attack flow involves:

1. Attacker identifies an exposed vCenter Server on the network
2. Attacker sends crafted HTTP requests to the vulnerable plug-in endpoints on port 443
3. The vSphere Client processes the requests without validating authentication
4. Attacker gains unauthorized access to perform plug-in operations

Since no verified proof-of-concept code is available in public repositories, organizations should refer to the VMware Security Advisory VMSA-2021-0010 for detailed technical information about the vulnerability mechanism and attack surface.

Detection Methods for CVE-2021-21986

Indicators of Compromise

• Unexpected or anomalous HTTP requests to vSphere Client plug-in endpoints without corresponding authentication events
• Access logs showing requests to Virtual SAN Health Check, Site Recovery, vSphere Lifecycle Manager, or Cloud Director Availability plug-in paths from unusual source IPs
• Unauthorized configuration changes to vSAN, lifecycle management, or disaster recovery settings
• Audit logs indicating plug-in operations without associated user sessions

Detection Strategies

• Implement network monitoring to detect unusual traffic patterns to vCenter Server port 443, particularly focusing on requests to plug-in-specific API endpoints
• Enable verbose logging on vCenter Server and review for authentication anomalies or requests processed without valid session tokens
• Deploy intrusion detection system (IDS) rules to identify patterns consistent with authentication bypass attempts against vSphere Client
• Correlate vCenter Server access logs with authentication logs to identify requests that bypass the normal authentication flow

Monitoring Recommendations

• Configure alerting for any access to critical plug-in functionality from untrusted network segments
• Monitor for changes to vSAN health configurations, lifecycle management policies, or site recovery settings that do not correlate with authorized administrative activity
• Implement network segmentation and monitor traffic crossing boundaries to vCenter Server management interfaces
• Review vCenter Server audit logs regularly for evidence of unauthorized plug-in access

How to Mitigate CVE-2021-21986

Immediate Actions Required

• Apply the security patches provided by VMware as documented in VMSA-2021-0010 immediately
• Restrict network access to vCenter Server port 443 to only trusted management networks and administrative workstations
• If patching is not immediately possible, consider disabling the affected plug-ins (Virtual SAN Health Check, Site Recovery, vSphere Lifecycle Manager, VMware Cloud Director Availability) until patches can be applied
• Conduct a security review of vCenter Server access logs to identify any potential prior exploitation

Patch Information

VMware has released patches addressing this vulnerability as part of security advisory VMSA-2021-0010. Organizations should update to the following minimum versions:

• vCenter Server 6.5 U3p or later
• vCenter Server 6.7 U3n or later
• vCenter Server 7.0 U2b or later

For VMware Cloud Foundation deployments, refer to the advisory for specific update bundles that include the fix.

Workarounds

• Disable the affected plug-ins if they are not required for operations until patches can be applied
• Implement strict firewall rules to limit access to vCenter Server port 443 from untrusted networks
• Use network segmentation to isolate vCenter Server management interfaces from general user networks
• Deploy a reverse proxy or web application firewall (WAF) to filter and monitor requests to vCenter Server endpoints

# Example: Restrict vCenter access using iptables (Linux-based appliance)
# Allow only from trusted management subnet
iptables -A INPUT -p tcp --dport 443 -s 10.0.1.0/24 -j ACCEPT
iptables -A INPUT -p tcp --dport 443 -j DROP