CVE-2026-84649 in Jenkins
Summary
by MITRE • 09/02/2026
In Stapler 1839.ved17667b_a_eb_5 through 2107.v8dfcb_e8ed317 (both inclusive), except 2088.2093.vd7c3e58008a_6, included in Jenkins 2.447 through 2.579 (both inclusive), LTS 2.452.1 through 2.568.2 (both inclusive), an HTTP endpoint serving dynamically generated JavaScript resources embeds the user's cross-site request forgery (CSRF) token (crumb) as a string literal, allowing attackers with control over a page hosted on the same site as Jenkins to obtain a valid crumb for the targeted user's session and perform actions on their behalf.
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Analysis
by VulDB Data Team • 09/02/2026
The vulnerability identified in Stapler versions 1839 through 2107, which are integral components of Jenkins releases ranging from version 2.447 to 2.579 as well as Long Term Support (LTS) branches 2.452.1 through 2.568.2, represents a critical flaw in the handling of Cross-Site Request Forgery protections. Stapler serves as the core web framework for Jenkins, managing HTTP requests and responses. The specific issue resides within an HTTP endpoint responsible for serving dynamically generated JavaScript resources. In these affected versions, the implementation fails to properly isolate or randomize security tokens when embedding them into client-side scripts. Instead of utilizing secure mechanisms such as httpOnly cookies combined with proper SameSite attributes or dynamic token generation per request context that is not directly exposed in plain text within script literals accessible via cross-origin reads, the system embeds the user's CSRF crumb directly as a string literal within the JavaScript code served by this endpoint. This architectural decision creates a direct vector for attackers who can host malicious content on any page sharing the same origin or domain context with Jenkins to extract sensitive authentication data.
The technical flaw centers on the exposure of the CSRF token, known in Jenkins terminology as the crumb, through client-side script execution. When a user interacts with Jenkins, their session is protected by this unique crumb which must be included in subsequent state-changing requests to prevent unauthorized actions. However, because Stapler embeds this value directly into JavaScript files that are served over HTTP or HTTPS without sufficient restrictions on how they can be accessed by other scripts, an attacker leveraging a Cross-Site Scripting (XSS) vulnerability or any mechanism allowing script injection within the same origin can read this variable. Once obtained, the crumb is no longer secret to the browser's security model regarding that specific page context. The attacker can then use this valid token to forge requests against Jenkins on behalf of the victim user. This bypasses the CSRF protection mechanisms intended to ensure that actions are initiated by legitimate users rather than automated scripts from malicious sites.
The operational impact of this vulnerability is severe, as it effectively nullifies one of the primary defenses against unauthorized state changes in web applications. An attacker who gains control over a page hosted on the same site or domain as Jenkins can exploit this flaw to perform arbitrary actions within the Jenkins instance using the victim's session credentials and CSRF token. This includes but is not limited to creating new users with administrative privileges, modifying job configurations, triggering builds that may execute malicious code in build environments, deleting critical projects, or altering system settings. The ability to act on behalf of a user implies full compromise of their access level within Jenkins. If the targeted user has elevated permissions, such as an administrator account, the attacker gains complete control over the CI/CD pipeline infrastructure. This can lead to supply chain attacks where malicious code is injected into software builds distributed by the organization, compromising downstream systems and applications dependent on those builds.
This vulnerability aligns with CWE-352, which describes Cross-Site Request Forgery (CSRF), specifically highlighting the failure to protect against CSRF via improper token handling or exposure of tokens in client-side scripts that are accessible to other origins. Furthermore, from an offensive security perspective as defined by MITRE ATT&CK, this exploitation technique falls under T1098, which covers Account Manipulation and potentially T1562, Impair Defenses, as it undermines the integrity checks designed to prevent unauthorized actions. The attack vector relies on the attacker having control over a page within the same origin context, often achieved through stored or reflected XSS vulnerabilities in other plugins or components of Jenkins that are not patched by this specific Stapler update alone. Therefore, while fixing the Stapler component is necessary, it must be part of a broader remediation strategy that addresses all potential entry points for script injection.
Mitigation strategies primarily involve upgrading to unaffected versions of Stapler and Jenkins where this embedding behavior has been corrected or removed. For organizations unable to immediately upgrade due to compatibility constraints, applying strict Content Security Policy (CSP) headers can help mitigate the risk by restricting how scripts are executed and preventing unauthorized access to sensitive variables within JavaScript contexts. Specifically, policies that prevent inline script execution without nonces or hashes can reduce the likelihood of an attacker successfully injecting code that reads these tokens. Additionally, ensuring that Jenkins instances do not host any user-controlled content on the same domain is crucial. If possible, segregating Jenkins traffic onto a subdomain with appropriate SameSite cookie attributes and restricting access to administrative functions through multi-factor authentication adds layers of defense. Regular audits for XSS vulnerabilities across all installed plugins are essential to prevent the initial script injection that enables this CSRF token theft.