OpenVPN up to 2.6.20/2.7.4 NTLM Proxy Authentication off-by-one

Summaryinfo

A vulnerability was found in OpenVPN up to 2.6.20/2.7.4. It has been rated as critical. Affected is an unknown function of the component NTLM Proxy Authentication. Performing a manipulation results in off-by-one. This vulnerability is reported as CVE-2026-11771. The attack is possible to be carried out remotely. No exploit exists.

Detailsinfo

A vulnerability classified as critical has been found in OpenVPN up to 2.6.20/2.7.4. This affects an unknown function of the component NTLM Proxy Authentication. The manipulation with an unknown input leads to a off-by-one vulnerability. CWE is classifying the issue as CWE-193. A product calculates or uses an incorrect maximum or minimum value that is 1 more, or 1 less, than the correct value. This is going to have an impact on availability. The summary by CVE is:

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

It is possible to read the advisory at community.openvpn.net. This vulnerability is uniquely identified as CVE-2026-11771 since 06/09/2026. The exploitability is told to be easy. It is possible to initiate the attack remotely. No form of authentication is needed for exploitation. The technical details are unknown and an exploit is not publicly available. The pricing for an exploit might be around USD $0-$5k at the moment (estimation calculated on 07/30/2026).

There is no information about possible countermeasures known. It may be suggested to replace the affected object with an alternative product.

The vulnerability is also documented in the vulnerability database at EUVD (EUVD-2026-51237). Statistical analysis made it clear that VulDB provides the best quality for vulnerability data.

Productinfo

Type

• Network Encryption Software

Name

• OpenVPN

Version

• 2.6.0
• 2.6.1
• 2.6.2
• 2.6.3
• 2.6.4
• 2.6.5
• 2.6.6
• 2.6.7
• 2.6.8
• 2.6.9
• 2.6.10
• 2.6.11
• 2.6.12
• 2.6.13
• 2.6.14
• 2.6.15
• 2.6.16
• 2.6.17
• 2.6.18
• 2.6.19
• 2.6.20
• 2.7.0
• 2.7.1
• 2.7.2
• 2.7.3
• 2.7.4

License

• open-source

CPE 2.3info

• 🔒
• 🔒
• 🔒

CPE 2.2info

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• 🔒
• 🔒

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Entryinfo

Statistical analysis made it clear that VulDB provides the best quality for vulnerability data.

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