CVE-2018-1999044 in Jenkins
Summary
by MITRE
A denial of service vulnerability exists in Jenkins 2.137 and earlier, 2.121.2 and earlier in CronTab.java that allows attackers with Overall/Read permission to have a request handling thread enter an infinite loop.
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Analysis
by VulDB Data Team • 03/17/2020
This vulnerability resides in the cron scheduling functionality of Jenkins continuous integration server, specifically within the CronTab.java file where the parsing and validation of cron expressions occurs. The flaw manifests when an attacker with minimal permissions - specifically Overall/Read access - can manipulate cron expression inputs to cause the system's request handling threads to enter an infinite loop. This represents a classic denial of service condition that can severely impact system availability and operational continuity. The vulnerability affects Jenkins versions up to 2.137 and 2.121.2, indicating a significant window of affected releases that organizations would need to address through patching or mitigation strategies.
The technical implementation of this vulnerability stems from inadequate input validation and processing logic within the cron tab parsing mechanism. When a maliciously crafted cron expression is submitted through the Jenkins web interface or API, the internal parsing routine fails to properly handle certain edge cases or malformed inputs that cause the thread to become trapped in iterative processing loops. This behavior directly violates the expected execution flow of the application and can lead to resource exhaustion as threads remain occupied indefinitely. The vulnerability can be classified under CWE-835 as an infinite loop without a valid exit condition, where the loop termination logic fails to properly evaluate the input parameters.
From an operational perspective, this vulnerability poses significant risks to continuous integration environments where Jenkins serves as a critical infrastructure component. Attackers can exploit this weakness to consume system resources and potentially bring down the entire Jenkins instance, affecting build pipelines, automated testing, and deployment workflows. The impact extends beyond simple service disruption as it can cause cascading failures in dependent systems that rely on Jenkins for their operational functions. The fact that this requires only Overall/Read permissions makes it particularly dangerous since it can be exploited by users who typically have limited access rights, potentially allowing for privilege escalation scenarios or resource exhaustion attacks that could affect other users or system components.
Organizations should implement immediate mitigations including updating to patched versions of Jenkins where available, as well as implementing input validation controls at the network level to restrict malformed cron expressions. The ATT&CK framework categorizes this as a resource exhaustion technique under T1499 where attackers consume system resources to prevent legitimate use of services. Additional defensive measures include monitoring for unusual thread behavior, implementing rate limiting on configuration updates, and ensuring proper access controls are in place to limit who can modify cron schedules. The vulnerability highlights the importance of validating user inputs in all system components and demonstrates how seemingly minor functionality can become a significant security risk when proper input sanitization and error handling are not implemented.