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CVE-2021-21985: VMware vCenter Server Improper Input Validation Vulnerability

100
Patch now
CRITICAL 9.8EXPLOITEDREMOTEDOWNGRADEUPGRADE

CVE-2021-21985 is a critical-severity flaw (CVSS 9.8) affecting vcenter server, cloud foundation, reported as exploited in the wild. Below: why it ranks where it does, which versions are affected, what to do, and the exact commands.

The vSphere Client (HTML5) contains a remote code execution vulnerability due to lack of input validation in the Virtual SAN Health Check plug-in which is enabled by default in vCenter Server. A malicious actor with network access to port 443 may exploit this issue to execute commands with unrestricted privileges on the underlying operating system that hosts vCenter Server.

Why this priority

  • Critical severity
  • EPSS top 1%: very likely to be exploited
  • Exploited in the wild (CISA KEV)
  • Used in ransomware campaigns
  • Public exploit reference
  • CISA SSVC: automatable with total technical impact
  • Remotely exploitable, no login, no user action

A fixed release or distro patch exists.

Risk facts

CVSS score9.8 (Critical)
Attack vectorNetwork (remote)
Attack complexityLow
Privileges neededNone
User interactionNone
EPSS exploit chance100.0% (top 1%)
CISA exploited listYes, fix due 2021-11-17, used in ransomware
WeaknessCWE-918, CWE-20, CWE-470
Published2021-05-26
Last modified2026-08-12
Publisher (CNA): VMware · Vendor: vmware

Share: LinkedIn · X · Reddit

Last updated 2026-08-12 · Sources: CVE Program, NVD, CISA KEV, EPSS and vendor advisories · How we score

Am I affected?

Timeline

vmware vcenter server DOWNGRADE
Affected: 6.5 · 6.5 a · 6.5 b · 6.5 c · 6.5 d · 6.5 e · 6.5 f · 6.5 update1
Fixed in: no fixed release known
Safe to downgrade to: a release before 6.5
1. Check installed version
apt-cache policy vcenter-server || rpm -q vcenter-server || vcenter-server --version
2. No fixed release: DOWNGRADE to a release before 6.5 (Debian / Ubuntu)
apt-cache madison vcenter-server sudo apt install vcenter-server=<VERSION_BEFORE_6.5> sudo apt-mark hold vcenter-server
2. No fixed release: DOWNGRADE (RHEL / Amazon Linux / Fedora)
dnf --showduplicates list vcenter-server sudo dnf downgrade vcenter-server-<VERSION_BEFORE_6.5> sudo dnf versionlock add vcenter-server
Downgrade risk
Downgrading can break dependent software and may reopen older bugs. Read the distro advisory first and test on a non-production host.
After downgrade
Restart the service, and keep the package held until a fixed release ships.
vmware cloud foundation UPGRADE
Affected: >= 3.0 and < 3.10.2.1 · >= 4.0 and < 4.2.1
Fixed in: 3.10.2.1, 4.2.1
Safe to downgrade to: a release before 3.0
1. Check installed version
apt-cache policy cloud-foundation || rpm -q cloud-foundation || cloud-foundation --version
2. UPGRADE to 3.10.2.1 (Debian / Ubuntu)
sudo apt update && sudo apt install --only-upgrade cloud-foundation
2. UPGRADE (RHEL / Amazon Linux / Fedora)
sudo dnf upgrade cloud-foundation
Note
Distros often backport fixes without changing the version number. Confirm with the distro advisory or changelog.
3. If no update is available yet, DOWNGRADE to a release before 3.0 (Debian / Ubuntu)
apt-cache madison cloud-foundation sudo apt install cloud-foundation=<VERSION_BEFORE_3.0> sudo apt-mark hold cloud-foundation
3. If no update is available yet, DOWNGRADE (RHEL / Amazon Linux / Fedora)
dnf --showduplicates list cloud-foundation sudo dnf downgrade cloud-foundation-<VERSION_BEFORE_3.0> sudo dnf versionlock add cloud-foundation
Downgrade risk
Downgrading can break dependent software and may reopen older bugs. Read the distro advisory first and test on a non-production host.
After downgrade
Restart the service, and keep the package held until a fixed release ships.

Exact fixes by distribution

A distro patch exists: prefer upgrading over downgrading.
Fedora 46 · bitcoin-core-selinux · FIXED · 0.3-2.fc46 · FEDORA-2026-51fc806272 · source
sudo dnf upgrade --advisory=FEDORA-2026-51fc806272
Fedora ELN · guestfs-tools · FIXED · 1.57.5-1.eln159 · FEDORA-2026-9577c5ec16 · source
sudo dnf upgrade --advisory=FEDORA-2026-9577c5ec16

Public exploits and detection

Public code exists for this CVE. Treat unpatched systems as at risk, and only run PoCs in a lab.

Verify at the official source NVD · CVE Program · CISA KEV catalog · Report a problem with this page
Scores and fixes here are computed automatically from those and other public sources. Confirm with the vendor advisory before changing production systems.

Quick answers

How do I fix CVE-2021-21985?

vcenter server: downgrade or apply the listed mitigation; cloud foundation: upgrade to 3.10.2.1, 4.2.1. Exact commands for each OS are on this page.

Is CVE-2021-21985 being exploited?

Yes. CVE-2021-21985 is on the CISA Known Exploited Vulnerabilities list, so treat it as patch-now.

What does CVE-2021-21985 affect?

CVE-2021-21985 affects vcenter server, cloud foundation. Severity is critical (CVSS 9.8). The vSphere Client (HTML5) contains a remote code execution vulnerability due to lack of input validation in the Virtual SAN Health Check plug-in which is enabled by default in vCenter Server. A malicious actor with network access to port 4

How urgent is CVE-2021-21985?

Priority 100/100. Critical severity; EPSS top 1%: very likely to be exploited; Exploited in the wild (CISA KEV); Used in ransomware campaigns; Public exploit reference; CISA SSVC: automatable with total technical impact; Remotely exploitable, no login, no user action

Software pages

VMware vulnerabilities

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References

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