Description
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
Published: 2026-07-30
Score: 7 High
EPSS: < 1% Very Low
KEV: No
Impact: n/a
Action: n/a
AI Analysis

Impact

An off‑by‑one buffer write occurs in the NTLM proxy authentication routine of OpenVPN when it parses a crafted NTLM response from a proxy server. The overflow can corrupt adjacent memory and crash the program, resulting in a denial of service. The flaw is a classic stack‑based buffer overflow (CWE‑121) with an off‑by‑one error (CWE‑193) and out‑of‑bounds write (CWE‑787). No elevation of privilege or code execution is achievable with the information provided.

Affected Systems

OpenVPN versions 2.1.0 through 2.6.20 and 2.7_alpha1 through 2.7.4 are affected.

Risk and Exploitability

The CVSS score of 7 indicates moderate to high severity, while the EPSS score of < 1% suggests a very low but non‑zero probability of exploitation. The vulnerability is not listed in the CISA KEV catalog. A crafted NTLM response could trigger the vulnerability, causing the OpenVPN process to crash. Because the exploit does not require elevated privileges or access to the OpenVPN server, the impact is limited to service interruption.

Generated by OpenCVE AI on August 3, 2026 at 10:41 UTC.

Remediation

No vendor fix or workaround currently provided.

OpenCVE Recommended Actions

  • Upgrade to the latest OpenVPN release that includes the NTLM proxy authentication fix, as referenced in the official release notes (e.g., 2.6.21 or newer).
  • If immediate upgrade is not feasible, disable NTLM proxy authentication or block proxy traffic until a patch is applied.
  • Monitor VPN logs for abnormal NTLM authentication responses and review network traffic for suspicious activity.

Generated by OpenCVE AI on August 3, 2026 at 10:41 UTC.

Tracking

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Advisories
Source ID Title
Debian DLA Debian DLA DLA-4666-1 openvpn security update
Debian DSA Debian DSA DSA-6376-1 openvpn security update
Ubuntu USN Ubuntu USN USN-8540-1 OpenVPN vulnerabilities
History

Sat, 01 Aug 2026 09:30:00 +0000

Type Values Removed Values Added
Metrics ssvc

{'options': {'Automatable': 'no', 'Exploitation': 'none', 'Technical Impact': 'partial'}, 'version': '2.0.3'}

Sat, 01 Aug 2026 00:15:00 +0000

Type Values Removed Values Added
Title openvpn: OpenVPN: Denial of Service via crafted NTLM proxy response
References
Metrics threat_severity

None

 cvssV3_1

{'score': 5.3, 'vector': 'CVSS:3.1/AV:N/AC:H/PR:N/UI:R/S:U/C:N/I:N/A:H'}

threat_severity

Moderate

Thu, 30 Jul 2026 18:45:00 +0000

Type Values Removed Values Added
First Time appeared Openvpn
Openvpn openvpn
Vendors & Products Openvpn
Openvpn openvpn

Thu, 30 Jul 2026 17:00:00 +0000

Type Values Removed Values Added
Description 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
Weaknesses CWE-121
CWE-193
CWE-787
References
Metrics cvssV4_0

{'score': 7, 'vector': 'CVSS:4.0/AV:N/AC:H/AT:P/PR:N/UI:P/VC:N/VI:N/VA:H/SC:N/SI:N/SA:H'}

Subscriptions

Openvpn Openvpn
cve-icon MITRE

Status: PUBLISHED

Assigner: OpenVPN

Published:

Updated: 2026-07-30T18:04:42.335Z

Reserved: 2026-06-09T11:44:10.570Z

Link: CVE-2026-11771

cve-icon Vulnrichment

Updated: 2026-07-30T18:04:32.695Z

cve-icon NVD

Status : Analyzed

Published: 2026-07-30T17:16:27.610

Modified: 2026-08-05T19:38:34.177

Link: CVE-2026-11771

cve-icon Redhat

Severity : Moderate

Publid Date: 2026-07-30T16:36:41Z

Links: CVE-2026-11771 - Bugzilla

cve-icon OpenCVE Enrichment

Updated: 2026-08-03T10:45:03Z

Weaknesses