CVE-2026-25520: Comprehensive Security Analysis

Executive Summary

CVE-2026-25520 represents a critical sandbox escape vulnerability in SandboxJS, a JavaScript sandboxing library, with a maximum CVSS score of 10.0. This vulnerability allows attackers to break out of the sandbox environment and execute arbitrary code in the host context, completely defeating the security boundary the library is designed to enforce.

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1. Vulnerability Assessment and Severity Evaluation

Severity Classification

• CVSS Score: 10.0 (Critical)
• Vulnerability Type: Sandbox Escape / Security Boundary Bypass
• Attack Complexity: Low
• Privileges Required: None
• User Interaction: None

Technical Assessment

The vulnerability stems from improper wrapping of function return values within the sandbox environment. This fundamental flaw allows attackers to:

1. Access the host environment's Function constructor
2. Execute arbitrary JavaScript code outside the sandbox
3. Completely bypass all sandbox security controls

Why CVSS 10.0 is Justified:

• Complete compromise of confidentiality, integrity, and availability
• No authentication or special privileges required
• Low attack complexity with reliable exploitation
• Defeats the entire purpose of the security control
• Potential for complete system compromise depending on host environment privileges

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2. Attack Vectors and Exploitation Methods

Exploitation Technique

The vulnerability exploits JavaScript's prototype chain and object introspection capabilities:

// Conceptual exploitation path:

// Step 1: Create an object with functions in the sandbox
const obj = {
    func1: function() {},
    func2: function() {}
};

// Step 2: Use Object.values() to extract functions
const values = Object.values(obj);

// Step 3: Access the unwrapped Array from host context
// The returned Array is from the host environment, not wrapped

// Step 4: Use Array.prototype.at to get Function constructor
const hostFunction = values.at.constructor;

// Step 5: Execute arbitrary code in host context
hostFunction('return process')().mainModule.require('child_process').exec('malicious_command');

Attack Vectors

1. Web Application Exploitation

   • User-supplied code execution features (code playgrounds, formula evaluators)
   • Plugin/extension systems using SandboxJS
   • Educational platforms with code execution capabilities

2. Server-Side Exploitation

   • Node.js applications using SandboxJS for untrusted code execution
   • Serverless functions with dynamic code evaluation
   • API endpoints accepting and executing user code

3. Supply Chain Attacks

   • Compromised dependencies executing code in sandboxed environments
   • Malicious npm packages designed to escape sandbox restrictions

Real-World Exploitation Scenarios

Scenario 1: Online Code Editor

Attacker submits code to an online JavaScript playground
→ Code executes in SandboxJS environment
→ Exploit triggers sandbox escape
→ Attacker gains access to server filesystem/environment
→ Data exfiltration or further system compromise

Scenario 2: Plugin System

Application allows third-party plugins in sandbox
→ Malicious plugin uses exploit
→ Accesses host application data/credentials
→ Establishes persistence or lateral movement

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3. Affected Systems and Software Versions

Affected Versions

• SandboxJS versions: All versions prior to 0.8.29
• Patched Version: 0.8.29 and later

Potentially Affected Systems

1. Web Applications

   • Online IDEs and code playgrounds
   • Formula/expression evaluators
   • Template engines with code execution
   • Low-code/no-code platforms

2. Node.js Backend Services

   • API services with dynamic code execution
   • Workflow automation platforms
   • Testing/CI-CD environments
   • Serverless function platforms

3. Desktop Applications

   • Electron-based applications using SandboxJS
   • Development tools with plugin systems
   • Educational software

4. IoT and Embedded Systems

   • Node.js-based IoT devices with scripting capabilities
   • Edge computing platforms

Dependency Chain Risk

Applications may be indirectly affected through:

• Direct dependency on SandboxJS
• Transitive dependencies in npm packages
• Bundled libraries in larger frameworks

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4. Recommended Mitigation Strategies

Immediate Actions (Priority 1)

1. Update to Patched Version

   npm update sandboxjs@0.8.29
   # or
   yarn upgrade sandboxjs@0.8.29
   

2. Verify Installation

   npm list sandboxjs
   # Ensure version is 0.8.29 or higher
   

3. Emergency Workarounds (if immediate patching is impossible)

   • Disable features using SandboxJS for untrusted code
   • Implement additional input validation and filtering
   • Deploy Web Application Firewall (WAF) rules to detect exploitation attempts
   • Increase monitoring and logging for suspicious activities

Short-Term Mitigations (Priority 2)

1. Defense in Depth

   • Implement OS-level sandboxing (containers, VMs)
   • Use process isolation for code execution
   • Apply principle of least privilege to application runtime
   • Implement resource limits (CPU, memory, execution time)

2. Security Controls

   // Add additional validation layers
   const executeUserCode = (code) => {
       // Input validation
       if (containsSuspiciousPatterns(code)) {
           throw new Error('Suspicious code detected');
       }
       
       // Execute in isolated process
       const result = executeInIsolatedEnvironment(code);
       
       // Output sanitization
       return sanitizeOutput(result);
   };
   

3. Monitoring and Detection

   • Log all sandbox execution attempts
   • Monitor for unusual function calls or patterns
   • Implement anomaly detection for code execution behavior
   • Set up alerts for potential exploitation attempts

Long-Term Strategies (Priority 3)

1. Architecture Review

   • Evaluate necessity of client-side code execution
   • Consider alternative sandboxing solutions:
     • VM2 (with awareness of its own security history)
     • Isolated-vm
     • WebAssembly-based sandboxes
     • Browser-native sandboxing (iframes with proper CSP)

2. Security Development Lifecycle

   • Implement dependency scanning in CI/CD pipeline
   • Regular security audits of third-party libraries
   • Automated vulnerability scanning
   • Security testing for sandbox escape scenarios

3. Incident Response Preparation

   • Develop incident response plan for sandbox escapes
   • Create rollback procedures
   • Establish communication protocols for security incidents

Detection and Response

Indicators of Compromise (IoCs):

• Unusual access to Object.values, Object.entries, or Array.prototype.at in sandbox context
• Attempts to access constructor properties on built-in objects
• Unexpected system calls or file access from sandboxed code
• Anomalous network connections from application processes

Detection Rules (Pseudo-code):

ALERT if (
    sandbox_execution AND
    (access_to_constructor OR
     access_to_Function OR
     suspicious_prototype_chain_traversal)
)

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5. Impact on Cybersecurity Landscape

Broader Implications

1. JavaScript Sandbox Security Concerns

   • Highlights fundamental challenges in JavaScript sandboxing
   • Demonstrates difficulty of creating secure execution boundaries in dynamic languages
   • Raises questions about reliance on JavaScript-based sandboxing solutions

2. Supply Chain Security

   • Emphasizes risks of third-party security libraries
   • Demonstrates how security-critical dependencies can become attack vectors
   • Reinforces need for continuous dependency monitoring

3. Web Application Security

   • Affects trust model for user-generated code execution
   • Impacts online learning platforms, code playgrounds, and development tools
   • May drive adoption of more robust isolation mechanisms

Industry Impact

1. Affected Sectors

   • EdTech platforms with code execution features
   • SaaS platforms with scripting capabilities
   • Development tool vendors
   • Cloud computing providers offering code execution services

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