ALERT: Critical RCE Bug in VMware vCenter Server Under Active Attack

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.

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.

CVE-2021-21985 is an improper input validation flaw (CWE-20, with related unsafe reflection CWE-470 and SSRF CWE-918 classifications) in the Virtual SAN Health Check plug-in of the VMware vSphere Client, which is enabled by default in vCenter Server. It is triggered by crafted requests sent to the plug-in over the network; VMware indicated that network access to vCenter's HTTPS port (443) is sufficient to reach the vulnerable component. A successful attacker gains remote code execution with unrestricted privileges on the underlying operating system hosting vCenter Server, a highly privileged position in the virtualization stack. Any organization running an affected VMware vCenter Server is affected; because vCenter is the default management plane for vSphere, this spans a very large share of enterprise virtualization estates, with tens of thousands of instances directly exposed to the internet. Exploitation is confirmed in the wild: CISA added the flaw to the Known Exploited Vulnerabilities catalog on 2021-11-03 with known ransomware use and requires applying vendor updates, and EPSS puts the 30-day exploitation probability at essentially 100%, though the source data lists no public PoC.

Do: Update vCenter Server per VMware's instructions; fixes shipped in May 2021 for the 6.5, 6.7, and 7.0 branches (e.g., 6.5 U3n, 6.7 U3o, and 7.0 U2c — verify your current build against the vendor advisory). Until patched, restrict access to vCenter's HTTPS (443) interface to trusted management networks rather than the open internet, and review appliance logs and running processes for indicators of exploitation, since ransomware operators are known to use this flaw after gaining network access.