CVE-2020-11810: OpenVPN Denial of Service Vulnerability

CVE-2020-11810 Overview

A race condition vulnerability exists in OpenVPN 2.4.x before version 2.4.9 that allows an attacker to inject a data channel v2 (P_DATA_V2) packet using a victim's peer-id, resulting in denial of service. The vulnerability exploits a timing window during the VPN connection initialization phase when Negotiable Cipher Parameters (NCP) is in use.

Critical Impact

An attacker can drop victim VPN connections by exploiting the small timing window between client connection initiation and server PUSH_REPLY response, disrupting secure communications.

Affected Products

• OpenVPN 2.4.x (versions prior to 2.4.9)
• Debian Linux 8.0, 9.0, and 10.0
• Fedora 30 and 32

Discovery Timeline

• 2020-04-27 - CVE-2020-11810 published to NVD
• 2024-11-21 - Last updated in NVD database

Technical Details for CVE-2020-11810

Vulnerability Analysis

This vulnerability is classified as a race condition (CWE-362) that occurs during the OpenVPN connection establishment phase. The flaw exists in how OpenVPN processes incoming P_DATA_V2 packets before the data channel crypto parameters have been fully initialized. Under normal circumstances, injected packets with a spoofed peer-id would be dropped. However, when an attacker carefully times the packet injection to arrive during the brief initialization window (typically a few seconds), the victim's VPN connection is terminated.

The attack requires network access and precise timing to exploit the narrow window between when a client initiates its connection and when the server sends the PUSH_REPLY response. This timing-dependent nature makes exploitation challenging but still feasible, particularly in environments where connection timing can be observed or predicted.

Root Cause

The root cause lies in the multi.c source file where the multi_process_float() function was being called without properly validating that the packet had been successfully decrypted. The vulnerability stems from insufficient validation during the client "float" operation—a mechanism that allows clients to change their source address during an active session. When a P_DATA_V2 packet arrives before crypto initialization completes, the code incorrectly processes the float operation, leading to connection termination.

Attack Vector

The attack exploits the network-accessible OpenVPN service by requiring the attacker to:

1. Obtain or predict a victim's peer-id value
2. Craft a malicious P_DATA_V2 packet with the spoofed peer-id
3. Time the packet injection to arrive during the crypto initialization window
4. The vulnerability only manifests when NCP (Negotiable Cipher Parameters) is enabled

The patch adds a crucial validation check to ensure packets have non-zero length (indicating successful decryption) before processing the float operation:

             orig_buf = c->c2.buf.data;
             if (process_incoming_link_part1(c, lsi, floated))
             {
-                if (floated)
+                /* nonzero length means that we have a valid, decrypted packed */
+                if (floated && c->c2.buf.len > 0)
                 {
                     multi_process_float(m, m->pending);
                 }

Source: OpenVPN Commit 37bc691e7d26

Detection Methods for CVE-2020-11810

Indicators of Compromise

• Unexpected VPN connection drops occurring during the initial connection handshake phase
• Multiple P_DATA_V2 packets arriving from sources other than legitimate client IP addresses
• Anomalous connection termination patterns correlating with new client connection attempts

Detection Strategies

• Monitor OpenVPN server logs for connection failures occurring within seconds of connection initiation
• Implement network-level packet inspection to detect P_DATA_V2 packets with potentially spoofed peer-ids
• Deploy anomaly detection for unusual patterns of connection drops affecting specific clients

Monitoring Recommendations

• Enable verbose logging on OpenVPN servers to capture connection establishment failures
• Set up alerting for abnormal rates of client connection terminations during the initialization phase
• Monitor for network traffic patterns indicative of timing-based attacks against VPN infrastructure

How to Mitigate CVE-2020-11810

Immediate Actions Required

• Upgrade OpenVPN to version 2.4.9 or later immediately on all affected systems
• Review VPN server logs for signs of exploitation attempts
• Consider temporarily disabling NCP if immediate patching is not possible (note: this reduces cipher flexibility)

Patch Information

OpenVPN has released version 2.4.9 which addresses this vulnerability. The fix adds validation to ensure that the data channel packet has been properly decrypted (indicated by non-zero buffer length) before processing client float operations. Detailed patch information is available through the OpenVPN Ticket #1272 and the OpenVPN Patch Submission #1079.

For Debian systems, refer to the Debian CVE Tracker for distribution-specific updates. Fedora users should check the Fedora package announcements for updated packages.

Workarounds

• If immediate patching is not feasible, consider temporarily disabling NCP by setting ncp-disable in the server configuration (impacts cipher negotiation flexibility)
• Implement network-level rate limiting on incoming connections to reduce attack surface
• Deploy intrusion detection systems to monitor for suspicious P_DATA_V2 packet patterns

# Configuration example - Temporary NCP disable (not recommended long-term)
# Add to OpenVPN server configuration
ncp-disable

# Verify OpenVPN version after upgrade
openvpn --version

# Restart OpenVPN service after configuration changes
systemctl restart openvpn@server