CVE-2026-84673 in Customizable Header Plugin
Summary
by MITRE • 09/02/2026
Jenkins Customizable Header Plugin 295.v2544b_ca_19b_97 and earlier allows overwriting the plugin's appearance configuration through Stapler data binding, allowing attackers to configure a custom SVG icon containing inline JavaScript, resulting in a stored cross-site scripting (XSS) vulnerability.
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Analysis
by VulDB Data Team • 09/03/2026
The Jenkins Customizable Header Plugin version 295.v2544b_ca_19b_97 and earlier contains a critical security flaw rooted in improper validation of user-supplied input during the configuration process. This vulnerability arises from the plugin's reliance on Stapler data binding, a framework feature that automatically maps HTTP request parameters to Java object properties. In this specific implementation, the system fails to adequately sanitize or restrict the types of content allowed within the fields used for customizing the plugin's appearance. Specifically, the application accepts and stores raw HTML input without sufficient filtering mechanisms to prevent the injection of executable scripts. This lack of robust input validation creates a direct pathway for attackers to manipulate the stored configuration data in ways that were not intended by the developers.
The operational impact of this flaw is severe, as it leads directly to a Stored Cross-Site Scripting vulnerability. An attacker with permission to modify the plugin's appearance settings can inject an SVG icon element that contains inline JavaScript code. Because Jenkins stores these configurations server-side and serves them back to users upon page load or interaction, the malicious script becomes persistent within the application context. When any user accesses a page where this customized header is rendered, their browser will execute the embedded JavaScript in the security context of the Jenkins instance. This allows the attacker to perform actions on behalf of the victim, such as stealing session cookies, capturing authentication tokens, or performing unauthorized administrative actions if the victim has elevated privileges.
From an industry standards perspective, this vulnerability aligns with CWE-79, which describes Improper Neutralization of Input During Web Page Generation commonly known as Cross-site Scripting. The specific mechanism involves storing malicious script in a database or configuration file and executing it later when retrieved by users. Furthermore, the exploitation technique maps to MITRE ATT&CK techniques related to Client-Side Injection and potentially Account Manipulation if used for persistence or privilege escalation within the Jenkins environment. The use of Stapler data binding without proper type checking or content validation is a common pattern in Java-based web frameworks that can lead to such injection flaws when developers assume automatic handling provides sufficient security, which it does not in this case due to the permissive nature of HTML rendering engines.
Mitigation for this vulnerability requires immediate action by system administrators and plugin maintainers. The primary remediation step is to upgrade the Jenkins Customizable Header Plugin to a version that includes proper input sanitization and output encoding mechanisms. Developers must ensure that any user-controlled data rendered in an HTML context is either strictly validated against a whitelist of allowed characters or encoded using appropriate libraries such as OWASP Java Encoder Project to prevent script execution. Additionally, implementing Content Security Policy headers can provide an additional layer of defense by restricting the sources from which scripts are allowed to execute, thereby mitigating the impact even if some injection attempts succeed. Regular security audits and static code analysis focused on data binding configurations should be conducted to identify similar vulnerabilities in other plugins or custom Jenkins extensions before they can be exploited in production environments.