How to safely render user-generated content in Jenkins

Introduction

Rendering user-generated content in Jenkins can be a tricky affair, given Jenkins’ strong focus on Continuous Integration/Continuous Deployment (CI/CD), and the potential security risks involved. User-generated content can include job descriptions, parameter inputs, or script blocks that users input into Jenkins. It’s essential to implement strategies that keep Jenkins secure while rendering this content. This tutorial aims to guide Jenkins administrators and users through various methods to safely render user-generated content, from basic HTML escapes to sandboxing and content policy implementation.

Basic HTML Escaping

The foundation of safely rendering user-generated content lies in HTML escaping. This method ensures that any HTML tags or scripts embedded in the content are treated as strings rather than executable HTML or JavaScript. Let’s explore how you could implement HTML escaping in Jenkins:

String safeContent = Util.escapeHtml(unsafeContent);

In the above example, unsafeContent represents the user-generated content, and Util.escapeHtml() is a Jenkins method that converts potentially unsafe characters into their HTML-encoded equivalents. This is a basic but effective way to prevent XSS (Cross-Site Scripting) attacks.

Using Markup Formatter

Jenkins comes with a markup formatter that allows more fine-grained control over what HTML tags are allowed. By default, Jenkins uses the PlainTextFormatter, which strips out all HTML tags. However, you can switch to the SafeHtmlFormatter to allow a curated list of tags:

import hudson.markup.RawHtmlMarkupFormatter;

MarkupFormatter formatter = new RawHtmlMarkupFormatter(false);
Jenkins.getInstance().setMarkupFormatter(formatter);

This approach provides a balance between security and functionality, allowing certain HTML tags while blocking potentially dangerous ones.

Content Security Policy (CSP)

Implementing a strong Content Security Policy (CSP) is an advanced strategy for ensuring the safety of user-generated content. CSP allows you to specify which resources Jenkins pages are allowed to load or execute. To customize Jenkins CSP, you can edit the JENKINS_HOME/jenkins.model.Jenkins.security.realm.XFrameOptionsPageDecorator.xml file to include your CSP directives:

<?xml version="1.0" encoding="UTF-8"?>
<hudson.model.Descriptor_-Decorator>
  <policy>default-src 'self'; script-src 'self' 'unsafe-inline';</policy>
</hudson.model.Descriptor_-Decorator>

This CSP example restricts script sources to only include scripts from the same domain and allows inline scripts. Adapting CSP to your needs can significantly improve security when rendering user-generated content.

Script Security Plugin

For Jenkins, the Script Security Plugin is a critical tool for safely executing user-generated scripts. It automatically applies sandboxing to Groovy scripts (a common type of script used in Jenkins). Here’s how to leverage this plugin:

def script = new SecureGroovyScript(unsafeScript, false, null);
script.configuringWithKeyItem()
    .useSandbox(true);

This code snippet uses the SecureGroovyScript class to encapsulate the user-generated script in a sandbox. Scripts run in this manner have limited access to Java methods and Jenkins internals, providing a robust layer of security.

Conclusion

Safely rendering user-generated content in Jenkins requires a comprehensive strategy combining HTML escaping, use of markup formatters, implementation of CSP, and leveraging plugins like Script Security for sandboxing. By following these methods, Jenkins administrators can maintain a high level of security while accommodating the dynamic and versatile nature of user content.