CVE-2026-27099: Jenkins Stored XSS Vulnerability

CVE-2026-27099 Overview

CVE-2026-27099 is a stored Cross-Site Scripting (XSS) vulnerability affecting Jenkins automation server. The vulnerability exists because Jenkins does not properly escape user-provided descriptions in the "Mark temporarily offline" offline cause feature. Attackers with Agent/Configure or Agent/Disconnect permission can inject malicious scripts that persist in the application and execute in the context of other users' sessions when viewing the affected agent status.

Critical Impact

Authenticated attackers can exploit this stored XSS vulnerability to execute arbitrary JavaScript in victims' browsers, potentially leading to session hijacking, credential theft, or unauthorized actions within the Jenkins environment.

Affected Products

• Jenkins 2.483 through 2.550 (both inclusive)
• Jenkins LTS 2.492.1 through 2.541.1 (both inclusive)

Discovery Timeline

• 2026-02-18 - CVE-2026-27099 published to NVD
• 2026-02-18 - Last updated in NVD database

Technical Details for CVE-2026-27099

Vulnerability Analysis

This stored XSS vulnerability (CWE-79) stems from insufficient input sanitization in Jenkins' agent management functionality. When an administrator or user with appropriate permissions marks an agent as temporarily offline, they can provide a description explaining the reason for the offline status. Jenkins fails to properly escape HTML and JavaScript content within this description field before rendering it to other users.

The vulnerability is particularly dangerous because the malicious payload persists in the Jenkins environment. Any user who subsequently views the agent status page will have the injected script executed in their browser context. This could enable attackers to steal session cookies, perform actions on behalf of authenticated users, or exfiltrate sensitive data from the Jenkins instance.

Root Cause

The root cause is improper output encoding of user-controlled input. When the offline cause description is displayed in the Jenkins web interface, the application fails to apply appropriate HTML entity encoding or JavaScript escaping. This allows raw HTML and script tags to be interpreted as executable code by the browser rather than being displayed as plain text.

Attack Vector

The attack requires network access and low-privilege authentication to the Jenkins instance. An attacker must possess either Agent/Configure or Agent/Disconnect permission to exploit this vulnerability. The attack flow involves:

1. The attacker authenticates to Jenkins with an account having Agent/Configure or Agent/Disconnect permissions
2. The attacker navigates to an agent configuration and selects "Mark temporarily offline"
3. In the offline cause description field, the attacker injects malicious JavaScript payload
4. The payload is stored in Jenkins and rendered without sanitization
5. When other users (including administrators) view the agent status, the malicious script executes in their browser session

The exploitation does require user interaction—a victim must view the page containing the malicious payload. However, since agent status pages are commonly accessed during routine administration, the likelihood of exploitation is elevated.

Detection Methods for CVE-2026-27099

Indicators of Compromise

• Unusual script tags or JavaScript code present in agent offline cause descriptions
• Unexpected HTML entities or encoded payloads in agent configuration fields
• Browser console errors related to blocked scripts or Content Security Policy violations on agent pages
• Audit log entries showing frequent modifications to agent offline status by low-privilege users

Detection Strategies

• Review Jenkins audit logs for suspicious modifications to agent offline causes, particularly those containing <script>, javascript:, or event handler attributes like onerror or onload
• Implement Web Application Firewall (WAF) rules to detect and block common XSS payloads targeting Jenkins endpoints
• Configure Content Security Policy headers to mitigate the impact of successful XSS exploitation
• Use automated security scanning tools to identify stored XSS payloads within Jenkins configuration data

Monitoring Recommendations

• Enable comprehensive audit logging in Jenkins and forward logs to a SIEM for analysis
• Monitor for unusual user session behavior that may indicate session hijacking resulting from XSS exploitation
• Set up alerts for modifications to agent configurations by users who don't typically perform such actions
• Periodically scan Jenkins agent configurations for suspicious content patterns

How to Mitigate CVE-2026-27099

Immediate Actions Required

• Upgrade Jenkins to a version beyond 2.550 or LTS version beyond 2.541.1 that contains the security fix
• Review all existing agent offline cause descriptions for potentially malicious content
• Restrict Agent/Configure and Agent/Disconnect permissions to trusted users only
• Implement Content Security Policy headers as a defense-in-depth measure

Patch Information

Jenkins has released security updates addressing this vulnerability. Organizations should upgrade to patched versions as documented in the Jenkins Security Advisory #SECURITY-3669. The patch implements proper HTML escaping for the offline cause description field to prevent script injection.

Workarounds

• Limit Agent/Configure and Agent/Disconnect permissions to highly trusted users only until patching is complete
• Implement strict Content Security Policy headers to prevent inline script execution
• Deploy a Web Application Firewall configured to filter XSS payloads targeting Jenkins
• Consider temporarily disabling the "Mark temporarily offline" feature through plugin restrictions if feasible

# Example: Review Jenkins permissions to identify users with Agent/Configure or Agent/Disconnect
# Access Jenkins > Manage Jenkins > Configure Global Security > Authorization
# Review users and groups with Agent-related permissions and restrict as necessary