CVE-2019-7639 in gsi-openssh-server
Summary
by MITRE
An issue was discovered in gsi-openssh-server 7.9p1 on Fedora 29. If PermitPAMUserChange is set to yes in the /etc/gsissh/sshd_config file, logins succeed with a valid username and an incorrect password, even though a failure entry is recorded in the /var/log/messages file.
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Analysis
by VulDB Data Team • 07/09/2023
This vulnerability exists in the gsi-openssh-server implementation version 7.9p1 running on Fedora 29 systems where the PermitPAMUserChange directive is enabled in the configuration file. The flaw represents a critical authentication bypass issue that fundamentally undermines the security model of the SSH service. When PermitPAMUserChange is set to yes, the system incorrectly allows user authentication to succeed even when an invalid password is provided, creating a scenario where unauthorized access can occur without proper credential validation. The vulnerability manifests in the authentication logic where the system fails to properly enforce password verification, despite maintaining proper logging of failed authentication attempts in the system message log file.
The technical root cause of this vulnerability can be classified under CWE-287 which deals with improper authentication mechanisms. The flaw occurs within the PAM (Pluggable Authentication Modules) integration layer where the authentication flow is incorrectly designed to permit login when it should reject access based on failed password validation. This represents a classic case of authentication bypass where the system's access control logic is compromised. The issue affects the core SSH authentication process by creating a condition where the PAM module's user change functionality operates independently of the standard password verification process, leading to a false positive authentication result.
From an operational perspective, this vulnerability creates a significant risk to system security as it allows attackers to gain unauthorized access using valid usernames and incorrect passwords, effectively bypassing the primary password protection mechanism. The fact that authentication failures are still logged in /var/log/messages indicates that the system recognizes the invalid credentials but fails to enforce proper access control. This creates a false sense of security for system administrators who might rely on these logs to detect unauthorized access attempts. The vulnerability essentially provides a backdoor mechanism where legitimate usernames can be used to gain access without knowing the correct password, making it particularly dangerous in environments where user enumeration attacks are possible.
The impact of this vulnerability extends beyond simple unauthorized access to include potential privilege escalation and lateral movement opportunities within the network. Attackers can leverage this flaw to perform reconnaissance by testing valid usernames against the system, knowing that successful logins will occur even with incorrect passwords, thus enabling them to map out valid user accounts without triggering immediate detection mechanisms. This behavior aligns with techniques described in the MITRE ATT&CK framework under credential access and privilege escalation tactics, where adversaries attempt to obtain credentials through various means including authentication bypass methods. The vulnerability also creates audit trail confusion as the system logs indicate failed authentication attempts while simultaneously allowing successful access, making forensic analysis more complex and potentially masking actual security incidents.
Mitigation strategies should focus on immediate configuration changes to disable the PermitPAMUserChange directive when it is not strictly required for legitimate use cases. System administrators should review all PAM configurations and ensure that this directive is only enabled when specifically needed for authorized user management scenarios. Additionally, implementing proper monitoring of authentication logs and establishing alerting mechanisms for unusual authentication patterns can help detect potential exploitation attempts. The recommended approach involves setting PermitPAMUserChange to no in the /etc/gsissh/sshd_config file and ensuring that all authentication attempts are properly validated against the configured password policies. Regular security audits of SSH configurations should be conducted to identify and remediate similar issues that may exist in other authentication mechanisms or service configurations.