CVE-2026-106555 in OpenSSH
Summary
by MITRE • 10/07/2026
In sshd in OpenSSH before 10.6, GSSAPIAuthentication authentication state can incorrectly be persisted across authentication attempts.
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Analysis
by VulDB Data Team • 10/07/2026
The vulnerability identified in versions of OpenSSH prior to release 10.6 involves a critical flaw within the Global Security and Authentication Service Application interface implementation, specifically regarding how the daemon manages session states during multi-factor or iterative authentication processes. In standard secure shell operations, when an administrator or user initiates a connection using GSSAPI for authentication, the server must validate credentials against a Kerberos Key Distribution Center or similar identity provider. The core technical flaw lies in the persistence of this authenticated state across distinct and separate authentication attempts within the same TCP session. Instead of resetting the internal flags that track successful GSSAPI verification after each failed attempt or upon initiation of a new credential exchange, sshd retains the positive validation status from previous interactions. This behavior violates fundamental security principles regarding state management in multi-step authentication protocols, where each step must be independently verified and isolated to prevent privilege escalation through session hijacking or replay mechanisms.
From an operational perspective, this flaw allows for potential unauthorized access scenarios that bypass intended security controls. An attacker who has previously failed GSSAPI authentication but left the connection open could potentially leverage the lingering authenticated state to gain shell access without providing valid credentials in subsequent attempts. Alternatively, if a user switches between different identity principals or keys during a single session, the server might incorrectly associate the permissions of one principal with another due to this stale state persistence. This undermines the integrity of the authentication mechanism and can lead to unauthorized privilege escalation, particularly in environments where GSSAPI is used for centralized access control based on specific group memberships or role-based attributes tied to individual Kerberos tickets. The impact is severe because it effectively neutralizes one layer of defense by allowing a connection that should have been terminated or reset to remain open with elevated trust levels derived from prior successful authentications.
This vulnerability aligns closely with CWE-20, which describes Improper Input Validation, as the server fails to properly validate the current authentication state against the actual credentials presented in each step of the process. Furthermore, it relates to CWE-613, Insufficient Session Expiration, because the session retains a privileged status beyond its intended validity period within the context of that specific authentication exchange. In terms of offensive security frameworks such as MITRE ATT&CK, this flaw facilitates techniques associated with Credential Access and Defense Evasion, specifically by allowing an attacker to bypass normal authentication checks through state manipulation rather than brute force or credential theft. The persistence of the GSSAPI authenticated flag creates a window where lateral movement within a network could be achieved more easily if the compromised session is used to pivot to other systems that trust the same Kerberos realm.
Mitigation strategies primarily involve upgrading OpenSSH to version 10.6 or later, which contains code corrections ensuring that authentication states are properly cleared and reset between attempts. For environments where immediate patching is not feasible due to compatibility constraints with legacy applications relying on older SSH implementations, administrators should enforce strict session timeouts via the ClientAliveInterval and ServerAliveInterval directives in sshd_config to ensure that long-lived connections do not remain open indefinitely. Additionally, restricting GSSAPI authentication to specific user groups or IP ranges using AllowUsers or Match blocks can limit the blast radius of this vulnerability by reducing the number of accounts exposed to potential state manipulation attacks. Regular auditing of SSH logs for unusual patterns in authentication failures followed by successes from the same source address may also help detect exploitation attempts before significant damage occurs, although reliance on detection alone is insufficient without addressing the underlying code defect through software updates.