CVE-2020-3976: VMware Cloud Foundation DoS Vulnerability

CVE-2020-3976 Overview

CVE-2020-3976 is a partial denial of service vulnerability affecting the authentication services in VMware ESXi and vCenter Server. This vulnerability allows an unauthenticated attacker with network access to disrupt authentication services, potentially impacting the availability of virtualization infrastructure management capabilities. VMware has evaluated this as a moderate severity issue that could affect enterprise environments running vulnerable versions of these critical virtualization platforms.

Critical Impact

Exploitation of this vulnerability can cause partial denial of service in VMware authentication services, potentially disrupting administrative access and management operations for virtualized infrastructure.

Affected Products

• VMware ESXi versions 6.5, 6.7, and 7.0
• VMware vCenter Server versions 6.5, 6.7, and 7.0
• VMware Cloud Foundation (multiple versions)

Discovery Timeline

• August 21, 2020 - CVE-2020-3976 published to NVD
• November 21, 2024 - Last updated in NVD database

Technical Details for CVE-2020-3976

Vulnerability Analysis

This vulnerability exists within the authentication services of VMware ESXi and vCenter Server. The flaw is classified under CWE-400 (Uncontrolled Resource Consumption), indicating that the authentication service fails to properly limit resource consumption when processing authentication requests. An attacker can exploit this weakness to exhaust system resources dedicated to authentication processing, resulting in a partial denial of service condition.

The vulnerability is network-accessible, requiring no authentication or user interaction to exploit. However, the impact is limited to availability, with no direct effect on confidentiality or integrity of the affected systems. The partial nature of the denial of service means that while authentication services may be degraded, complete system unavailability is not expected.

Root Cause

The root cause of CVE-2020-3976 is improper resource consumption control (CWE-400) in the authentication service implementation. The service lacks adequate rate limiting or resource management controls when handling incoming authentication requests. This allows an attacker to send a high volume of requests that consume processing resources, memory, or connection pools beyond sustainable levels, leading to service degradation.

Attack Vector

The attack vector for this vulnerability is network-based, allowing remote exploitation without any form of authentication. An attacker positioned on the network with access to the authentication service endpoints can launch the attack by:

1. Identifying vulnerable VMware ESXi or vCenter Server instances exposed on the network
2. Sending crafted or high-volume authentication requests to the authentication service
3. Overwhelming the service's ability to process legitimate requests
4. Causing degradation or partial unavailability of authentication functionality

This attack does not require any privileges on the target system and can be executed without user interaction, making it relatively straightforward to exploit from a network perspective.

Detection Methods for CVE-2020-3976

Indicators of Compromise

• Unusual spikes in authentication request volume from single or multiple source IP addresses
• Authentication service performance degradation or increased response times
• Log entries indicating resource exhaustion or connection limit warnings in VMware authentication services
• Failed authentication attempts correlating with service availability issues

Detection Strategies

• Monitor VMware ESXi and vCenter Server logs for anomalous authentication patterns and resource exhaustion indicators
• Implement network traffic analysis to detect high-volume request patterns targeting authentication endpoints
• Configure SIEM rules to alert on authentication service degradation correlated with unusual network activity
• Deploy intrusion detection signatures for known denial of service attack patterns against VMware services

Monitoring Recommendations

• Enable detailed logging for authentication services on ESXi hosts and vCenter Server
• Monitor system resource utilization (CPU, memory, network connections) on management interfaces
• Implement baseline alerting for authentication request volumes to detect anomalous spikes
• Review VMware service health metrics through vCenter for early warning signs of resource exhaustion

How to Mitigate CVE-2020-3976

Immediate Actions Required

• Apply VMware security patches as outlined in VMware Security Advisory VMSA-2020-0018
• Restrict network access to management interfaces using firewall rules to limit exposure
• Implement rate limiting on network devices for traffic destined to VMware management services
• Monitor authentication services for signs of exploitation while patch deployment is in progress

Patch Information

VMware has released patches to address this vulnerability across all affected product versions. Organizations should consult VMware Security Advisory VMSA-2020-0018 for specific patch versions and update procedures for ESXi, vCenter Server, and Cloud Foundation deployments. The advisory provides detailed guidance on obtaining and applying the appropriate updates for each affected product version.

Workarounds

• Segment VMware management networks from general network traffic using VLANs and firewalls
• Implement network-level rate limiting for connections to ESXi and vCenter management interfaces
• Deploy a reverse proxy or load balancer with request throttling capabilities in front of vCenter Server
• Restrict management interface access to trusted IP ranges and administrative jump hosts only

# Example ESXi firewall configuration to restrict management access
# Run on ESXi host via SSH or DCUI

# List current firewall rules
esxcli network firewall ruleset list

# Enable the firewall if disabled
esxcli network firewall set --enabled true

# Restrict vSphere Client access to specific IP ranges
esxcli network firewall ruleset set --allowed-all false --ruleset-id vSphereClient
esxcli network firewall ruleset allowedip add --ip-address 10.0.0.0/24 --ruleset-id vSphereClient

# Apply similar restrictions to other management services
esxcli network firewall ruleset set --allowed-all false --ruleset-id webAccess
esxcli network firewall ruleset allowedip add --ip-address 10.0.0.0/24 --ruleset-id webAccess