Client crash via NTLM response off-by-one
Published Jul 30, 2026 · Updated Jul 30, 2026
Stack-based buffer overflow in OpenVPN 2.1.0 through 2.6.20 and 2.7_alpha1 through 2.7.4 allows attackers to crash clients. In ntlm_phase_3, the target-information length check permits a response that exactly fills ntlmv2_response, leaving a subsequent one-byte write past the stack buffer. Exploitation requires NTLMv2 HTTP proxy authentication and a malicious proxy that returns a crafted NTLM response; the consequence is a client-process crash.
Summary
What happened
Stack-based buffer overflow in OpenVPN 2.1.0 through 2.6.20 and 2.7_alpha1 through 2.7.4 allows attackers to crash clients. In ntlm_phase_3, the target-information length check permits a response that exactly fills ntlmv2_response, leaving a subsequent one-byte write past the stack buffer. Exploitation requires NTLMv2 HTTP proxy authentication and a malicious proxy that returns a crafted NTLM response; the consequence is a client-process crash.
The record
- CVE
- CVE-2026-11771
- Published
- Jul 30, 2026
- Updated
- Jul 30, 2026
- Vendor
- OpenVPN Inc.
- Product
- OpenVPN
- Classifications
- CWE-193, CWE-121, CWE-787, T1499.004
- Attack vector
- network
- Privileges
- unauthenticated
Timeline
How it unfolded
- Jul 30, 2026CVE publishedPublication date reported by the CVE source.
- Jul 30, 2026Record updatedLatest update available in the CVE record.
Exploitability
Present is not the same as exploitable
Compare your product and version with the public record. A matching version still requires validation against your environment.
Is a vulnerable build present?
Compare these published version ranges with your installed build and any vendor patches.
- Affected versionversion=2.1.0 <=2.6.20
- Affected versionversion=2.7_alpha1 <=2.7.4
What conditions does exploitation require?
What is affected?
Published CVSS scores
CVSS describes severity. EPSS estimates exploitation probability.
Attacks
What attackers are doing with it
Daily unique IPs observed by Shadowserver honeypots for known exploited vulnerabilities (KEVs). Missing observations do not establish an absence of attacks.
Weakness, pattern, technique
Public exploit references
No public exploit references are available in this record.
Labels summarize the accepted research assessment. They do not indicate a test against your environment.
Technologies
Your stack
See the directory against your own environment.
Your stack
Check the software in your environment
Book a demo to see how Hinoki identifies affected software and validates exploitability in your environment.
Book a demo