CVE-2000-1096 in Cron
Summary
by MITRE
crontab by Paul Vixie uses predictable file names for a temporary file and does not properly ensure that the file is owned by the user executing the crontab -e command, which allows local users with write access to the crontab spool directory to execute arbitrary commands by creating world-writeable temporary files and modifying them while the victim is editing the file.
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Analysis
by VulDB Data Team • 12/08/2024
The vulnerability described in CVE-2000-1096 represents a critical privilege escalation flaw in the crontab utility developed by Paul Vixie, which forms a fundamental component of Unix-like operating systems for scheduling automated tasks. This issue resides in the temporary file handling mechanism of crontab -e command, which is used to edit crontab files. The flaw stems from predictable filename generation for temporary files, creating a window of opportunity for malicious local users to exploit the system's trust model. When a user executes crontab -e, the system creates temporary files in the crontab spool directory to facilitate the editing process, but these temporary files are generated with predictable names that can be anticipated by attackers.
The technical implementation of this vulnerability exploits the principle of least privilege and proper file ownership verification within Unix security models. The crontab utility fails to validate that the temporary files it creates are exclusively owned by the user executing the command, instead relying on predictable naming conventions that allow attackers to pre-create files with the same names. This design flaw enables attackers with write access to the crontab spool directory to create world-writeable temporary files and subsequently modify them while a victim user is actively editing their crontab configuration. The vulnerability specifically targets the timing window between when the temporary file is created and when it is processed, allowing for arbitrary code execution with the privileges of the victim user.
The operational impact of CVE-2000-1096 extends beyond simple privilege escalation, as it can be leveraged to establish persistent access to compromised systems. An attacker can manipulate the temporary files to inject malicious commands that will execute whenever the victim user saves their crontab modifications, effectively creating a backdoor that persists across system reboots and user sessions. This vulnerability directly violates the security principle of file integrity and demonstrates a failure in proper temporary file management practices that aligns with common weakness patterns identified in CWE-377. The attack vector requires only local access with write permissions to the crontab spool directory, making it particularly dangerous in multi-user environments where users may have varying levels of system access.
Mitigation strategies for this vulnerability should focus on implementing proper file ownership verification and secure temporary file creation mechanisms. System administrators should ensure that temporary files are created with unique, unpredictable names and proper file permissions that prevent unauthorized modification. The recommended approach involves using secure temporary file creation functions that guarantee exclusive access and proper ownership, aligning with best practices from the Open Group's Base Specifications and industry standards for secure programming. Additionally, regular system auditing should verify that crontab spool directories maintain appropriate permissions and that no world-writeable files exist within these critical system areas. This vulnerability also highlights the importance of implementing defense-in-depth strategies and proper access controls, as outlined in the MITRE ATT&CK framework for privilege escalation techniques, particularly those involving file system manipulation and scheduled task exploitation.