CVE-2008-5138 in Libpam Mount
Summary
by MITRE
passwdehd in libpam-mount 0.43 allows local users to overwrite arbitrary files via a symlink attack on a /tmp/passwdehd.##### temporary file.
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Analysis
by VulDB Data Team • 08/28/2019
The vulnerability identified as CVE-2008-5138 represents a critical security flaw in the libpam-mount 0.43 library that affects Unix-like operating systems implementing PAM (Pluggable Authentication Modules) authentication frameworks. This issue specifically targets the passwdehd utility component within the library, creating a dangerous condition where local attackers can exploit weak temporary file handling mechanisms to execute arbitrary file overwrites. The vulnerability arises from improper handling of temporary files during the authentication process, particularly when the system creates temporary files in the /tmp directory without sufficient security measures to prevent symbolic link attacks.
The technical flaw manifests through a race condition vulnerability where the passwdehd utility creates temporary files in the /tmp directory without proper atomic operations or security checks. When a local user can predict or influence the creation of temporary files in this location, they can establish symbolic links that point to sensitive system files or configuration files. This symlink attack allows the attacker to manipulate the contents of files that should normally be protected from unauthorized modification, effectively bypassing normal file system permissions and access controls. The vulnerability operates under CWE-377, which classifies insecure temporary file handling as a significant security weakness, and specifically relates to CWE-378, which addresses the creation of temporary files with insecure permissions.
The operational impact of this vulnerability extends beyond simple file overwrites to potentially enable privilege escalation and system compromise. Local attackers who exploit this weakness can target critical system files such as password databases, configuration files, or even system binaries, depending on the context in which the vulnerable utility operates. The attack vector is particularly dangerous because it requires minimal privileges and can be executed by any user with access to the system, making it an attractive target for malicious actors seeking to establish persistent access or disrupt system operations. This vulnerability directly maps to ATT&CK technique T1068, which covers local privilege escalation through exploitation of system vulnerabilities, and T1059, which involves the use of system commands to manipulate file systems.
Mitigation strategies for CVE-2008-5138 should focus on immediate remediation through software updates and proper temporary file handling practices. System administrators must ensure that all instances of libpam-mount are updated to versions that properly implement secure temporary file creation mechanisms, typically through the use of atomic file creation operations that prevent symbolic link attacks. The recommended approach involves implementing proper file permissions, using secure temporary file creation functions such as mkstemp(), and avoiding predictable temporary file names that can be manipulated by attackers. Additionally, system hardening measures should include restricting write access to the /tmp directory for non-root users, implementing proper file system auditing, and monitoring for suspicious file creation patterns that may indicate exploitation attempts. Organizations should also consider implementing process monitoring to detect and prevent unauthorized file system modifications that could indicate successful exploitation of this vulnerability.