CVE-2018-6558 in fscrypt
Summary
by MITRE
The pam_fscrypt module in fscrypt before 0.2.4 may incorrectly restore primary and supplementary group IDs to the values associated with the root user, which allows attackers to gain privileges via a successful login through certain applications that use Linux-PAM (aka pam).
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Analysis
by VulDB Data Team • 05/04/2023
The pam_fscrypt module vulnerability identified as CVE-2018-6558 represents a critical privilege escalation flaw within the Linux Pluggable Authentication Modules framework. This vulnerability affects fscrypt versions prior to 0.2.4 and specifically targets the module's handling of group identification during authentication processes. The flaw manifests when certain applications utilize Linux-PAM for authentication, creating a pathway for malicious actors to exploit improper privilege management. The vulnerability stems from the module's incorrect restoration of primary and supplementary group IDs to root-level values during authentication procedures, effectively allowing unauthorized users to assume elevated privileges within the system.
The technical implementation of this vulnerability lies in the pam_fscrypt module's failure to properly manage group context during authentication transitions. When a user successfully authenticates through applications that rely on Linux-PAM, the module incorrectly resets the group identification to root-associated values rather than maintaining the authenticating user's proper group memberships. This mismanagement creates a persistent security boundary violation where the system temporarily grants root-level group privileges to non-root users. The flaw operates at the intersection of privilege management and authentication frameworks, specifically targeting the group ID restoration mechanism within the PAM authentication flow. This behavior aligns with CWE-276, which addresses improper privilege management, and represents a classic case of privilege escalation through incorrect access control enforcement.
The operational impact of this vulnerability extends beyond simple privilege escalation to encompass potential system compromise and data exposure. Attackers can leverage this flaw to gain unauthorized access to resources that should be restricted to root users, potentially enabling them to modify system files, access sensitive data, or establish persistent access. The vulnerability is particularly dangerous because it operates silently during the authentication process, making detection difficult. Applications that use Linux-PAM for authentication become potential attack vectors, including common services such as ssh, sudo, and various network authentication mechanisms. The privilege restoration issue affects both primary and supplementary group memberships, potentially granting access to multiple system resources simultaneously. This vulnerability directly relates to ATT&CK technique T1068, which covers privilege escalation through the exploitation of system vulnerabilities, and T1548, which addresses privilege escalation through the modification of system processes.
Mitigation strategies for CVE-2018-6558 require immediate patching of the fscrypt module to version 0.2.4 or later, which addresses the group ID restoration flaw. System administrators should also implement monitoring for unauthorized privilege escalation attempts and review group membership configurations across all authentication services. Additional protective measures include restricting access to applications that utilize the vulnerable pam_fscrypt module, implementing proper user privilege separation, and conducting regular security audits of authentication frameworks. The vulnerability highlights the importance of proper privilege management within authentication modules and demonstrates the critical need for thorough testing of security-sensitive components. Organizations should also consider implementing automated patch management systems to ensure timely updates of security-critical libraries and modules. Given the nature of the vulnerability, it is essential to validate that all applications using Linux-PAM are properly configured to prevent unauthorized group ID manipulation during authentication processes.