CVE-2021-21986 in vCenter Server
Summary
by MITRE • 05/26/2021
The vSphere Client (HTML5) contains a vulnerability in a vSphere authentication mechanism for the Virtual SAN Health Check, Site Recovery, vSphere Lifecycle Manager, and VMware Cloud Director Availability plug-ins. A malicious actor with network access to port 443 on vCenter Server may perform actions allowed by the impacted plug-ins without authentication.
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Analysis
by VulDB Data Team • 05/29/2021
The vulnerability identified as CVE-2021-21986 represents a critical authentication bypass flaw within VMware's vSphere Client ecosystem, specifically targeting the HTML5 interface. This vulnerability affects multiple enterprise-grade plug-ins including Virtual SAN Health Check, Site Recovery, vSphere Lifecycle Manager, and VMware Cloud Director Availability, creating a significant attack surface for malicious actors. The flaw exists within the authentication mechanism that governs access to these specialized vSphere components, undermining the fundamental security posture of VMware's virtualization infrastructure. The vulnerability is particularly concerning because it allows unauthenticated access to privileged operations through a well-known and widely used port, specifically port 443 which is the standard HTTPS port for secure web communications. This exposure means that any attacker with network access to the vCenter Server can exploit this vulnerability without requiring valid credentials, effectively bypassing the authentication layer that should protect sensitive administrative functions.
The technical implementation of this vulnerability stems from improper access controls within the vSphere Client's authentication framework. The flaw enables an attacker to execute unauthorized operations within the impacted plug-ins, potentially allowing for full administrative control over virtualized environments. This authentication bypass occurs at the application layer where the system fails to properly validate user credentials before granting access to sensitive functionalities. The vulnerability is classified under CWE-287 which addresses improper authentication issues in software systems, specifically targeting the lack of proper authorization checks. From a threat modeling perspective, this vulnerability aligns with ATT&CK technique T1078 which covers valid accounts and legitimate credentials as a means of gaining access to systems, though in this case the flaw allows bypassing the credential requirement entirely rather than simply using stolen credentials.
The operational impact of CVE-2021-21986 extends far beyond simple unauthorized access, as it can lead to complete compromise of virtualized environments managed by vCenter Server. Attackers can leverage this vulnerability to perform operations such as modifying virtual machine configurations, accessing sensitive data, manipulating backup and recovery processes, and potentially escalating privileges within the broader network infrastructure. The affected plug-ins are integral to enterprise virtualization management, making this vulnerability particularly dangerous for organizations that rely heavily on VMware's vSphere platform for their data center operations. Organizations running affected versions of vCenter Server are at risk of data breaches, service disruption, and potential lateral movement within their networks, as the compromised system could serve as a launching point for further attacks. The vulnerability's exposure through port 443 makes it particularly attractive to attackers who can exploit it remotely without requiring physical access or prior authentication credentials.
Mitigation strategies for CVE-2021-21986 should prioritize immediate patching of affected vCenter Server installations, as VMware has released security updates to address this vulnerability. Organizations should also implement network segmentation to limit access to port 443 on vCenter Server, particularly restricting access to only trusted administrative networks. Additional defensive measures include monitoring for unusual activity on port 443, implementing network access controls, and conducting thorough vulnerability assessments of their VMware environments. Security teams should also consider disabling unnecessary plug-ins and features until patches are applied, following the principle of least privilege. The vulnerability highlights the importance of proper access control implementation and the need for regular security assessments of enterprise infrastructure components. Organizations should also review their incident response procedures to ensure readiness for potential exploitation of this type of authentication bypass vulnerability, as the impact can be severe and far-reaching across virtualized environments.