CVE-2021-21980: VMware Cloud Foundation Info Disclosure

CVE-2021-21980 Overview

CVE-2021-21980 is an unauthorized arbitrary file read vulnerability affecting the vSphere Web Client (FLEX/Flash) component of VMware vCenter Server and VMware Cloud Foundation. A malicious actor with network access to port 443 on vCenter Server may exploit this issue to gain access to sensitive information stored on the server.

This vulnerability poses a significant risk to organizations running VMware virtualization infrastructure, as vCenter Server is a critical management component. Successful exploitation could allow attackers to read configuration files, credentials, certificates, and other sensitive data without requiring authentication.

Critical Impact

Unauthenticated attackers with network access to vCenter Server port 443 can read arbitrary files, potentially exposing sensitive configuration data, credentials, and encryption keys used in the virtualization environment.

Affected Products

• VMware vCenter Server 6.5 (all updates through 6.5 Update 3q)
• VMware vCenter Server 6.7 (all updates through 6.7 Update 3o)
• VMware Cloud Foundation 3.x

Discovery Timeline

• November 24, 2021 - CVE-2021-21980 published to NVD
• November 21, 2024 - Last updated in NVD database

Technical Details for CVE-2021-21980

Vulnerability Analysis

The vulnerability exists within the vSphere Web Client, specifically the legacy FLEX/Flash-based interface that VMware has been phasing out in favor of the HTML5-based vSphere Client. The FLEX client contains an unauthorized arbitrary file read flaw that can be exploited remotely.

The attack does not require any privileges or user interaction, making it particularly dangerous in environments where vCenter Server is exposed to untrusted networks. An attacker only needs network connectivity to port 443 on the vCenter Server to attempt exploitation. The vulnerability exclusively impacts confidentiality, allowing unauthorized read access to files on the vCenter Server appliance or Windows host.

This type of arbitrary file read vulnerability can be leveraged to extract critical information including database credentials, LDAP/AD integration configurations, SSL/TLS private keys, VMware configuration files, and potentially cached credentials or session tokens.

Root Cause

The root cause of CVE-2021-21980 is improper access control within the vSphere Web Client (FLEX/Flash) component. The vulnerable component fails to properly validate and authorize file read requests, allowing unauthenticated users to access files that should be restricted. This represents a breakdown in the security boundary between the web interface and the underlying file system.

The legacy FLEX/Flash client architecture may have contributed to this vulnerability, as older web technologies often lack modern security controls and input validation mechanisms that would prevent such file access issues.

Attack Vector

The attack vector for CVE-2021-21980 is network-based, requiring the attacker to have TCP/IP connectivity to port 443 on the vulnerable vCenter Server. The exploitation path involves sending specially crafted requests to the vSphere Web Client (FLEX/Flash) endpoint that bypasses authorization checks.

Since the vulnerability requires no authentication or user interaction, it can be exploited opportunistically by any attacker who can reach the vCenter Server over the network. This makes internet-exposed vCenter Server instances particularly vulnerable to attack.

The attack workflow typically involves:

1. Network reconnaissance to identify vCenter Server instances
2. Crafting HTTP/HTTPS requests to the vulnerable FLEX client endpoint
3. Specifying target files to read through path manipulation
4. Receiving file contents in the server response

Due to the sensitive nature of this vulnerability and the lack of verified proof-of-concept code in public repositories, specific exploitation details are not provided here. Organizations should refer to the VMware Security Advisory VMSA-2021-0027 for technical details.

Detection Methods for CVE-2021-21980

Indicators of Compromise

• Unusual HTTP/HTTPS requests to the vSphere Web Client FLEX endpoints on port 443
• Access log entries showing requests for sensitive file paths such as /etc/passwd, configuration files, or VMware credential stores
• Network traffic from unexpected sources targeting vCenter Server management interfaces
• Evidence of data exfiltration containing vCenter configuration or credential information

Detection Strategies

• Monitor vCenter Server access logs for anomalous requests to FLEX client endpoints
• Implement network intrusion detection rules to identify file read exploitation attempts targeting vCenter
• Deploy web application firewall rules to detect and block path traversal patterns in requests
• Analyze outbound network traffic for large data transfers from vCenter Server to unusual destinations

Monitoring Recommendations

• Enable verbose logging on vCenter Server to capture detailed request information
• Configure SIEM alerts for suspicious access patterns to vCenter management interfaces
• Monitor network flows to/from vCenter Server for connections from unauthorized IP ranges
• Regularly review vCenter Server access logs for evidence of exploitation attempts

How to Mitigate CVE-2021-21980

Immediate Actions Required

• Apply the VMware security patches referenced in VMSA-2021-0027 immediately
• Restrict network access to vCenter Server port 443 to only authorized management networks
• Ensure vCenter Server is not directly exposed to the internet
• Migrate from the legacy FLEX/Flash client to the HTML5-based vSphere Client

Patch Information

VMware has released security updates to address this vulnerability. Administrators should apply the patches specified in VMware Security Advisory VMSA-2021-0027. The advisory provides version-specific remediation guidance for affected vCenter Server and Cloud Foundation deployments.

For vCenter Server 6.5 and 6.7 environments, upgrade to the latest patched versions as specified in the advisory. Cloud Foundation 3.x customers should apply the appropriate async patch bundle.

Workarounds

• Disable the FLEX/Flash-based vSphere Web Client if not required for operations and use only the HTML5 vSphere Client
• Implement strict firewall rules limiting access to vCenter Server management ports to authorized administrator workstations only
• Deploy a reverse proxy or web application firewall in front of vCenter Server to filter malicious requests
• Enable enhanced monitoring and logging to detect exploitation attempts while awaiting patching

# Example firewall rule to restrict vCenter access (iptables)
# Allow access only from trusted management subnet
iptables -A INPUT -p tcp --dport 443 -s 10.0.10.0/24 -j ACCEPT
iptables -A INPUT -p tcp --dport 443 -j DROP

# Verify vCenter Server version to determine patch status
# Connect to VCSA via SSH and run:
# cat /etc/vmware/version