CVE-2011-1075 in FreeBSD
Summary
by MITRE • 10/19/2021
FreeBSD's crontab calculates the MD5 sum of the previous and new cronjob to determine if any changes have been made before copying the new version in. In particular, it uses the MD5File() function, which takes a pathname as an argument, and is called with euid 0. A race condition in this process may lead to an arbitrary MD5 comparison regardless of the read permissions.
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Analysis
by VulDB Data Team • 10/23/2021
The vulnerability described in CVE-2011-1075 resides within FreeBSD's crontab utility implementation, specifically addressing how the system handles cron job modifications through MD5 checksum comparisons. This flaw manifests in the way crontab manages file integrity verification during update operations, creating a potential security risk that could be exploited by malicious actors. The system employs the MD5File() function to compute checksums of cron job files, using the effective user ID of root (euid 0) during these operations, which introduces a critical security consideration.
The core technical flaw stems from a race condition present in the crontab utility's file handling mechanism. When a user attempts to modify cron jobs, the system calculates MD5 hashes of both the existing and new cron job files to determine if any actual changes have occurred. This comparison process occurs before the new version is copied into place, but the race condition allows for manipulation of the file system state between the hash calculation and the actual file replacement. The vulnerability specifically affects the MD5File() function which accepts a pathname argument and executes with elevated privileges, making the race condition particularly dangerous.
This race condition enables attackers to perform arbitrary MD5 comparisons that bypass normal read permission checks, effectively allowing unauthorized modification of cron job configurations. The operational impact of this vulnerability extends beyond simple privilege escalation, as it could enable persistent access to compromised systems through scheduled task manipulation. Attackers could exploit this weakness to inject malicious code into cron jobs, potentially establishing backdoors or maintaining long-term system access. The vulnerability's severity is amplified by the fact that it operates at the system level where cron jobs typically execute with elevated privileges.
The security implications align with CWE-367, which addresses Time-of-Check to Time-of-Use (TOCTOU) vulnerabilities, and relates to ATT&CK technique T1053.003 for scheduled task/job hijacking. Organizations using FreeBSD systems should implement immediate mitigations including updating to patched versions of the operating system, implementing proper file access controls, and monitoring for unauthorized cron job modifications. Additionally, system administrators should consider restricting cron job permissions and implementing more robust file integrity checking mechanisms that eliminate the race condition by using atomic file operations or proper locking mechanisms. The vulnerability demonstrates the importance of careful privilege management and race condition prevention in system utilities that handle sensitive configuration files.