CVE-2026-42521: Jenkins Plugin Information Disclosure

CVE-2026-42521 Overview

CVE-2026-42521 is an insecure deserialization vulnerability in the Jenkins Matrix Authorization Strategy Plugin affecting versions 2.0-beta-1 through 3.2.9. The vulnerability exists in the deserialization of inheritance strategies, where the plugin invokes parameterless constructors of classes specified in configuration without restricting which classes can be instantiated. This flaw allows attackers with Item/Configure permission to instantiate arbitrary types, potentially leading to information disclosure or other security impacts depending on the classes available on the classpath.

Critical Impact

Authenticated attackers with Item/Configure permission can exploit insecure deserialization to instantiate arbitrary classes, potentially leading to information disclosure or further system compromise depending on available classpath resources.

Affected Products

• Jenkins Matrix Authorization Strategy Plugin versions 2.0-beta-1 through 3.2.9 (inclusive)

Discovery Timeline

• 2026-04-29 - CVE-2026-42521 published to NVD
• 2026-04-29 - Last updated in NVD database

Technical Details for CVE-2026-42521

Vulnerability Analysis

This vulnerability is classified as CWE-502 (Deserialization of Untrusted Data). The Jenkins Matrix Authorization Strategy Plugin fails to implement proper restrictions when deserializing inheritance strategy configurations. During the deserialization process, the plugin blindly invokes parameterless constructors of classes specified in the configuration data without validating whether those classes are safe to instantiate.

The attack requires network access and low privilege (Item/Configure permission), but does not require user interaction. The primary impact is on integrity, as attackers can manipulate the instantiation of arbitrary objects within the Jenkins environment.

Root Cause

The root cause is insufficient class validation during deserialization of inheritance strategies. The plugin accepts class names from configuration input and directly instantiates them using parameterless constructors without maintaining an allowlist of permitted classes. This violates the principle of least privilege and allows untrusted input to control object instantiation.

Attack Vector

The vulnerability is exploitable over the network by authenticated users who possess the Item/Configure permission in Jenkins. An attacker can craft malicious configuration data that specifies arbitrary class names to be instantiated during the inheritance strategy deserialization process.

The exploitation mechanism involves:

1. An attacker with Item/Configure permission accesses the plugin's configuration interface
2. The attacker submits a malicious configuration containing references to dangerous classes available on the classpath
3. During deserialization, the plugin instantiates the specified classes via their parameterless constructors
4. Depending on the classes available on the Jenkins classpath, this could result in information disclosure, denial of service, or other security impacts

For detailed technical information about this vulnerability, refer to the Jenkins Security Advisory #2026-04-29.

Detection Methods for CVE-2026-42521

Indicators of Compromise

• Unusual configuration changes to Matrix Authorization Strategy Plugin settings
• Unexpected class instantiation errors in Jenkins logs related to inheritance strategies
• Access to plugin configuration endpoints by users who don't typically modify authorization settings
• Error messages indicating failed instantiation of non-standard classes during deserialization

Detection Strategies

• Monitor Jenkins audit logs for configuration changes to the Matrix Authorization Strategy Plugin
• Implement alerting on unusual Item/Configure permission usage patterns
• Review Jenkins system logs for deserialization exceptions or unexpected class loading
• Deploy application-level monitoring to detect anomalous class instantiation attempts

Monitoring Recommendations

• Enable detailed Jenkins audit logging for plugin configuration changes
• Implement SIEM rules to correlate Item/Configure permission usage with plugin configuration modifications
• Monitor for exploitation attempts by tracking unusual class loading patterns in Jenkins
• Review user permissions regularly to ensure Item/Configure access is appropriately restricted

How to Mitigate CVE-2026-42521

Immediate Actions Required

• Update Jenkins Matrix Authorization Strategy Plugin to a patched version (newer than 3.2.9)
• Review and restrict Item/Configure permissions to only trusted users
• Audit recent configuration changes to the Matrix Authorization Strategy Plugin
• Monitor Jenkins logs for signs of exploitation attempts

Patch Information

Jenkins has released a security update addressing this vulnerability. Organizations should upgrade the Matrix Authorization Strategy Plugin to a version newer than 3.2.9 as soon as possible. Detailed patch information is available in the Jenkins Security Advisory #2026-04-29.

Workarounds

• Restrict Item/Configure permissions to only essential, trusted users until patching is complete
• Implement network segmentation to limit access to Jenkins instances
• Enable additional audit logging to detect potential exploitation attempts
• Consider temporarily disabling the Matrix Authorization Strategy Plugin if not critical to operations