CVE-2009-2939 in postfix
Summary
by MITRE
The postfix.postinst script in the Debian GNU/Linux and Ubuntu postfix 2.5.5 package grants the postfix user write access to /var/spool/postfix/pid, which might allow local users to conduct symlink attacks that overwrite arbitrary files.
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Analysis
by VulDB Data Team • 08/23/2021
The vulnerability described in CVE-2009-2939 represents a critical privilege escalation issue within the postfix mail transfer agent package distribution on Debian and Ubuntu systems. This flaw exists in the post installation script (postinst) of the postfix package version 2.5.5, where the script incorrectly configures file permissions during the package installation process. The issue specifically affects the /var/spool/postfix/pid directory, which is created with overly permissive access controls that allow the postfix user to write to this location. The root cause stems from improper handling of directory creation and permission assignment within the package management workflow, creating a window of opportunity for local attackers to exploit the system's privilege structure.
The technical exploitation of this vulnerability relies on the principle of symbolic link manipulation and file system race conditions. When the postfix package is installed or upgraded, the postinst script creates the /var/spool/postfix/pid directory without proper security considerations. This allows the postfix user, who typically runs with limited privileges, to establish symbolic links within this directory structure. An attacker can then leverage these symbolic links to overwrite arbitrary files on the system, potentially targeting critical system files or configuration data. The vulnerability operates under CWE-276, which addresses improper file permissions, and specifically relates to CWE-59, which covers symlink attacks and race conditions. This type of attack falls under the ATT&CK technique T1059.007 for execution through command and script interpreters, and T1548.001 for abuse of system permissions.
The operational impact of CVE-2009-2939 extends beyond simple privilege escalation to encompass potential system compromise and data integrity violations. Local users who can execute the postinst script or manipulate the installation process gain the ability to overwrite critical system files, potentially leading to complete system compromise. The vulnerability is particularly dangerous because it occurs during legitimate package installation processes, making detection more difficult. Attackers can use this weakness to modify system binaries, configuration files, or even escalate privileges to root level access. The attack vector is relatively straightforward since it requires only local access and knowledge of the package installation process, making it accessible to both casual attackers and more sophisticated threat actors. This vulnerability demonstrates the importance of proper file system permission management and the risks associated with inadequate privilege separation in system administration scripts.
Mitigation strategies for CVE-2009-2939 must address both immediate remediation and long-term prevention of similar issues. The primary solution involves upgrading to patched versions of the postfix package where the postinst script properly handles directory creation and permission assignment. System administrators should implement strict monitoring of package installation activities and verify file permissions immediately after any postfix package updates. The recommended approach includes implementing proper directory creation with restrictive permissions, ensuring that only the necessary processes can write to critical system directories. Organizations should also establish security policies that prevent local users from executing package installation scripts without proper authorization. Additionally, regular security audits of system packages and their installation scripts should be conducted to identify and remediate similar permission-related vulnerabilities. The implementation of principle of least privilege concepts and proper access control lists can significantly reduce the risk of exploitation, aligning with ATT&CK tactics that focus on privilege escalation and lateral movement within compromised systems.