# CVE-2026-11771 - NTLM proxy auth stack off-by-one write

OpenVPN 2.x 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.

OpenVPN version 2.1.0 through 2.6.20 and 2.7_alpha1 through 2.7.4 are affected. This is fixed in versions 2.6.21 and 2.7.5.

CVE Record: [CVE-2026-11771](https://www.cve.org/CVERecord?id=CVE-2026-11771)

Github:
* [OpenVPN/openvpn-private-issues#116](https://github.com/OpenVPN/openvpn-private-issues/issues/116)

Release notes:
* [openvpn-2.7.5](https://community.openvpn.net/ReleaseHistory#openvpn-275-released-1-july-2026)
* [openvpn-2.6.21](https://community.openvpn.net/ReleaseHistory#openvpn-2621-released-1-july-2026)

Reported-By: Tristan Madani (@TristanInSec)