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OpenSSH CVE-2026-59998

| EUVDEUVD-2026-42141 MEDIUM
Improper Following of Specification by Caller (CWE-573)
2026-07-08 mitre GHSA-3j2g-cqmq-cjw9
6.5
CVSS 3.1 · NVD
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Severity by source

Vendor (mitre) PRIMARY
MEDIUM
qualitative
NVD
6.5 MEDIUM
AV:N/AC:L/PR:N/UI:N/S:U/C:L/I:L/A:N
vuln.today AI
4.8 MEDIUM

Network-accessible SSH service; AC:H for mandatory Windows AD integration and explicit GSSAPI configuration; C:L and I:L for authentication control bypass; no availability impact.

3.1 AV:N/AC:H/PR:N/UI:N/S:U/C:L/I:L/A:N
4.0 AV:N/AC:H/AT:P/PR:N/UI:N/VC:L/VI:L/VA:N/SC:N/SI:N/SA:N
SUSE
4.8 MEDIUM
AV:N/AC:H/PR:N/UI:N/S:U/C:L/I:L/A:N
Red Hat
4.8 MEDIUM
qualitative

Primary rating from Vendor (mitre).

CVSS VectorNVD

Attack Vector
Network
Attack Complexity
Low
Privileges Required
None
User Interaction
None
Scope
Unchanged
Confidentiality
Low
Integrity
Low
Availability
None

Lifecycle Timeline

4
CVSS changed
Jul 09, 2026 - 17:07 NVD
4.8 (MEDIUM) 6.5 (MEDIUM)
Patch available
Jul 08, 2026 - 02:01 EUVD
Analysis Generated
Jul 08, 2026 - 01:27 vuln.today
CVE Published
Jul 08, 2026 - 00:11 cve.org
MEDIUM 4.8

DescriptionNVD

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

AnalysisAI

OpenSSH sshd before version 10.4 silently ignores the GSSAPIStrictAcceptorCheck configuration directive when the server is integrated with Windows Active Directory, defeating a security control that administrators rely on to enforce GSSAPI acceptor name validation during Kerberos-based SSH authentication. This undocumented behavior means AD-joined SSH servers may accept GSSAPI authentications against unintended Kerberos service principals regardless of how the option is configured, yielding limited confidentiality and integrity impact. No public exploits or active exploitation have been identified; the CVSS AC:H rating reflects the specific environmental prerequisites required for exploitation.

Technical ContextAI

GSSAPI (Generic Security Services Application Programming Interface) is the abstraction layer OpenSSH uses to support Kerberos-based single sign-on, most commonly when servers are joined to a Windows Active Directory domain. The GSSAPIStrictAcceptorCheck sshd_config directive is documented to enforce that the GSSAPI service principal name presented during authentication matches the server's actual principal, preventing cross-principal authentication. CWE-573 (Improper Following of Specification by Caller) identifies the root cause: the OpenSSH implementation does not correctly apply the strict-checking specification in the Windows AD code path, leaving the control silently inoperative regardless of its configured value. All OpenSSH versions prior to 10.4 are affected per CPE cpe:2.3:a:openbsd:openssh:*:*:*:*:*:*:*:*. The issue was disclosed on the oss-security mailing list and the openssh-unix-dev list before being resolved in the 10.4p1 release.

RemediationAI

The primary and recommended fix is to upgrade to OpenSSH 10.4p1 or later, which resolves the undocumented behavior; release notes are available at https://www.openssh.org/releasenotes.html#10.4p1. For systems that cannot be immediately patched, administrators may disable GSSAPI authentication entirely by setting GSSAPIAuthentication no in sshd_config and reloading sshd - this eliminates the vulnerable code path but also removes Kerberos single sign-on capability, which may impact usability in AD environments. A less disruptive workaround is to restrict SSH access at the network level (firewall or security group rules) to only trusted Kerberos client subnets, reducing the population of potential authenticators; this does not fix the control bypass but limits exposure. Additional context is available in the oss-security disclosure at https://www.openwall.com/lists/oss-security/2026/07/06/5 and the openssh-unix-dev thread at https://marc.info/?l=openssh-unix-dev&m=178333966933090&w=2.

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CVE-2026-59998 vulnerability details – vuln.today

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