CVE-2015-3246 in libuser
Summary
by MITRE
libuser before 0.56.13-8 and 0.60 before 0.60-7, as used in the userhelper program in the usermode package, directly modifies /etc/passwd, which allows local users to cause a denial of service (inconsistent file state) by causing an error during the modification. NOTE: this issue can be combined with CVE-2015-3245 to gain privileges.
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Analysis
by VulDB Data Team • 06/25/2024
The vulnerability identified as CVE-2015-3246 represents a critical flaw in the libuser library version 0.56.13-8 and earlier versions, as well as 0.60-7 and earlier versions, specifically within the userhelper program of the usermode package. This issue manifests when the program directly modifies the /etc/passwd file without proper error handling or atomic operations, creating a dangerous condition where local attackers can manipulate the system's user account management functionality. The flaw exists in the core user management infrastructure that handles user account creation, modification, and deletion operations, making it a fundamental security weakness in Unix-like operating systems that rely on proper user privilege management.
The technical implementation of this vulnerability stems from the direct manipulation of the /etc/passwd file, which is a critical system file containing user account information and password hashes. When the userhelper program attempts to modify this file, it does not implement proper transactional mechanisms or rollback capabilities. This means that if an error occurs during the modification process, the file can be left in an inconsistent state where some changes are applied while others are not, resulting in a corrupted user database. The absence of proper file locking mechanisms and atomic write operations creates a window of opportunity for attackers to exploit this inconsistency, potentially leading to denial of service conditions where legitimate user management operations fail or become unreliable.
The operational impact of this vulnerability extends beyond simple denial of service, as it creates a pathway for privilege escalation when combined with CVE-2015-3245, which typically involves a separate vulnerability in the same software ecosystem. Attackers who can trigger the inconsistent file state condition can potentially manipulate user account permissions, create unauthorized accounts, or corrupt existing user entries in ways that allow them to gain elevated privileges. This vulnerability aligns with CWE-116, which addresses improper encoding or handling of structured data, and specifically relates to CWE-362, which deals with concurrent execution using shared resources with improper synchronization. The flaw demonstrates poor adherence to secure coding practices where system-critical files are modified without proper error recovery mechanisms.
The attack surface for this vulnerability is primarily limited to local users who have access to the userhelper program or can trigger its execution, but the implications are severe as it affects the fundamental user account management infrastructure. System administrators should implement immediate mitigations including updating to patched versions of the usermode package, implementing proper monitoring for /etc/passwd file modifications, and ensuring that all user management operations are performed through secure, well-tested interfaces. The vulnerability also highlights the importance of following ATT&CK framework techniques related to privilege escalation and defense evasion, as attackers can exploit this flaw to maintain persistent access or hide their activities within the compromised system. Organizations should conduct thorough security assessments to identify any systems running vulnerable versions of libuser and ensure that all user management operations are properly audited and secured against such atomicity failures.