# CVE-2026-11771: OpenVPN off-by-one buffer write in NTLM proxy authentication

Source: https://junglewise.ai/threats/cve-2026-11771-openvpn-off-by-one-buffer-write-in-ntlm-proxy-authentication  
Publisher: Junglewise Threat Intelligence, https://junglewise.ai/threats  
Published: 2026-07-30. Updated: 2026-07-30  
License: CC BY 4.0 with a link back

CVE-2026-11771 · Severity: info · CVSS 7 · Published 2026-07-30

OpenVPN is a widely used software for creating secure private network connections. A vulnerability in how it handles authentication with certain proxy servers could allow a malicious proxy to crash the OpenVPN application. This could lead to a denial of service, preventing users from connecting to their corporate networks or secure resources.

Technologies: [Openvpn](https://junglewise.ai/threats/technologies/openvpn). Vendors: [Openvpn](https://junglewise.ai/threats/vendors/openvpn).

## Quick answers

**Is CVE-2026-11771 exploited in the wild?** No. Junglewise Threat Intelligence has no report of CVE-2026-11771 being exploited in the wild.

**How severe is CVE-2026-11771?** Informational severity, with a CVSS score of 7, published 30 July 2026.

**Which products and versions are affected?** Openvpn 2.1.0 through 2.6.20, 2.7_alpha1 through 2.7.4.

**Is there a fix for CVE-2026-11771?** A patch was published on 2026-07-01: Versions 2.6.21 and 2.7.5 released to address the issue. Junglewise finds every place you run it, drafts the upgrade and applies it once you approve.

**How is CVE-2026-11771 attacked?** It is attacked over the network, in a software library.

**Am I affected by CVE-2026-11771?** Check whether you run Openvpn at an affected version. Junglewise does that check for you around the clock: connect your code, cloud and domains, and it matches new vulnerabilities like this one against what you actually run and tells you when one reaches you.

## Technical details

An off-by-one error (CWE-193) leading to a stack-based buffer overflow (CWE-121) exists in OpenVPN's NTLM proxy authentication component. The vulnerability is triggered when the OpenVPN client receives a specially crafted NTLM response from a malicious or compromised proxy server. While the primary impact is a denial of service (application crash), the underlying root cause is an out-of-bounds write. Exploitation requires the client to be configured to use NTLM authentication through a proxy. The issue has been addressed in OpenVPN versions 2.6.21 and 2.7.5.

## Affected products

- OpenVPN OpenVPN 2.1.0 through 2.6.20, 2.7_alpha1 through 2.7.4

## Timeline

- 2026-07-01: patched: Versions 2.6.21 and 2.7.5 released to address the issue.
- 2026-07-30: disclosed: CVE-2026-11771 published.

## References

- [https://community.openvpn.net/ReleaseHistory](https://community.openvpn.net/ReleaseHistory)
- [https://community.openvpn.net/ReleaseHistory](https://community.openvpn.net/ReleaseHistory)
- [https://community.openvpn.net/Security%20Announcements/CVE-2026-11771](https://community.openvpn.net/Security%20Announcements/CVE-2026-11771)

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