CVE-2021-21986: 
vSphere vCenter Server vulnerability analysis and mitigation

CVE-2021-21986 is a vulnerability discovered in VMware vCenter Server's vSphere Client (HTML5) that affects the authentication mechanism for multiple plugins including Virtual SAN Health Check, Site Recovery, vSphere Lifecycle Manager, and VMware Cloud Director Availability. The vulnerability was disclosed on May 25, 2021, affecting vCenter Server versions 6.5, 6.7, and 7.0, as well as Cloud Foundation versions 3.x and 4.x (VMware Advisory).

The vulnerability stems from an authentication mechanism flaw in the vSphere Client (HTML5) that affects multiple server plug-ins. It has been assigned a CVSS v3.1 base score of 6.5 (Moderate severity), indicating a significant but not critical security risk. The vulnerability specifically impacts the authentication mechanisms of several plugins including Virtual SAN Health Check, Site Recovery, vSphere Lifecycle Manager, and VMware Cloud Director Availability (VMware Advisory).

When exploited, this vulnerability allows a malicious actor with network access to port 443 on vCenter Server to perform actions allowed by the impacted plug-ins without proper authentication. This means an attacker could potentially execute unauthorized operations through the affected plugins without needing valid credentials (VMware Advisory, Hacker News).

VMware released patches to address this vulnerability in the following versions: vCenter Server 7.0 U2b, 6.7 U3n, and 6.5 U3p. For Cloud Foundation, patches were released in versions 4.2.1 and 3.10.2.1. If immediate patching is not possible, VMware provided workarounds that involve disabling the affected plug-ins, though this results in a loss of management and monitoring capabilities (VMware Advisory).

Security researchers highlighted the severity of the vulnerability, particularly due to the large number of exposed vCenter Servers. A study by Trustwave revealed that as of May 31, 2021, approximately 4,019 (80.88%) of publicly accessible vCenter Servers remained vulnerable to this and related vulnerabilities (Trustwave).