EUVD-2026-5589: Critical Sandbox Escape Vulnerability Analysis

Executive Summary

EUVD-2026-5589 represents a critical severity sandbox escape vulnerability in SandboxJS, a JavaScript sandboxing library. With a CVSS 3.1 base score of 10.0 (Critical), this vulnerability enables complete sandbox escape through type confusion and object coercion exploitation, allowing arbitrary code execution outside the intended security boundary.

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1. Vulnerability Assessment and Severity Evaluation

Severity Classification

• CVSS 3.1 Score: 10.0/10.0 (Critical)
• Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:C/C:H/I:H/A:H

CVSS Breakdown Analysis

Metric	Value	Implication
Attack Vector (AV:N)	Network	Exploitable remotely without physical access
Attack Complexity (AC:L)	Low	No specialized conditions required for exploitation
Privileges Required (PR:N)	None	No authentication needed
User Interaction (UI:N)	None	Fully automated exploitation possible
Scope (S:C)	Changed	Impact extends beyond the vulnerable component
Confidentiality (C:H)	High	Complete information disclosure possible
Integrity (I:H)	High	Complete data manipulation possible
Availability (A:H)	High	Complete system disruption possible

Risk Assessment

This vulnerability represents a complete failure of the security boundary that SandboxJS is designed to enforce. The maximum CVSS score reflects:

• Total sandbox escape capability
• No prerequisites for exploitation
• Network-accessible attack surface
• Scope change indicating compromise beyond the sandbox

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2. Potential Attack Vectors and Exploitation Methods

Technical Vulnerability Mechanism

The vulnerability stems from a Time-of-Check to Time-of-Use (TOCTOU) race condition combined with JavaScript object coercion abuse:

Root Cause

// Vulnerable pattern (conceptual)
if (hasOwnProperty(key)) {  // Check: key coerces to string "safe"
    return object[key];      // Use: key coerces to string "__proto__"
}

The vulnerability exploits the mismatch between:

1. Validation phase: Key is checked using hasOwnProperty(key)
2. Access phase: Same key object is used for property access

Exploitation Technique

Attackers can craft malicious objects with custom toString() or valueOf() methods that return different values depending on context:

// Proof-of-Concept exploitation pattern
const maliciousKey = {
    _toggle: true,
    toString: function() {
        // Return different values on alternating calls
        this._toggle = !this._toggle;
        return this._toggle ? "safeProperty" : "__proto__";
    }
};

// First call (validation): returns "safeProperty" - passes check
// Second call (access): returns "__proto__" - escapes sandbox

Attack Vectors

1. Prototype Pollution

   • Access __proto__, constructor, or prototype properties
   • Modify Object.prototype to inject malicious properties globally
   • Achieve arbitrary code execution in parent context

2. Direct Property Access

   • Access restricted internal properties
   • Bypass security checks and access control mechanisms
   • Read sensitive configuration or credentials

3. Function Constructor Access

   • Obtain reference to Function constructor
   • Execute arbitrary JavaScript outside sandbox
   • Break out to Node.js environment (if applicable)

4. Module System Exploitation

   • Access require() in Node.js environments
   • Import native modules (fs, child_process, etc.)
   • Execute system commands

Exploitation Scenarios

Scenario 1: Web Application Context

Attacker → Malicious Input → SandboxJS → Prototype Pollution → 
XSS/RCE in Parent Application

Scenario 2: Server-Side Execution

Attacker → API Request → Sandboxed Code Execution → 
Sandbox Escape → System Command Execution

Scenario 3: Plugin/Extension Systems

Malicious Plugin → Sandboxed Environment → Escape → 
Access to Host Application Resources

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3. Affected Systems and Software Versions

Directly Affected

• Product: SandboxJS (nyariv/SandboxJS)
• Vulnerable Versions: All versions < 0.8.29
• Fixed Version: 0.8.29 and later
• Vendor: nyariv (GitHub)

Dependency Chain Impact

Organizations should identify systems using SandboxJS:

1. Direct Dependencies

   • Applications explicitly importing SandboxJS
   • Custom sandboxing implementations

2. Transitive Dependencies

   • Frameworks incorporating SandboxJS
   • Build tools and development environments
   • Testing frameworks using sandboxed execution

3. Deployment Contexts

   • Node.js server applications
   • Browser-based applications
   • Serverless/Lambda functions
   • Content Management Systems (CMS)
   • Low-code/no-code platforms
   • Online code editors and REPLs
   • Educational platforms with code execution

Detection Methods

Package.json Analysis

# Check for vulnerable versions
npm list sandboxjs
yarn why sandboxjs

# Search for version constraints
grep -r "sandboxjs.*0\.[0-7]\." package.json
grep -r "sandboxjs.*0\.8\.[0-2][0-9]" package.json

Runtime Detection

// Check installed version
const version = require('sandboxjs/package.json').version;
console.log('SandboxJS version:', version);

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4. Recommended Mitigation Strategies

Immediate Actions (Priority 1 - Critical)

1. Update to Patched Version

# NPM
npm update sandboxjs@latest
npm install sandboxjs@^0.8.29

# Yarn
yarn upgrade sandboxjs@^0.8.29

# Verify installation
npm list sandboxjs

2. Emergency Workarounds (if immediate patching impossible)

Input Sanitization

// Ensure all keys are primitive strings
function sanitizeKey(key) {
    if (typeof key !== 'string') {
        return String(key); // Force immediate coercion
    }
    return key;
}

// Validate keys before use
function safePropertyAccess(obj, key) {
    const sanitized = sanitizeKey(key);
    // Perform additional validation
    if (sanitized.startsWith('__') || 
        sanitized === 'constructor' || 
        sanitized === 'prototype') {
        throw new Error('Forbidden property access');
    }
    return obj[sanitized];
}

Disable Sandboxed Code Execution

• Temporarily disable features relying on SandboxJS
• Implement alternative security controls
• Queue untrusted code for manual review

3. Network Segmentation

• Isolate systems running vulnerable versions
• Implement strict firewall rules
• Monitor for exploitation attempts

Medium-Term Actions (Priority 2)

1. Dependency Management

// package.json - Pin to safe version
{
  "dependencies": {
    "sandboxjs": ">=0.8.29"
  }
}

2. Automated Vulnerability Scanning

# Integrate into CI/CD pipeline
npm audit
yarn audit
snyk test

3. Security Testing

• Conduct penetration testing on sandboxed environments
• Implement fuzzing for input validation
• Review all code paths using SandboxJS

Long-Term Strategic Measures (Priority 3)

1. Defense in Depth

Implement multiple security layers:

// Multi-layer security approach
class SecureSandbox {
    constructor()