CVE-2021-21606 in Jenkins
Summary
by MITRE • 01/14/2021
Jenkins 2.274 and earlier, LTS 2.263.1 and earlier improperly validates the format of a provided fingerprint ID when checking for its existence allowing an attacker to check for the existence of XML files with a short path.
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Analysis
by VulDB Data Team • 02/14/2021
The vulnerability identified as CVE-2021-21606 affects Jenkins versions up to 2.274 and LTS versions up to 2.263.1, representing a critical validation flaw in the fingerprint ID handling mechanism. This issue stems from insufficient input sanitization within Jenkins' file existence checking functionality, specifically when processing XML files through the fingerprinting system. The vulnerability allows malicious actors to exploit a path traversal pattern that bypasses proper validation checks, enabling unauthorized enumeration of XML file paths within the Jenkins environment.
The technical flaw manifests in the improper validation of fingerprint ID formats during existence checks, where Jenkins fails to adequately sanitize user-provided input before processing file system queries. This validation gap creates a condition where attackers can craft specific fingerprint IDs that, when processed, reveal information about the existence of XML files in the system. The vulnerability operates by leveraging the way Jenkins handles file paths internally, where the system does not properly verify that the provided fingerprint ID corresponds to legitimate file system entries before performing existence checks.
From an operational perspective, this vulnerability poses significant risks to Jenkins environments as it enables attackers to perform reconnaissance activities without proper authentication. The ability to check for XML file existence provides threat actors with valuable information about the system structure, potentially revealing sensitive configuration files, build artifacts, or other XML-based data that could be exploited in subsequent attacks. This reconnaissance capability aligns with ATT&CK technique T1083 (File and Directory Discovery) and represents a precursor to more sophisticated attacks targeting Jenkins infrastructure.
The impact of this vulnerability extends beyond simple information disclosure, as it can facilitate privilege escalation and lateral movement within Jenkins environments. Attackers can use the reconnaissance data to identify critical XML files such as credentials.xml, config.xml, or job configuration files that may contain sensitive information. This weakness particularly affects organizations using Jenkins for continuous integration and deployment processes where XML files often contain build scripts, deployment configurations, and security credentials that could be targeted by adversaries.
Mitigation strategies for CVE-2021-21606 should prioritize immediate patching of affected Jenkins installations to versions 2.275 or later, where the fingerprint ID validation has been properly addressed. Organizations should also implement network segmentation and access controls to limit exposure of Jenkins instances to untrusted networks. Additional defensive measures include enabling Jenkins security features such as IP whitelisting, implementing proper authentication mechanisms, and conducting regular security audits of Jenkins configurations. The vulnerability demonstrates the importance of proper input validation and aligns with CWE-20, which addresses improper input validation issues that can lead to various security exploits including path traversal and information disclosure scenarios.
Organizations should also consider implementing monitoring solutions that can detect unusual file existence checking patterns or fingerprint ID validation attempts that may indicate exploitation attempts. Regular security training for Jenkins administrators and developers is crucial to prevent configuration errors that could exacerbate the impact of such vulnerabilities. The remediation process should include thorough testing of patched environments to ensure that the vulnerability has been properly addressed without introducing regressions in Jenkins functionality.