CVE-2026-11771 in OpenVPN
Summary
by MITRE • 07/30/2026
OpenVPN version 2.1.0 through 2.6.20 and 2.7_alpha1 through 2.7.4 allows attackers via an off-by-one buffer write in the NTLM proxy authentication to potentially cause a crash via a crafted NTLM response from a malicious proxy server
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Analysis
by VulDB Data Team • 07/30/2026
This vulnerability represents a critical buffer overflow condition within the OpenVPN authentication framework that specifically affects versions ranging from 2.1.0 through 2.6.20 and 2.7_alpha1 through 2.7.4. The flaw exists in the NTLM proxy authentication implementation where an off-by-one error occurs during memory allocation and data processing when handling NTLM responses from proxy servers. This type of vulnerability falls under CWE-121, which encompasses stack-based buffer overflow conditions, and more specifically aligns with CWE-787, representing out-of-bounds write operations that can lead to memory corruption and system instability.
The technical execution of this vulnerability requires an attacker to position themselves in a man-in-the-middle role or compromise a proxy server within the network path between the OpenVPN client and authentication server. When the vulnerable client processes a crafted NTLM response from a malicious proxy, the buffer overflow occurs during the parsing of authentication tokens, specifically when the application attempts to write data beyond the allocated memory boundaries for storing NTLM challenge-response data structures. This off-by-one condition typically manifests when the application incorrectly calculates buffer sizes or fails to properly validate the length of incoming authentication data before copying it into fixed-size memory buffers.
The operational impact of this vulnerability extends beyond simple denial-of-service conditions, as successful exploitation can potentially lead to arbitrary code execution within the OpenVPN client process. Attackers could leverage this weakness to crash the VPN client application, causing connectivity disruption for legitimate users, or in more sophisticated scenarios, they might be able to inject malicious code that executes with the privileges of the OpenVPN client process. This represents a significant risk in enterprise environments where VPN connectivity is critical for remote access and business continuity. The vulnerability particularly affects organizations using NTLM authentication proxies as part of their network infrastructure, making it relevant to both corporate networks and cloud-based VPN deployments.
Mitigation strategies should prioritize immediate patching of affected OpenVPN versions to the latest stable releases that contain memory safety fixes and proper bounds checking for NTLM response handling. Network administrators should also implement additional monitoring for unusual proxy authentication traffic patterns and consider deploying network segmentation controls to limit exposure to potentially malicious proxy servers. Organizations may need to temporarily disable NTLM proxy authentication mechanisms until patches are fully deployed and validated, while also implementing proper input validation and length checking protocols at the application level. This vulnerability demonstrates the importance of maintaining up-to-date security patches and following secure coding practices as outlined in the OWASP Top Ten and NIST Cybersecurity Framework guidelines for preventing buffer overflow exploits that can be leveraged for privilege escalation and system compromise attacks.