CVE-2018-9853 in FreeSSHD
Summary
by MITRE
Insecure access control in freeSSHd version 1.3.1 allows attackers to obtain the privileges of the freesshd.exe process by leveraging the ability to login to an unprivileged account on the server.
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Analysis
by VulDB Data Team • 03/02/2020
The vulnerability identified as CVE-2018-9853 represents a critical access control flaw within freeSSHd version 1.3.1 that fundamentally undermines the security posture of affected systems. This issue stems from improper privilege management during the authentication process, where attackers can exploit the system's failure to properly enforce access controls even when users log in with unprivileged accounts. The vulnerability specifically targets the freesshd.exe process which operates with elevated privileges, creating a pathway for privilege escalation that directly violates fundamental security principles of least privilege and separation of concerns.
The technical implementation of this flaw involves the SSH daemon's insufficient validation of user sessions and their associated privilege levels. When legitimate users authenticate through the SSH service, the system fails to properly isolate their sessions from the underlying privileged operations performed by the freesshd.exe process. This creates an environment where authenticated users can potentially manipulate session contexts or exploit implementation gaps in the privilege management system. The vulnerability operates at the application level within the SSH service implementation, specifically affecting how the service handles user authentication tokens and session management. According to CWE classification, this represents a weakness in privilege management and access control enforcement, categorized under CWE-284 for improper access control and CWE-787 for out-of-bounds write vulnerabilities that can lead to privilege escalation.
The operational impact of CVE-2018-9853 extends beyond simple unauthorized access, as it enables attackers to achieve full system compromise through privilege escalation. Once an attacker successfully authenticates with an unprivileged account, they can leverage this vulnerability to execute commands with the elevated privileges of the freesshd.exe process, effectively bypassing the normal security boundaries that separate user sessions from system-level operations. This vulnerability aligns with ATT&CK technique T1068 for privilege escalation and T1566 for social engineering through credential access, as it allows adversaries to obtain system-level privileges without requiring additional exploitation vectors. The affected environment becomes vulnerable to complete system compromise, data exfiltration, and persistent access through the elevated privileges gained.
Mitigation strategies for CVE-2018-9853 must address both immediate remediation and long-term architectural improvements. The primary recommendation involves upgrading to a patched version of freeSSHd that properly implements access control mechanisms and privilege separation. Organizations should also implement network segmentation to limit access to SSH services and employ additional authentication layers such as multi-factor authentication to reduce the attack surface. Security configurations should enforce strict session isolation and privilege management policies, ensuring that authenticated user sessions cannot manipulate the underlying privileged processes. The vulnerability demonstrates the importance of proper access control implementation and highlights the need for security testing that specifically validates privilege management behaviors in authentication systems. Organizations should also consider implementing monitoring solutions that can detect anomalous privilege escalation attempts and establish incident response procedures specifically addressing such access control violations.