CVE-2019-9704 in Vixie cron
Summary
by MITRE
Vixie Cron before the 3.0pl1-133 Debian package allows local users to cause a denial of service (daemon crash) via a large crontab file because the calloc return value is not checked.
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Analysis
by VulDB Data Team • 07/31/2023
The vulnerability identified as CVE-2019-9704 affects Vixie Cron versions prior to 3.0pl1-133 in Debian packaging, representing a classic memory management flaw that can be exploited for denial of service attacks. This issue stems from inadequate error handling within the cron daemon's memory allocation routines, specifically concerning the calloc function which is responsible for allocating memory and initializing it to zero. The vulnerability manifests when a malicious local user submits a crontab file of substantial size, triggering the daemon to crash due to unhandled memory allocation failures.
The technical root cause of this vulnerability aligns with CWE-704, which categorizes improper handling of memory allocation failures as a significant security concern. In the affected versions of Vixie Cron, the code does not properly validate the return value from calloc calls, meaning that when memory allocation fails due to resource constraints or the size of the crontab file exceeding available memory, the program continues execution without proper error handling. This failure to check allocation results leads to undefined behavior and ultimately daemon termination, creating a denial of service condition that disrupts legitimate cron job scheduling operations.
From an operational perspective, this vulnerability presents a significant risk to system availability and service continuity, particularly in environments where cron jobs are critical for system maintenance, monitoring, and automated tasks. The impact extends beyond simple service disruption as it can affect the entire system's ability to execute scheduled operations, potentially leading to missed backups, security updates, and other time-sensitive administrative tasks. The local privilege requirement means that any user with access to modify crontab files can exploit this vulnerability, making it particularly dangerous in multi-user environments where privilege escalation or unauthorized access might occur.
The attack vector for this vulnerability is straightforward yet effective, requiring only local access to submit a maliciously crafted crontab file containing an excessive amount of data. This approach aligns with ATT&CK technique T1499.004, which covers network denial of service attacks through resource exhaustion. The vulnerability can be exploited in various scenarios including compromised user accounts, insider threats, or misconfigured access controls that allow unauthorized modifications to crontab files. System administrators should note that this issue affects the core cron daemon functionality rather than network services, making it particularly challenging to detect through traditional network monitoring approaches.
Mitigation strategies for CVE-2019-9704 primarily involve updating to the patched version of Vixie Cron 3.0pl1-133 or later, which properly implements memory allocation validation checks. Organizations should also implement additional controls such as monitoring crontab file modifications, implementing automated alerting for unusual cron activity, and establishing proper access controls to limit who can modify crontab entries. System hardening measures including limiting the maximum size of crontab files and implementing resource quotas for user processes can provide additional defense in depth. Regular vulnerability assessments and penetration testing should include verification of cron daemon configurations to ensure proper memory management practices are in place and that no similar allocation flaws exist in other system components. The fix implemented in the patched version demonstrates proper error handling practices that should be adopted across similar system utilities to prevent analogous vulnerabilities from manifesting in other software components.