Cyber Resilience

CVE-2026-59998

Openbsd Openssh ≤ 10.4

Published
08 July 2026
Modified
09 July 2026
CVSS Score v3.1 4.8
Click a component to see what it means
Raw vectorCVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:L/I:L/A:N
EPSS Score 0.0018 8th percentile
Risk Priority 35 floored blend · peak EPSS

Summary

CVE-2026-59998 is a medium-severity Improper Following of Specification by Caller (CWE-573) vulnerability in Openbsd Openssh. Its CVSS base score is 4.8 (Medium).

Operationally, exploitation aligns with the MITRE ATT&CK technique Domain Controller Authentication (T1556.001); ranked at the 8th percentile by exploit likelihood (below the median); it is not currently listed in the CISA KEV catalog.

The strongest mitigations our analysis identified map to AC-3 (Access Enforcement) and CM-6 (Configuration Settings) — see the control section below for these in your framework.

EU & UK References

Vulnerability Data

sshd in OpenSSH before 10.4 has an undocumented security-relevant behavior: GSSAPIStrictAcceptorCheck has no value if the server is in Windows Active Directory.

CWE(s)
CWE-573

Related Threats

MITRE ATT&CK Enterprise TechniquesAI

T1556.001 Domain Controller Authentication Defense Impairment
Adversaries may patch the authentication process on a domain controller to bypass the typical authentication mechanisms and enable access to accounts.
attack.mitre.org →
Why these techniques?

The vulnerability describes a failure in GSSAPI authentication enforcement on Windows Active Directory environments, which could allow adversaries to bypass domain controller authentication checks (T1556.001).

Confidence: MEDIUM · MITRE ATT&CK Enterprise v19.0

CVEs Like This One

CVE-2026-35414Same product: Openbsd Openssh
CVE-2026-59997Same product: Openbsd Openssh
CVE-2026-59995Same product: Openbsd Openssh
CVE-2026-60001Same product: Openbsd Openssh
CVE-2026-35388Same product: Openbsd Openssh
CVE-2026-60000Same product: Openbsd Openssh
CVE-2026-35385Same product: Openbsd Openssh
CVE-2026-60002Same product: Openbsd Openssh
CVE-2026-35387Same product: Openbsd Openssh
CVE-2026-35386Same product: Openbsd Openssh

Affected Assets

openbsd
openssh
≤ 10.4

Mitigating Controls

Control response

Prevent
Stop it (NIST 800-53)
  • AC-3 Access Enforcement
  • CM-6 Configuration Settings
  • SI-2 Flaw Remediation
Detect
Catch it (NIST detect / respond)

—

Harden
Shrink the surface (DISA STIG)

—

Validate
Prove the fix (OWASP ASVS)

—

Mitigating Controls (NIST 800-53 r5) AI

AC-3 Access Enforcement good match
prevent

AC-3 enforces that the GSSAPI acceptor check is actually applied, preventing the undocumented bypass when sshd runs against Active Directory.

CM-6 Configuration Settings partial match
prevent

CM-6 requires explicit configuration settings for GSSAPIStrictAcceptorCheck and verification that the setting is honored on Windows AD hosts.

SI-2 Flaw Remediation partial match
prevent

SI-2 drives timely patching or configuration updates once the undocumented behavior is corrected in OpenSSH 10.4.

Mitigating Controls (NIST CSF 2.0) AI

Derived directly from the weakness types (CWEs) cited in the NVD entry via our AI-authored CWE→CSF cross-walk (authority under review) — links open the control.

PR.PS-06 mostly match
prevents

Secure SDLC practices directly enforce correct adherence to language, framework, protocol and platform specifications during implementation.

Mitigating Controls (ISO/IEC 27001:2022 Annex A) AI

Derived directly from the weakness types (CWEs) cited in the NVD entry via our AI-authored CWE→ISO cross-walk (authority under review) — links open the control.

A.8.29 Security testing in development and acceptance mostly match
detects

Security testing can detect specification non-compliance but does not itself prevent the weakness.

A.8.25 Secure development life cycle partial match
prevents

Secure development life cycle mandates adherence to language, framework and protocol specifications, directly reducing improper caller behavior.

A.8.26 Application security requirements partial match
prevents

Application security requirements explicitly capture specification compliance obligations that callers must follow.

A.8.27 Secure system architecture and engineering principles partial match
prevents

Secure system architecture and engineering principles include specification adherence as a design constraint.

A.8.28 Secure coding partial match
prevents

Secure coding standards enforce correct use of APIs, protocols and language features, mitigating caller specification violations.

A.8.32 Change management partial match
prevents

Change management can catch specification drift during updates but is not a primary mitigation.

References