CVE-2021-21985

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.

What to 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.