CVE-2015-3245 in libuser
Summary
by MITRE
Incomplete blacklist vulnerability in the chfn function in libuser before 0.56.13-8 and 0.60 before 0.60-7, as used in the userhelper program in the usermode package, allows local users to cause a denial of service (/etc/passwd corruption) via a newline character in the GECOS field.
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Analysis
by VulDB Data Team • 06/25/2024
The vulnerability identified as CVE-2015-3245 represents a critical incomplete blacklist flaw within the chfn function of libuser library versions prior to 0.56.13-8 and 0.60 before 0.60-7. This issue specifically affects the userhelper program within the usermode package, creating a pathway for local attackers to exploit system integrity and availability. The vulnerability stems from inadequate input validation mechanisms that fail to properly sanitize user-supplied data, particularly when processing the GECOS field during user account modifications. The GECOS field, traditionally used to store user information such as full names and contact details, becomes a vector for exploitation when newline characters are introduced, leading to unpredictable system behavior and potential data corruption.
The technical implementation of this vulnerability involves the chfn function's insufficient filtering of special characters, specifically newline characters, which are not properly escaped or removed from the GECOS field input. When a local user provides a GECOS value containing newline characters, the system's parsing mechanisms become confused during the modification of the /etc/passwd file, resulting in malformed entries that can cause the passwd file to become corrupted. This corruption manifests as a denial of service condition where legitimate user account operations fail, and the system may become unstable or unusable. The vulnerability is classified under CWE-20 as "Improper Input Validation" and represents a classic example of insecure input sanitization that enables arbitrary code execution or system compromise through data manipulation.
The operational impact of CVE-2015-3245 extends beyond simple denial of service to encompass potential system instability and data integrity concerns within Unix-like operating systems. Local attackers can exploit this vulnerability to corrupt the /etc/passwd file, which serves as the primary database for user account information and authentication. The consequences include authentication failures, inability to create or modify user accounts, and potential privilege escalation opportunities for determined attackers. The vulnerability operates at the system level and requires local access to exploit, making it particularly dangerous in environments where multiple users have shell access. This flaw directly aligns with ATT&CK technique T1068, which covers "Local Privilege Escalation" through system weaknesses, and T1078, covering "Valid Accounts" exploitation through system-level modifications.
Mitigation strategies for CVE-2015-3245 involve immediate patching of affected libuser library versions, ensuring that all systems running usermode package components are updated to versions containing proper input validation and sanitization. System administrators should implement comprehensive input filtering mechanisms that prevent newline and other special characters from being processed in GECOS fields during user account modifications. Additional protective measures include monitoring for unusual patterns in user account modifications, implementing proper access controls to limit local user privileges, and establishing regular system integrity checks to detect corruption in critical system files. The vulnerability demonstrates the importance of robust input validation practices and proper sanitization of user-provided data, particularly in system-level applications that handle sensitive configuration files. Organizations should also consider implementing automated patch management systems to ensure timely deployment of security updates and maintain compliance with security standards such as those outlined in NIST SP 800-53 for system security controls.