CVERadar
CRITICAL · CVSS 3.1 9.8CISA KEV · added 2021-11-03Metasploit modulePublic PoCKnown ransomware useEPSS 100.00% (p100)Threat level 99

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

VMware · vCenter Server · NVD published 2021-05-26 · posted 2026-10-21 · last modified 2026-08-12

What it is

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.

WeaknessCWE-918CWE-20CWE-470
CVSS vectorCVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H (source: [email protected])
CISA due date2021-11-17 (for US federal agencies under BOD; a good deadline for everyone)
ATT&CK tacticsInitial Access → Execution → Command and Control

Exploitation evidence

First exploit evidence seen: 2021-05-25 (earliest date across the sources below).

Exploited in the wild

  • CISA KEV: listed since 2021-11-03. Source

Public exploit code

Metasploit

GitHub proof-of-concept repositories

10 public repositories from 10 accounts, first seen 2021-05-27. Most starred: alt3kx/CVE-2021-21985_PoC (214 stars), view repository.

Verify before running: public PoCs can be malicious. Read the code and only run it in an isolated lab. A PoC means exploit code is public, not that this CVE is exploited in the wild.

Sources: CISA KEV · Rapid7 Metasploit Framework module metadata · nomi-sec/PoC-in-GitHub.

Defend the Chain

The attack chain for CVE-2021-21985 laid across the MITRE ATT&CK Enterprise tactics. Solid cards come from the MITRE CTID KEV mappings; dashed cards are inferred by CVERadar from the CWE or description. Correct defences are real ATT&CK mitigations for that technique.

Threat level 99/100
Defender level 1 · 0 XP
Chain links broken: 0/3

Reconnaissance

Resource Development

Initial Access

Execution

Persistence

Privilege Escalation

Stealth

Defense Impairment

Credential Access

Discovery

Lateral Movement

Collection

Command and Control

Exfiltration

Impact

Pick a red technique card on the board, then choose the defence that stops it.

Defend against T1190 Exploit Public-Facing Application Inferred

Inferred from CWE CWE-918

How to fix it

CISA required action

Apply updates per vendor instructions. (source: CISA KEV)

Fixed versions

  • vmware cloud foundation: upgrade to 3.10.2.1 or later (affected from 3.0)
  • vmware cloud foundation: upgrade to 4.2.1 or later (affected from 4.0)

Derived from the NVD CPE configuration (versionEndExcluding / versionEndIncluding). Always confirm with the vendor advisory.

Vendor advisories and patches

MITRE ATT&CK mitigations for this chain

  • M1048 Application Isolation and Sandboxing (for T1190, T1203) : Application isolation will limit what other processes and system features the exploited target can access.
  • M1037 Filter Network Traffic (for T1190, T1090) : Restrict outbound network traffic from public-facing servers to prevent unauthorized connections from initiating communications with attacker-controlled infrastructure. While this may not prevent the initial exploitation, it limits the attacker's ability to verify and control the compromised server post-exploit, reduci
  • M1030 Network Segmentation (for T1190) : Segment externally facing servers and services from the rest of the network with a DMZ or on separate hosting infrastructure.
  • M1016 Vulnerability Scanning (for T1190) : Regularly scan externally facing systems for vulnerabilities and establish procedures to rapidly patch systems when critical vulnerabilities are discovered through scanning and through public disclosure.
  • M1026 Privileged Account Management (for T1190) : Use least privilege for service accounts will limit what permissions the exploited process gets on the rest of the system.
  • M1050 Exploit Protection (for T1190, T1203) : Web Application Firewalls may be used to limit exposure of applications to prevent exploit traffic from reaching the application.
  • M1035 Limit Access to Resource Over Network (for T1190) : Ensure that all publicly exposed services are actually intended to be so, and restrict access to any that should only be available internally.
  • M1051 Update Software (for T1190, T1203) : Update software regularly by employing patch management for externally exposed applications.
  • M1031 Network Intrusion Prevention (for T1090) : Network intrusion detection and prevention systems that use network signatures to identify traffic for specific adversary malware can be used to mitigate activity at the network level. Signatures are often for unique indicators within protocols and may be based on the specific C2 protocol used by a particular adversary
  • M1020 SSL/TLS Inspection (for T1090) : If it is possible to inspect HTTPS traffic, the captures can be analyzed for connections that appear to be domain fronting.

Detection Generic guidance

  • Initial Access: Alert on unusual requests to the affected service (odd paths, oversized payloads, exploit strings) in WAF, reverse-proxy and application logs.
  • Execution: Watch for the service process spawning shells or script interpreters (cmd.exe, powershell, sh, bash, python) via EDR process-tree rules.
  • Command and Control: Alert on outbound connections from the server to new or rare destinations.

Response checklist Generic guidance

  1. Inventory: find every instance of the affected product and version (include internet-facing and forgotten ones).
  2. Patch or mitigate per the vendor advisory; if you cannot patch, remove internet exposure or apply the vendor workaround.
  3. Assume compromise if the system was exposed before patching: hunt for the indicators in the advisory and review logs back to the disclosure date.
  4. Rotate credentials and secrets stored on or used by the affected system if compromise cannot be ruled out.

Sources