VMware vCenter Server Improper Input Validation Vulnerability

Exploited · KEV 2021-11-03RansomwareMetasploit moduleCVE-2021-21985CWE-20CWE-470

Exploited since 2021-11-03. No WAF vendor names it yet. Our CRS test: not blocked.

Edge rules

0/8

Our test

Not blocked

CVSS

9.8

EPSS

100.0% Top 0.1%

Posts, 14d

Published

2021-05-26

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.

Edge coverage

Claimed by vendors

CloudflareBefore feed historyOlder than its feedWe measured: passed AWSNo public claimNot namedWe measured: passed Microsoft AzureNo public claimNot named Google CloudBefore feed historyOlder than its feedWe measured: passed FastlyBefore feed historyOlder than its feed F5Before feed historyOlder than its feed AkamaiNo public sourceNo public changelog OWASP CRSBefore feed historyOlder than its feed

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Ask about CVE-2021-21985: who's covered, what to patch first, how vendors compare. I cite sources and won't help with attacks.

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Severity, decoded

CVSS 3.1

9.8Critical

Critical · 9.8 / 10

Exploitable over the network, with no privileges and no user interaction, leading to full loss of confidentiality, integrity and availability.

Over the internetNo login neededNo user actionWorks reliablyFull compromise

How the score is built8 metrics

Attack vectorAn attacker can reach it over the internet; no access to your network needed.
Network
Attack complexityNo special conditions are needed; the attack works reliably.
Low
Privileges requiredNo account or login is needed.
None
User interactionNo one has to click or open anything; the attack works on its own.
None
ScopeThe damage stays within the vulnerable component.
Unchanged
Confidentiality impactAn attacker can read all data the affected component holds, or the most sensitive parts of it.
High
Integrity impactAn attacker can change any data or files the affected component protects.
High
Availability impactAn attacker can take the component down completely or keep it from serving anyone.
High

CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H

CISA's assessmentSSVC, from the CVE record

ExploitationActive

Attackers are exploiting this in the wild.

AutomatableYes

Attackers can script the attack end to end and hit many targets at once.

Technical impactTotal

A successful attack gives full control over the affected software or all of its data.

Public exploits

2 found

1 Metasploit0 Exploit-DB1 Scanner template0 GitHub PoCs

MetasploitVMware vCenter Server Virtual SAN Health Check Plugin RCE2021-05-25 · exploit · rank Excellent · has check
NucleiCVE-2021-21985.yamldetection template, replayed in our lab

Links are to third-party code: never run it outside an isolated lab.

Timeline

2021-05-25
Metasploit module published
VMware vCenter Server Virtual SAN Health Check Plugin RCE
2021-05-26
CVE published
CVE List
2021-11-03
Added to CISA KEV
Confirmed exploitation in the wild
2026-11-02
Our test: OWASP Core Rule Set let it through
paranoia level 1, anomaly threshold 5, default CRS settings
2026-11-02
Our test: OWASP Core Rule Set let it through
paranoia level 2, anomaly threshold 5, default CRS settings