CVE-2026-2738: ovpn-dco-win Buffer Overflow Vulnerability

CVE-2026-2738 Overview

A buffer overflow vulnerability exists in ovpn‑dco‑win version 2.8.0, the Windows kernel driver for OpenVPN Data Channel Offload. This vulnerability allows local attackers to cause a system crash by sending oversized packets to a remote peer when the AEAD (Authenticated Encryption with Associated Data) tag appears at the end of the encrypted packet. The flaw stems from improper size calculation when handling encrypted packet buffers.

Critical Impact

Local attackers can exploit this buffer overflow to cause system crashes and denial of service conditions, potentially affecting system availability and connected VPN sessions.

Affected Products

• ovpn‑dco‑win version 2.8.0
• OpenVPN installations using the ovpn-dco-win driver for data channel offload

Discovery Timeline

• 2026-02-19 - CVE-2026-2738 published to NVD
• 2026-02-19 - Last updated in NVD database

Technical Details for CVE-2026-2738

Vulnerability Analysis

This buffer overflow vulnerability is classified under CWE-131 (Incorrect Calculation of Buffer Size). The vulnerability occurs in the ovpn-dco-win kernel driver when processing encrypted packets where the AEAD authentication tag is positioned at the end of the packet payload. Due to improper buffer size calculations, sending packets that exceed expected boundaries can trigger a buffer overflow condition within the kernel driver.

The local attack vector requires attacker presence on the target system, and the vulnerability specifically manifests when transmitting data to a remote VPN peer. When exploited, the overflow condition leads to a system crash, resulting in denial of service. The vulnerability affects both the local system's availability and can cascade to impact connected VPN infrastructure.

Root Cause

The root cause of CVE-2026-2738 is an incorrect calculation of buffer size (CWE-131) within the ovpn-dco-win driver. When handling encrypted packets with AEAD tags positioned at the packet's end, the driver fails to properly account for the total size required to accommodate both the encrypted payload and the authentication tag. This miscalculation results in insufficient buffer allocation, leading to an out-of-bounds write condition when processing oversized packets.

Attack Vector

The attack requires local access to a system running the vulnerable ovpn-dco-win driver. An attacker must be able to send crafted packets through the OpenVPN data channel with packet sizes that exceed the incorrectly calculated buffer boundaries. The attack specifically targets scenarios where AEAD encryption is in use and the authentication tag is appended to the end of the encrypted packet data.

The vulnerability is triggered during the packet transmission process to a remote peer, meaning the attacker needs an active VPN connection or the ability to initiate one. Upon successful exploitation, the kernel driver experiences a buffer overflow that results in a system crash (Blue Screen of Death on Windows systems).

For detailed technical information about this vulnerability, refer to the OpenVPN Security Announcement.

Detection Methods for CVE-2026-2738

Indicators of Compromise

• Unexpected system crashes or Blue Screen of Death (BSOD) errors referencing the ovpn-dco-win driver
• Windows Event Log entries indicating kernel driver failures related to ovpn-dco-win.sys
• Crash dump files showing buffer overflow conditions in VPN driver components
• Unusual patterns of oversized packet transmissions through OpenVPN connections

Detection Strategies

• Monitor Windows Event Logs for system crashes associated with the ovpn-dco-win kernel driver
• Implement endpoint detection rules to identify abnormal packet sizes transmitted through the VPN data channel
• Deploy crash dump analysis tools to identify buffer overflow signatures in ovpn-dco-win driver memory
• Use SentinelOne's kernel-level monitoring to detect anomalous driver behavior and memory violations

Monitoring Recommendations

• Enable enhanced logging for OpenVPN connections to track packet size anomalies
• Configure Windows crash dump collection to capture full memory dumps for forensic analysis
• Monitor system stability metrics for endpoints running the affected driver version
• Implement alerting for repeated system crashes on VPN-enabled endpoints

How to Mitigate CVE-2026-2738

Immediate Actions Required

• Identify all systems running ovpn‑dco‑win version 2.8.0 and prioritize them for patching
• Review the OpenVPN Security Announcement for vendor guidance and patch availability
• Consider temporarily disabling the ovpn-dco-win driver on critical systems until patches are applied
• Implement network segmentation to limit the potential impact of system crashes on VPN infrastructure

Patch Information

Consult the OpenVPN Security Announcement for official patch information and updated driver versions that address this buffer overflow vulnerability. Ensure all affected systems are updated to a patched version of the ovpn-dco-win driver as soon as patches become available from OpenVPN.

Workarounds

• Disable the ovpn-dco-win driver and fall back to the standard OpenVPN TAP driver, which does not use kernel-level data channel offload
• Restrict local access to systems running the vulnerable driver to trusted users only
• Implement application control policies to limit which users can interact with VPN connections
• Monitor and limit packet sizes at the network level where feasible

# Disable ovpn-dco-win driver (Windows PowerShell - run as Administrator)
# Stop the driver service
sc.exe stop ovpn-dco-win

# Disable the driver from starting automatically
sc.exe config ovpn-dco-win start= disabled

# Verify the driver is disabled
sc.exe query ovpn-dco-win