CVE-2021-21972

CVE-2021-21972 is a remote code execution vulnerability in a plugin of the vSphere Client in VMware vCenter Server, underpinned by a path traversal flaw (CWE-23) in the plugin's file-upload functionality. It is triggered over the network through port 443: an attacker who can reach the vCenter web interface can submit crafted file-upload requests, traverse to arbitrary filesystem paths, and plant executable files on the underlying operating system. Successful exploitation yields unrestricted privileges on the vCenter host OS, giving attackers control of the central management platform for an organization's entire VMware vSphere virtualized estate. Any organization running an affected vCenter Server release whose port 443 is reachable from untrusted networks is exposed. The flaw is listed in CISA's Known Exploited Vulnerabilities catalog (added 2021-11-03) with known ransomware use and a 99.9% EPSS score (100th percentile), indicating active, widespread exploitation in the wild, though no public PoC is recorded in this data.

What to do: Apply the vCenter Server updates published in VMware's advisory for CVE-2021-21972 as soon as possible, prioritizing internet-facing or partner-reachable vCenter instances. Until patched, restrict access to vCenter on port 443 to trusted management networks and review access logs for unauthenticated file-upload activity against the vSphere Client upload endpoint. Because ransomware operators have actively exploited this bug, hunt for signs of compromise such as webshells or unexpected new local accounts on vCenter appliances.