CVE-2021-21973: VMware Cloud Foundation SSRF Vulnerability

CVE-2021-21973 Overview

CVE-2021-21973 is a Server-Side Request Forgery (SSRF) vulnerability affecting the vSphere Client (HTML5) due to improper validation of URLs in a vCenter Server plugin. A malicious actor with network access to port 443 may exploit this issue by sending a POST request to the vCenter Server plugin, potentially leading to information disclosure from internal network resources.

Critical Impact

This vulnerability is listed in CISA's Known Exploited Vulnerabilities (KEV) catalog, indicating active exploitation in the wild. Attackers can leverage this SSRF flaw to access internal services, probe internal networks, and potentially exfiltrate sensitive information from systems not directly accessible from the internet.

Affected Products

• VMware vCenter Server 7.x before 7.0 U1c
• VMware vCenter Server 6.7 before 6.7 U3l
• VMware vCenter Server 6.5 before 6.5 U3n
• VMware Cloud Foundation 4.x before 4.2
• VMware Cloud Foundation 3.x before 3.10.1.2

Discovery Timeline

• 2021-02-24 - CVE-2021-21973 published to NVD
• 2025-10-30 - Last updated in NVD database

Technical Details for CVE-2021-21973

Vulnerability Analysis

This SSRF vulnerability exists within the vSphere Client's HTML5 interface, specifically in how a vCenter Server plugin handles URL validation. The vulnerability stems from insufficient input validation when processing user-supplied URLs, allowing attackers to craft malicious requests that cause the server to make unintended HTTP requests to arbitrary destinations.

When exploited, the vCenter Server acts as a proxy, making requests on behalf of the attacker. This can expose internal services that would otherwise be inaccessible from external networks. The vulnerability requires network access to port 443 (HTTPS) on the vCenter Server, which is commonly exposed in enterprise environments for legitimate management purposes.

The information disclosure impact allows attackers to:

• Probe internal network infrastructure
• Access metadata services in cloud environments
• Retrieve sensitive configuration data from internal APIs
• Map internal network topology

Root Cause

The root cause is classified as CWE-918 (Server-Side Request Forgery). The vCenter Server plugin fails to properly validate and sanitize URLs provided in POST requests before using them to make server-side HTTP requests. This lack of validation allows attackers to specify arbitrary URLs, including internal IP addresses and hostnames that should not be accessible from external networks.

Attack Vector

The attack is network-based and requires no authentication or user interaction. An attacker with network access to port 443 on the vCenter Server can send specially crafted POST requests to the vulnerable plugin endpoint. The server then processes these requests and makes HTTP requests to the attacker-specified URLs, returning the responses or error information that can be used for reconnaissance.

The attack flow involves:

1. Attacker identifies an exposed vCenter Server instance
2. Attacker crafts a malicious POST request with an internal URL as the target
3. The vCenter Server processes the request and fetches content from the internal URL
4. Response data or error messages reveal information about the internal resource

Detection Methods for CVE-2021-21973

Indicators of Compromise

• Unusual outbound HTTP/HTTPS requests originating from vCenter Server to internal IP ranges
• POST requests to vCenter Server containing internal IP addresses or hostnames in request parameters
• Abnormal access patterns to internal metadata services (e.g., 169.254.169.254 in cloud environments)
• Error logs showing connection attempts to unexpected internal endpoints

Detection Strategies

• Monitor vCenter Server access logs for POST requests containing suspicious URL parameters pointing to internal resources
• Implement network-level monitoring to detect vCenter Server initiating connections to internal services it normally wouldn't access
• Configure SIEM rules to alert on requests containing RFC 1918 private IP addresses in URL parameters
• Review vCenter Server audit logs for unusual plugin activity patterns

Monitoring Recommendations

• Enable verbose logging on vCenter Server to capture detailed request information
• Deploy network traffic analysis tools to monitor egress traffic from vCenter Server instances
• Implement security monitoring for access to sensitive internal services from vCenter Server hosts
• Configure alerts for any vCenter Server connections to cloud metadata endpoints

How to Mitigate CVE-2021-21973

Immediate Actions Required

• Apply VMware security patches immediately to all affected vCenter Server and Cloud Foundation installations
• Restrict network access to vCenter Server port 443 to only authorized management networks
• Implement network segmentation to limit the impact of potential SSRF exploitation
• Review firewall rules to ensure vCenter Server cannot reach sensitive internal services unnecessarily

Patch Information

VMware has released security patches addressing this vulnerability. Organizations should update to the following versions or later:

• vCenter Server 7.0 U1c or later for 7.x installations
• vCenter Server 6.7 U3l or later for 6.7 installations
• vCenter Server 6.5 U3n or later for 6.5 installations
• Cloud Foundation 4.2 or later for 4.x installations
• Cloud Foundation 3.10.1.2 or later for 3.x installations

Refer to the VMware Security Advisory VMSA-2021-0002 for complete patch details and download links.

Workarounds

• Implement network-level access controls to restrict which IP addresses can reach vCenter Server on port 443
• Deploy a web application firewall (WAF) to inspect and filter malicious requests targeting the vulnerable plugin
• Use network segmentation to isolate vCenter Server from sensitive internal resources
• Configure egress filtering on vCenter Server hosts to block outbound connections to unnecessary internal services

# Example: Restrict vCenter Server access using iptables
# Allow only specific management subnet to access vCenter
iptables -A INPUT -p tcp --dport 443 -s 10.10.10.0/24 -j ACCEPT
iptables -A INPUT -p tcp --dport 443 -j DROP

# Block vCenter from accessing internal metadata services
iptables -A OUTPUT -d 169.254.169.254 -j DROP