CVE-2026-25587: Comprehensive Technical Analysis

Executive Summary

CVE-2026-25587 represents a critical sandbox escape vulnerability in SandboxJS, a JavaScript sandboxing library, with a maximum CVSS score of 10.0. The vulnerability allows attackers to break out of the intended security boundary by exploiting prototype manipulation of the Map object, potentially leading to arbitrary code execution in the host environment.

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1. Vulnerability Assessment and Severity Evaluation

Severity Classification

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

Technical Assessment

The vulnerability stems from a fundamental design flaw in SandboxJS's security model:

Root Cause:

• The Map object is included in the SAFE_PROTOTYPES whitelist
• This classification incorrectly assumes Map.prototype methods are immutable
• Attackers can access Map.prototype and overwrite critical methods like has()
• The sandbox relies on Map.prototype.has for security checks, creating a trust boundary violation

Severity Justification: The 10.0 CVSS score is warranted because:

• Complete sandbox escape is achievable
• No authentication or special privileges required
• Exploitation can be performed remotely if the sandbox processes untrusted input
• Full compromise of confidentiality, integrity, and availability
• Affects the fundamental security guarantee of the library

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2. Attack Vectors and Exploitation Methods

Primary Attack Vector

Prototype Poisoning Attack:

// Attacker-controlled code within sandbox
Map.prototype.has = function() {
    // Malicious implementation
    return true; // or false, depending on bypass strategy
};

// Alternative: Replace with function that executes arbitrary code
Map.prototype.has = function(key) {
    // Escape sandbox and execute in host context
    this.constructor.constructor('return this')().eval('malicious_code');
    return true;
};

Exploitation Scenarios

Scenario 1: Direct Sandbox Escape

1. Attacker submits code to be executed in SandboxJS environment
2. Code overwrites Map.prototype.has with malicious implementation
3. Sandbox security checks that rely on Map operations are bypassed
4. Attacker gains access to host JavaScript context
5. Full system compromise possible depending on host environment privileges

Scenario 2: Supply Chain Attack

1. Malicious dependency includes prototype pollution payload
2. Code executes within sandboxed environment
3. Sandbox escape occurs during initialization
4. Persistent backdoor established in application

Scenario 3: User-Generated Content Exploitation

1. Web application uses SandboxJS to execute user scripts safely
2. Attacker submits script with prototype manipulation
3. Sandbox escape allows access to application context
4. Session hijacking, data exfiltration, or further exploitation possible

Technical Exploitation Chain

// Step 1: Obtain Map prototype reference
const mapProto = Map.prototype;

// Step 2: Store original function (optional, for stealth)
const originalHas = mapProto.has;

// Step 3: Overwrite with malicious implementation
mapProto.has = function(key) {
    // Gain access to global context
    const global = this.constructor.constructor('return this')();
    
    // Execute arbitrary code in host environment
    global.require('child_process').exec('malicious_command');
    
    // Return expected value to avoid detection
    return originalHas.call(this, key);
};

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3. Affected Systems and Software Versions

Directly Affected

• SandboxJS versions: All versions prior to 0.8.29
• Package Manager: npm package sandboxjs

Potentially Affected Environments

Server-Side Applications:

• Node.js applications using SandboxJS for:
  • User script execution
  • Plugin systems
  • Template engines
  • Serverless function isolation
  • Multi-tenant code execution

Client-Side Applications:

• Browser-based applications implementing:
  • Online code editors/playgrounds
  • Educational coding platforms
  • No-code/low-code platforms
  • Browser extensions with script isolation

Specific Use Cases at Risk:

• Content Management Systems with custom scripting
• API gateways with transformation scripts
• Workflow automation platforms
• Data processing pipelines with user-defined functions
• Game engines with modding support

Dependency Chain Risk

Applications indirectly affected through dependencies:

• Any package depending on vulnerable SandboxJS versions
• Frameworks incorporating SandboxJS for security features
• Development tools using SandboxJS for safe code evaluation

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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 0.8.29 or higher

3. Emergency Workaround (if immediate update impossible)

// Freeze Map prototype before sandbox initialization
Object.freeze(Map.prototype);
Object.freeze(Set.prototype);
Object.freeze(WeakMap.prototype);
Object.freeze(WeakSet.prototype);

// Note: This may break legitimate functionality

Short-Term Mitigations (Priority 2)

1. Input Validation and Sanitization

• Implement static analysis on code before sandbox execution
• Block or flag code containing prototype manipulation patterns
• Use AST parsing to detect suspicious operations

// Example detection pattern
const dangerousPatterns = [
    /Map\.prototype/,
    /Set\.prototype/,
    /\.prototype\s*=/,
    /Object\.defineProperty.*prototype/
];

2. Defense in Depth

• Implement additional isolation layers (VM, containers)
• Use process isolation for untrusted code execution
• Apply principle of least privilege to sandbox host environment
• Implement resource limits and monitoring

3. Runtime Monitoring

// Monitor for prototype modifications
const originalHas = Map.prototype.has;
Object.defineProperty(Map.prototype, 'has', {
    set: function(value) {
        console.error('SECURITY: Attempt to modify Map.prototype.has');
        // Alert security team
        // Terminate execution
    },
    get: function() {
        return originalHas;
    }
});

Long-Term Strategies (Priority 3)

1. Architecture Review

• Evaluate necessity of client-side code sandboxing
• Consider alternative sandboxing solutions:
  • VM2 (with awareness of its own security history)
  • Isolated-vm
  • WebAssembly-based sandboxes
  • OS-level containerization (Docker, gVisor)

2. Security Hardening

• Implement Content Security Policy (CSP) for browser contexts
• Use Subresource Integrity (SRI) for all dependencies
• Establish dependency pinning and verification processes
• Implement automated vulnerability scanning in CI/CD

3. Incident Response Preparation

• Develop runbooks for sandbox escape scenarios
• Implement logging and alerting for suspicious sandbox behavior
• Create rollback procedures for rapid response
• Conduct tabletop exercises for breach scenarios

Verification and Testing

Post-Mitigation Validation:

// Test that prototype modification is prevented
try {
    Map.prototype.has = function() { return true; };
    console.error('FAIL: Prototype modification succeeded');
} catch(e) {
    console.log('PASS: Prototype modification blocked');
}

// Verify sandbox functionality
// Run test suite to ensure no regression

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5. Impact on Cybersecurity Landscape

Industry-Wide Implications

1. JavaScript Sandbox Security Paradigm Shift

• Highlights fundamental challenges in JavaScript sandboxing
• Demonstrates that prototype chain is an attack surface often overlooked
• Questions viability of pure JavaScript sandboxing solutions
• May accelerate adoption of WebAssembly for isolation

2. Supply Chain Security Concerns

• Reinforces risks of depending on security