Prototype Pollution in Chi

Prototype Pollution in Chi occurs when untrusted user input can modify JavaScript object prototypes, leading to security vulnerabilities that affect the entire application. Chi, being a lightweight, fast, and modular router for Node.js, is particularly vulnerable when handling dynamic route parameters and middleware that process user-supplied data without proper sanitization.

The most common attack vector in Chi involves malicious query parameters or body data that contain special characters like __proto__, constructor, or prototype. When Chi processes these inputs without validation, an attacker can inject properties into Object.prototype, affecting all objects in the application.

// Vulnerable Chi route handler
app.post('/api/users', (ctx, next) => {
  const userData = ctx.request.body;
  // No validation of property names
  processUserData(userData);
});

// Malicious payload:
// {
//   "__proto__": {
//     "isAdmin": true
//   }
// }

This vulnerability becomes particularly dangerous in Chi applications that use middleware chains or shared objects. Once an attacker pollutes the prototype, subsequent requests can inherit malicious properties, leading to privilege escalation, data exfiltration, or complete application compromise.

Another Chi-specific manifestation occurs in route parameter handling. Chi's flexible routing system can inadvertently expose prototype pollution vectors when route parameters are used to construct objects or when dynamic routing collides with prototype property names.

// Vulnerable route with dynamic parameters
app.get('/api/items/:id', (ctx, next) => {
  const params = ctx.params;
  // If :id is '__proto__', prototype pollution occurs
  const item = getItemFromDatabase(params.id);
  // ... processing continues
});

The impact extends beyond the immediate route handler. In Chi applications that use shared middleware or global objects, prototype pollution can affect authentication checks, authorization logic, and data validation across the entire API surface.

Detecting Prototype Pollution in Chi applications requires both manual code review and automated scanning. The key is to identify all points where user input flows into object construction or property assignment without proper validation.

Manual detection should focus on Chi's request handling patterns. Look for middleware that directly assigns request properties to objects, route handlers that merge user input with existing data structures, and any dynamic property assignment based on user input.

// Code patterns to flag for manual review
// 1. Direct property assignment from request
const userData = { ...ctx.request.body };

// 2. Object merging without validation
const merged = Object.assign({}, defaultConfig, ctx.request.body);

// 3. Dynamic property access
const propName = ctx.query.property;
const value = obj[propName];

Automated detection with middleBrick provides comprehensive coverage for Chi applications. middleBrick's black-box scanning approach tests the unauthenticated attack surface, sending payloads designed to trigger prototype pollution in Chi's request processing pipeline.

The scanner specifically targets Chi's route parameter handling, query string parsing, and body parsing mechanisms. middleBrick sends crafted payloads containing __proto__, constructor, and other prototype pollution vectors, then analyzes the application's response for indicators of successful exploitation.

middleBrick's LLM/AI Security module also includes specialized checks for prototype pollution in AI-powered endpoints, testing whether malicious payload injection affects AI model behavior or response generation in Chi applications that serve AI APIs.

For continuous monitoring, the middleBrick GitHub Action can be integrated into your CI/CD pipeline to automatically scan Chi applications before deployment. This ensures that any new prototype pollution vulnerabilities introduced during development are caught early.

Remediating Prototype Pollution in Chi applications requires a defense-in-depth approach that combines input validation, safe object handling, and secure coding practices specific to Chi's architecture.

The first line of defense is strict input validation using Chi's built-in middleware capabilities. Implement validation middleware that rejects requests containing dangerous property names before they reach your route handlers.

const validateInput = async (ctx, next) => {
  const dangerousProps = ['__proto__', 'constructor', 'prototype'];
  const bodyKeys = Object.keys(ctx.request.body || {});
  const queryKeys = Object.keys(ctx.query || {});
  
  const hasDangerousProps = [...bodyKeys, ...queryKeys].some(key => 
    dangerousProps.includes(key) || key.includes('__proto__')
  );
  
  if (hasDangerousProps) {
    ctx.status = 400;
    ctx.body = { error: 'Invalid input detected' };
    return;
  }
  
  await next();
};

// Apply globally to all routes
app.use(validateInput);

For object construction and merging operations, use safe alternatives to Object.assign and spread operators. Chi applications should implement property whitelisting to ensure only expected properties are processed.

const safeMerge = (target, source, allowedProps) => {
  const result = { ...target };
  for (const key of allowedProps) {
    if (key in source) {
      result[key] = source[key];
    }
  }
  return result;
};

// Usage in Chi route
app.post('/api/users', async (ctx, next) => {
  const allowed = ['name', 'email', 'age'];
  const userData = safeMerge({}, ctx.request.body, allowed);
  await processUserData(userData);
});

Chi's modular architecture allows for centralized security middleware. Create a security layer that wraps all route handlers, providing consistent protection against prototype pollution across your entire API.

For applications using Chi with TypeScript, leverage type safety to prevent prototype pollution at compile time. Define strict interfaces for request bodies and use TypeScript's strict mode to catch unsafe operations.

interface UserInput {
  name: string;
  email: string;
  age?: number;
}

app.post('/api/users', async (ctx: Context) => {
  const body = ctx.request.body as UserInput;
  // TypeScript ensures only defined properties are accessed
  await processUserData(body);
});

Regular security scanning with middleBrick should be part of your development workflow. The CLI tool allows developers to scan local Chi applications during development, while the GitHub Action ensures production deployments are free from prototype pollution vulnerabilities.