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Fix Improper Error Handling in LoopBack

LoopBack's default error responses are a goldmine for recon. Leaking stack traces, internal file paths, and database schema details via raw Error objects is an invitation for exploitation. To harden the application, you must intercept the response pipeline using a custom Reject Provider to sanitize every outbound error payload.

The Vulnerable Pattern

import {get, param} from '@loopback/rest';

export class UserController {
@get(‘/users/{id}’)
async findById(@param.path.string(‘id’) id: string): Promise {
try {
return await this.userRepository.findById(id);
} catch (err) {
// VULNERABILITY: Throwing raw error leaks stack traces and DB internals in dev mode
// and provides too much detail in production.
throw err;
}
}
}

The Secure Implementation

The fix moves error handling from individual controllers to a centralized 'Reject Provider'. This provider intercepts all exceptions before they are serialized to the client. By mapping internal errors to a generic structure and explicitly stripping properties like 'stack' and 'details', you prevent information disclosure. The logic ensures that 500-level errors return a non-descript message, while 400-level errors remain functional for the API consumer without exposing the underlying infrastructure.

import {Reject, HandlerContext, RestBindings} from '@loopback/rest';
import {Provider} from '@loopback/core';

export class CustomRejectProvider implements Provider {
value() {
return (context: HandlerContext, error: Error) => {
const {response} = context;
const err = error as any;


  const statusCode = err.statusCode || err.status || 500;
  
  // Log the full error internally for debugging
  console.error(`[Internal Error] ${err.stack || err}`);

  // Sanitize response: No stack traces, no internal paths
  const publicError = {
    error: {
      message: statusCode === 500 ? 'Internal Server Error' : err.message,
      code: err.code || 'INTERNAL_ERROR',
      status: statusCode
    }
  };

  response.status(statusCode).send(publicError);
};


}
}


// In application.ts:
// this.bind(RestBindings.SequenceActions.REJECT).toProvider(CustomRejectProvider);

System Alert • ID: 8574
Target: LoopBack API
Potential Vulnerability

Your LoopBack API might be exposed to Improper Error Handling

74% of LoopBack apps fail this check. Hackers use automated scanners to find this specific flaw. Check your codebase before they do.

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