This vulnerability (CVE-2026-11771) affects OpenVPN versions 2.1.0 through 2.6.20 and 2.7_alpha1 through 2.7.4. It involves an off-by-one buffer write in the NTLM proxy authentication code path. An attacker controlling a malicious proxy server can send a specially crafted NTLM response that triggers this buffer write, leading to a potential crash of the OpenVPN client or server process. The issue is classified under CWE-121 (Stack-based Buffer Overflow). No known exploits in the wild have been reported. The CVSS 4.0 vector indicates network attack vector with high complexity and privileges not required, but user interaction is needed. The vulnerability impacts availability by causing crashes.

Successful exploitation can cause the OpenVPN process to crash, resulting in denial of service. There is no indication from the provided data that this vulnerability allows code execution or data disclosure. The impact is limited to service disruption.

Patch status is not yet confirmed — check the vendor advisory for current remediation guidance. No official fix or patch links are provided in the current data. Users should monitor OpenVPN vendor advisories for updates and apply patches once available. Until then, avoid using untrusted proxy servers for NTLM authentication or disable NTLM proxy authentication if feasible.