CVE-2019-9705 in Vixie cron
Summary
by MITRE
Vixie Cron before the 3.0pl1-133 Debian package allows local users to cause a denial of service (memory consumption) via a large crontab file because an unlimited number of lines is accepted.
Be aware that VulDB is the high quality source for vulnerability data.
Analysis
by VulDB Data Team • 07/31/2023
The vulnerability identified as CVE-2019-9705 affects Vixie Cron versions prior to 3.0pl1-133 in Debian package distributions, representing a significant denial of service weakness that can be exploited by local attackers to consume excessive system memory resources. This issue stems from the cron daemon's lack of proper input validation when processing crontab files, allowing malicious users to craft exceptionally large crontab entries that can overwhelm system memory allocation mechanisms. The flaw specifically manifests when the cron daemon accepts an unlimited number of lines in crontab files without imposing reasonable size constraints, creating a scenario where memory consumption grows uncontrollably during processing operations.
From a technical perspective, the vulnerability operates through the cron daemon's handling of crontab file parsing, where no upper limits are imposed on the number of lines that can be processed within a single crontab entry. This design flaw enables attackers to submit crontab files containing thousands or even millions of lines, which the daemon processes sequentially without memory bounds checking. The memory consumption grows linearly with the number of lines in the crontab file, potentially leading to system resource exhaustion that can cause the system to become unresponsive or crash entirely. This behavior constitutes a classic resource exhaustion attack pattern that can be classified under CWE-400, which specifically addresses unchecked resource consumption vulnerabilities.
The operational impact of this vulnerability extends beyond simple denial of service conditions, as it can severely compromise system availability and stability within environments where cron jobs are frequently used for automated tasks. Local users who can write to crontab files or modify existing crontab entries can exploit this weakness to consume system resources, potentially affecting other critical services running on the same system. The vulnerability is particularly concerning in multi-user environments where privilege escalation or unauthorized access to crontab modification capabilities could allow attackers to systematically consume memory resources across multiple processes. This type of attack aligns with ATT&CK technique T1499.001, which involves resource exhaustion via malicious cron jobs and scheduled tasks.
Mitigation strategies for CVE-2019-9705 primarily involve updating to the patched version of Vixie Cron (3.0pl1-133 or later) where the developers have implemented proper input validation and memory consumption limits for crontab file processing. System administrators should also implement additional monitoring and alerting mechanisms to detect unusual memory consumption patterns in cron daemon processes, which could indicate exploitation attempts. Network segmentation and privilege restrictions on crontab file access can reduce the attack surface by limiting which users can modify scheduled tasks. The patch addresses the root cause by introducing bounds checking on crontab file line counts and implementing memory consumption limits during parsing operations, effectively preventing the unlimited memory growth that previously occurred. Organizations should also consider implementing automated crontab file size monitoring and regular security audits to ensure that scheduled tasks do not inadvertently create resource consumption issues.