Open Redirect in Loopback with Mongodb

An open redirect in a Loopback application that uses Mongodb can occur when an endpoint accepts a user-supplied URL or path and performs an HTTP redirect without strict validation. If the redirect target is derived from request parameters and later combined with data retrieved from a Mongodb collection, the vector becomes both storage-backed and runtime-driven.

Consider a profile settings endpoint that stores a user-supplied redirect URL in Mongodb and later redirects the browser to that value. An attacker can register a malicious URL in their profile (persisted in the database) and trigger a redirect for any user who visits the settings confirmation page. Because the stored URL is rendered directly from trusted-seeming database records, developers may incorrectly assume safety, but the persistence layer does not neutralize malicious payloads.

When Loopback resolves route handlers and pulls user data from Mongodb, the framework’s dynamic model binding can inadvertently pass attacker-controlled values to redirect logic if input validation is weak. This is especially risky when the redirect URL is built from multiple sources — such as concatenating a user-supplied host with a path stored in Mongodb — because subtle parsing issues may allow schemes like javascript: or protocol-relative URLs that browsers interpret as navigations.

An unauthenticated scan using middleBrick can detect open redirect patterns in Loopback routes that accept redirect parameters and return 3xx responses without allowlisting. The tool checks whether response headers or body content can cause browser navigation to attacker-controlled hosts, and it correlates findings with Mongodb-backed data flows shown in the OpenAPI spec. This helps surface stored versus reflected variants, which is critical for prioritization because stored vectors can affect multiple users.

In a typical workflow, an API defined in an OpenAPI specification might declare a redirectUrl parameter without strict format constraints. middleBrick’s LLM/AI Security checks do not apply here, but its standard authentication and BOLA/IDOR analyses can highlight endpoints where redirect parameters interact with per-user data stored in Mongodb. The resulting finding includes severity, a description of how the combination of Loopback routing, Mongodb persistence, and browser behavior enables abuse, and remediation guidance that emphasizes strict allowlists and context-aware validation.

To remediate open redirect issues when Loopback interacts with Mongodb, enforce strict allowlists on redirect targets and avoid storing untrusted URLs for later use. Where storage is necessary, validate and normalize the URL before persisting, and re-validate on retrieval before any redirect.

Below is a syntactically correct example of a Loopback model and repository method that stores a redirect URL in Mongodb only after validation. The validation ensures an allowed host and rejects dangerous schemes.

import {inject} from '@loopback/core';
import {getModelSchemaRef, repository} from '@loopback/repository';
import {HttpErrors, post} from '@loopback/rest';
import {UrlValidator} from '../validation/url.validator';
import {RedirectLogRepository} from '../repositories';

export class RedirectController {
  constructor(
    @repository(RedirectLogRepository)
    public redirectLogRepository: RedirectLogRepository,
  ) {}

  @post('/redirects', {responses: {}})
  async createRedirect(
    @requestBody({
      content: {'application/json': {schema: {type: 'object', properties: {'url': {type: 'string'}}}}},
    })
    url: string,
  ): Promise<{message: string}> {
    if (!UrlValidator.isSafeRedirect(url)) {
      throw new HttpErrors.BadRequest('Invalid redirect URL');
    }
    const entry = await this.redirectLogRepository.create({url});
    return {message: `Stored safe redirect id ${entry.id}`};
  }
}

The validator implements host and scheme allowlisting, normalizes input, and rejects javascript or data URIs. Here is a minimal implementation suitable for Loopback models using Mongodb-backed connectors.

// validation/url.validator.ts
import {URL} from 'url';

const ALLOWED_HOSTS = new Set(['example.com', 'trusted.org']);
const ALLOWED_PROTOCOLS = new Set(['https:', 'http:']);

export class UrlValidator {
  static isSafeRedirect(input: string): boolean {
    let parsed: URL;
    try {
      parsed = new URL(input);
    } catch {
      return false;
    }
    if (!ALLOWED_PROTOCOLS.has(parsed.protocol)) {
      return false;
    }
    if (!ALLOWED_HOSTS.has(parsed.hostname)) {
      return false;
    }
    // Reject potential open redirect bypass patterns
    if (parsed.searchParams.has('redirect') || parsed.hash.includes('redirect')) {
      return false;
    }
    return true;
  }
}

When reading back stored URLs for redirects, repeat the same validation. Do not rely on format checks alone; confirm that the resolved hostname matches the intended destination domain. middleBrick’s OpenAPI/Swagger analysis can highlight endpoints where redirect parameters map to Mongodb properties, helping you identify places where re-validation is necessary.

For existing data, consider a cleanup script that validates stored URLs against the current allowlist and either removes or updates risky entries. Combine this with runtime checks in Loopback middleware so that even if invalid data exists in the database, it cannot be used to pivot users to attacker-controlled sites.