CVE-2026-57300: Jenkins MCP Server Plugin Missing Permission Check Vulnerability

Technical summary

The vulnerability is rooted in CWE-862 (Missing Authorization), wherein the MCP Server Plugin fails to validate permissions before returning Pipeline replay script contents to users. An authenticated user with Item/Read permission on a job can make requests to retrieve replay scripts without the plugin enforcing additional access controls. The CVSS v3.1 score of 4.3 (MEDIUM) reflects that exploitation requires prior authentication and does not impact integrity or availability—only confidentiality of the replay script data.

Business impact

For Jenkins deployments using MCP Server Plugin, this vulnerability exposes a data confidentiality risk. Organizations managing multi-team or multi-tenant Jenkins instances may find that job developers or operators can view replay script details for jobs outside their team's typical scope, potentially leaking process logic or hardcoded values. The impact is not critical because it requires pre-existing access and does not enable lateral movement or system compromise; however, it violates principle of least privilege and may violate compliance policies around data isolation.

Affected systems

Jenkins MCP Server Plugin version 0.177.v629fdb_2557fe and all earlier versions are affected. Users should verify their exact plugin version in Jenkins' Plugin Manager (Manage Jenkins > Plugin Manager > Installed tab, search for 'MCP Server'). Only Jenkins instances with the MCP Server Plugin installed and in use are at risk.

Exploitability

Exploitation requires the attacker to be an authenticated Jenkins user with Item/Read permission on at least one job. The attack is straightforward once access is gained: request the replay script via the MCP Server Plugin's API or UI. No special tools, complex exploitation chains, or authentication bypass are needed. However, the barrier to exploitation is non-trivial—the attacker must already have a valid Jenkins account and specific permissions, limiting this to insider threats or compromised accounts.

Remediation

Upgrade Jenkins MCP Server Plugin to a patched version released after June 2026. Check the Jenkins plugin repository or the plugin vendor's advisory for the fixed version number. Verify the patched version from the official Jenkins plugin update channel and apply the upgrade through Jenkins' built-in plugin manager or manual installation. After patching, restart Jenkins to load the corrected plugin code.

Patch guidance

1. Log into your Jenkins instance as an administrator. 2. Navigate to Manage Jenkins > Plugin Manager. 3. Search for 'MCP Server' in the Updates tab if a patch is available. 4. Click 'Download now and install after restart' or manually upload the patched .hpi file if obtained from Jenkins' official repository. 5. Restart Jenkins. 6. After restart, confirm the plugin version has been updated by checking Manage Jenkins > Plugin Manager > Installed tab and verifying the version number is newer than 0.177.v629fdb_2557fe. Always verify patches against the official Jenkins security advisory and the plugin's release notes.

Detection guidance

Review Jenkins audit logs and plugin interaction logs for unusual requests to the MCP Server Plugin's replay script endpoints, especially from users not typically associated with those jobs. If your Jenkins version supports it, enable verbose logging for the MCP Server Plugin and monitor for API calls retrieving replay script data. In a security investigation, audit which users have accessed replay scripts and cross-reference against their job-level permissions to identify out-of-scope access patterns.

Why prioritize this

Although CVSS 4.3 is medium severity, prioritization should reflect your organization's sensitivity to data leakage and your use of Pipeline replay features. If you use replay scripts to store environment-specific logic or credential references, or if your Jenkins environment is multi-tenant, prioritize this patch within 1–2 weeks. If replay scripts contain minimal sensitive data and your user base is homogeneous and trusted, it can be scheduled into the next normal maintenance window. In all cases, patching should precede major organizational changes that expand Jenkins user access.

Risk score, explained

CVSS 4.3 reflects a medium-severity confidentiality issue affecting only authenticated users with pre-existing permissions. The vector CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:L/I:N/A:N breaks down as: Network-accessible (AV:N), Low attack complexity (AC:L), requires Low privileges or prior authentication (PR:L), no user interaction needed (UI:N), Unchanged scope (S:U), Low confidentiality impact (C:L), no integrity or availability impact (I:N, A:N). The score avoids a higher rating because the attacker cannot modify data, cause denial of service, or escalate privileges—only read what they should not see.

Frequently asked questions