CVE-2026-27099 is a stored cross-site scripting (XSS) vulnerability affecting Jenkins versions 2.483 through 2.550 and LTS versions 2.492.1 through 2.541.1. The vulnerability stems from Jenkins failing to properly escape user-supplied input in the description field used when marking an agent as temporarily offline. Specifically, users with Agent/Configure or Agent/Disconnect permissions can inject malicious JavaScript code into the offline cause description. This malicious script is then stored on the Jenkins server and executed in the browsers of users who view the affected page, potentially leading to session hijacking, credential theft, or unauthorized actions within the Jenkins interface. The vulnerability requires the attacker to have specific agent-related permissions, limiting the attack surface to insiders or compromised accounts with these privileges. No public exploits have been reported yet, but the flaw could be leveraged in targeted attacks against Jenkins environments. The lack of a CVSS score indicates the need for an independent severity assessment. The vulnerability impacts confidentiality and integrity primarily, with availability less likely affected. The scope is limited to Jenkins instances with affected versions and relevant permissions granted. The vulnerability is stored XSS, meaning user interaction is required only to view the malicious content. The flaw highlights the importance of proper input validation and output encoding in web applications, especially in CI/CD tools that are critical to software supply chains.

For European organizations, the impact of CVE-2026-27099 can be significant due to Jenkins' widespread use in software development and continuous integration/continuous deployment (CI/CD) pipelines. Exploitation could allow attackers to execute arbitrary scripts in the context of Jenkins users, potentially leading to credential theft, session hijacking, or unauthorized pipeline manipulation. This could compromise the integrity of build processes, leading to the injection of malicious code into software releases. Confidentiality of sensitive project information and credentials stored or accessible via Jenkins could also be at risk. While the vulnerability requires specific agent permissions, insider threats or compromised accounts could exploit it to escalate privileges or move laterally within the development environment. Disruption of development workflows could result in operational delays and reputational damage. Given the critical role of Jenkins in software supply chains, exploitation could have cascading effects on downstream applications and services. European organizations with complex Jenkins setups and multiple agents are particularly vulnerable. The absence of known exploits suggests a window for proactive mitigation before active attacks emerge.

To mitigate CVE-2026-27099, European organizations should: 1) Monitor Jenkins advisories and apply security patches promptly once Jenkins releases updates addressing this vulnerability. 2) Restrict Agent/Configure and Agent/Disconnect permissions to the minimum necessary users, enforcing the principle of least privilege. 3) Audit existing agent permissions to identify and remediate any unnecessary or excessive privileges. 4) Implement web application firewalls (WAFs) with rules to detect and block suspicious script injections in Jenkins interfaces. 5) Employ Content Security Policy (CSP) headers in Jenkins to limit the execution of unauthorized scripts. 6) Educate Jenkins administrators and users about the risks of stored XSS and safe handling of input fields. 7) Regularly review Jenkins logs for unusual activities related to agent configuration or disconnection events. 8) Consider isolating Jenkins agents and servers within segmented network zones to limit lateral movement in case of compromise. 9) Use multi-factor authentication (MFA) for Jenkins accounts to reduce the risk of account takeover. 10) Evaluate the use of Jenkins plugins that enhance security and input validation capabilities.