CVE-2026-27100 is a security vulnerability affecting Jenkins versions 2.550 and earlier, including LTS 2.541.1 and earlier. The flaw arises from Jenkins accepting Run Parameter values that reference builds to which the submitting user does not have access rights. Specifically, users with Item/Build and Item/Configure permissions can submit build parameters that refer to other builds they are unauthorized to view. This allows them to confirm whether certain jobs or builds exist and retrieve the display names of those builds. While this does not grant access to the build artifacts or code, it leaks metadata that can be leveraged for further reconnaissance or social engineering attacks. The vulnerability is rooted in insufficient access control validation on parameter inputs related to build references. No CVSS score has been assigned yet, and no public exploits have been reported. The vulnerability requires authenticated users with at least Item/Build and Item/Configure permissions, which means it cannot be exploited by anonymous users. The flaw is primarily an information disclosure issue, potentially aiding attackers in mapping the Jenkins environment and identifying targets for more severe attacks. Jenkins is widely used in continuous integration and continuous delivery (CI/CD) pipelines, making this vulnerability relevant for organizations relying on automated build and deployment processes. The absence of patch links suggests that fixes may still be pending or in development. Organizations should monitor Jenkins advisories and prepare to apply updates promptly. The vulnerability highlights the importance of strict permission management and validation of user inputs in CI/CD tools.

For European organizations, the impact of CVE-2026-27100 centers on information disclosure within Jenkins environments. While the vulnerability does not allow direct compromise of build artifacts or code integrity, leaking job and build existence and display names can facilitate targeted attacks, such as spear phishing or privilege escalation attempts. Organizations with complex Jenkins setups and multiple teams may inadvertently expose sensitive project metadata to users who should not have that visibility. This can undermine confidentiality and increase the risk of insider threats or lateral movement within the network. The impact is more pronounced in environments where permissions are broadly assigned or not regularly audited. Since Jenkins is integral to software development and deployment, any compromise or reconnaissance advantage can disrupt development workflows and delay releases. European companies in sectors like finance, manufacturing, and technology that rely heavily on Jenkins for CI/CD pipelines could face operational risks if attackers use this information to plan further attacks. However, the requirement for authenticated users with specific permissions limits the scope to internal or trusted users, reducing the likelihood of external attackers exploiting this vulnerability directly. Overall, the impact is moderate but should not be underestimated in sensitive or high-value environments.

To mitigate CVE-2026-27100, European organizations should implement the following specific measures: 1) Conduct a thorough audit of Jenkins user permissions, ensuring that only trusted users have Item/Build and Item/Configure permissions, and apply the principle of least privilege rigorously. 2) Restrict the ability to submit Run Parameter values to users who absolutely require it, and consider disabling or limiting parameterized builds where feasible. 3) Monitor Jenkins logs for unusual parameter submissions or attempts to reference unauthorized builds, enabling early detection of exploitation attempts. 4) Implement network segmentation and access controls to limit Jenkins access to trusted internal users and systems. 5) Stay informed on Jenkins security advisories and apply patches or updates promptly once available for this vulnerability. 6) Consider deploying additional security controls such as Web Application Firewalls (WAFs) or runtime application self-protection (RASP) solutions that can detect and block suspicious parameter manipulation. 7) Educate Jenkins administrators and developers about secure configuration practices and the risks of excessive permissions. These targeted actions go beyond generic advice by focusing on permission hygiene, monitoring, and proactive defense tailored to this vulnerability’s characteristics.