CVE-2017-9525 in Cron
Summary
by MITRE
In the cron package through 3.0pl1-128 on Debian, and through 3.0pl1-128ubuntu2 on Ubuntu, the postinst maintainer script allows for group-crontab-to-root privilege escalation via symlink attacks against unsafe usage of the chown and chmod programs.
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Analysis
by VulDB Data Team • 12/08/2022
The vulnerability identified as CVE-2017-9525 represents a critical privilege escalation flaw within the cron package ecosystem affecting Debian and Ubuntu distributions. This security issue stems from improper handling of file permissions during the package installation process, specifically within the postinst maintainer script that executes after package installation. The flaw allows malicious actors to exploit symbolic link attacks against the chown and chmod commands, creating a pathway for unauthorized users to gain root privileges. The vulnerability is particularly dangerous because it leverages the inherent trust placed in system administration scripts during package updates, making it difficult to detect and prevent through standard security measures.
The technical implementation of this vulnerability occurs when the postinst script processes crontab files without proper validation of symbolic link targets. When the script executes chown and chmod commands on files that could be manipulated through symbolic links, attackers can create malicious symlinks pointing to critical system files such as /etc/passwd or /etc/shadow. This unsafe usage pattern directly violates the principle of least privilege and creates a race condition where the attacker can manipulate file ownership and permissions before the system properly establishes them. The flaw is classified as a privilege escalation vulnerability under CWE-276, specifically addressing improper file permissions and insecure temporary file handling. The vulnerability operates at the system level and can be exploited by users with minimal privileges to achieve elevated access.
The operational impact of CVE-2017-9525 extends beyond simple privilege escalation, as it can be leveraged for persistent system compromise and data exfiltration. Attackers can use this vulnerability to establish backdoors, modify system configurations, or gain access to sensitive user data. The vulnerability affects all versions of the cron package through 3.0pl1-128 on Debian and 3.0pl1-128ubuntu2 on Ubuntu, representing a substantial attack surface across multiple system distributions. Security analysts should note that this vulnerability aligns with ATT&CK technique T1068, which describes the exploitation of local privilege escalation vulnerabilities, and T1548.001, covering abuse of system permissions for privilege escalation. The vulnerability's exploitation requires minimal technical skill and can be automated, making it particularly attractive to threat actors seeking persistent access to compromised systems.
Mitigation strategies for CVE-2017-9525 should focus on immediate package updates to versions that address the symbolic link handling issue. System administrators should implement comprehensive patch management procedures to ensure all affected systems receive timely updates. Additional protective measures include monitoring for unauthorized changes to crontab files and implementing file integrity monitoring solutions to detect malicious symlinks. The vulnerability highlights the importance of secure coding practices in system administration scripts, particularly around file operations and privilege handling. Organizations should also consider implementing privilege separation mechanisms and regularly auditing system permissions to prevent similar vulnerabilities from being exploited. The fix typically involves modifying the postinst script to use absolute paths and proper file validation before executing chown and chmod commands, thereby eliminating the race condition that enables the privilege escalation attack.