Injection Flaws in Loopback with Hmac Signatures

Loopback is a widely used Node.js framework for building APIs, and it commonly uses Hmac Signatures to verify the integrity and origin of incoming requests. When Hmac Signatures are implemented incorrectly, they can inadvertently enable injection flaws rather than prevent them.

An injection flaw occurs when untrusted data is interpreted as part of a command or query. In Loopback, if the signature verification logic does not strictly validate and sanitize input before using it in database queries, file operations, or dynamic object construction, attackers can inject malicious payloads. For example, an attacker might manipulate a query parameter or header that is included in the signed string, and if the server reuses or partially trusts that data after verification, it can lead to prototype pollution or NoSQL injection.

Consider a scenario where the Hmac signature is computed over a concatenated string of user-supplied parameters without strict schema validation:

const crypto = require('crypto');
const algorithm = 'sha256';
const secret = process.env.HMAC_SECRET;

function verifySignature(req, res, next) {
  const receivedSignature = req.headers['x-api-signature'];
  const { userId, filter } = req.query;
  const computedSignature = crypto.createHmac(algorithm, secret)
    .update(userId + filter)
    .digest('hex');

  if (computedSignature !== receivedSignature) {
    return res.status(401).send('Invalid signature');
  }
  next();
}

If filter is later used directly in a Loopback model query such as MyModel.find({ where: filter }), an attacker who can guess or leak part of the signature scheme might inject { "status": { "$ne": null } } as the filter, bypassing intended access controls. This is a BOLA/IDOR-style injection enabled by insufficient input validation after signature verification.

Another vector involves the deserialization of signed JSON payloads. If Loopback accepts a signed JSON body and reconstructs objects without sanitizing nested properties, attackers can exploit getters, setters, or constructor functions embedded in the payload to execute unintended logic. This ties into unsafe consumption patterns where the framework dynamically interprets data types, potentially leading to server-side request forgery (SSRF) or remote code execution when combined with malicious endpoints.

Because middleBrick scans the unauthenticated attack surface and tests input validation as one of its 12 parallel checks, it can detect whether injection-prone data flows exist after Hmac verification. The scanner checks whether user-controlled fields participating in signature computation are later used in sensitive operations without sanitization, highlighting risks such as property authorization bypass or data exposure.

To prevent injection flaws when using Hmac Signatures in Loopback, you must enforce strict input validation, avoid direct use of user data in queries, and ensure signature scope is minimal and well-defined.

First, validate and sanitize all inputs before including them in the signature computation. Use a schema that explicitly defines allowed fields and types:

const Joi = require('joi');

const querySchema = Joi.object({
  userId: Joi.string().hex().length(24).required(),
  status: Joi.string().valid('active', 'inactive', 'pending').optional()
});

function verifyAndSanitize(req, res, next) {
  const { error, value } = querySchema.validate(req.query);
  if (error) {
    return res.status(400).send('Invalid query parameters');
  }
  req.sanitizedQuery = value;
  next();
}

Then compute the Hmac over a canonical representation of the sanitized data, such as a sorted JSON string, rather than raw concatenation:

const crypto = require('crypto');
const algorithm = 'sha256';
const secret = process.env.HMAC_SECRET;

function computeSignature(data) {
  const canonical = JSON.stringify(data, Object.keys(data).sort());
  return crypto.createHmac(algorithm, secret).update(canonical).digest('hex');
}

function verifySignature(req, res, next) {
  const receivedSignature = req.headers['x-api-signature'];
  const expectedSignature = computeSignature(req.sanitizedQuery);

  if (!crypto.timingSafeEqual(Buffer.from(receivedSignature), Buffer.from(expectedSignature))) {
    return res.status(401).send('Invalid signature');
  }
  next();
}

When using the data in Loopback queries, always use parameterized conditions or whitelisted fields instead of passing user input directly:

MyModel.find({
  where: {
    id: req.sanitizedQuery.userId,
    status: req.sanitizedQuery.status || 'active'
  }
}).then(results => {
  res.json(results);
});

This approach ensures that even if an attacker influences the signature, they cannot alter the structure of the query. It also aligns with the principle of least privilege by limiting which fields participate in signature verification.

Using middleBrick’s CLI (middlebrick scan <url>) or its GitHub Action can help automate detection of weak Hmac usage and injection-prone query patterns. The Pro plan enables continuous monitoring of these endpoints so that any regression in signature handling is flagged early. For AI-assisted development, the MCP Server allows you to scan APIs directly from your coding assistant, helping catch insecure patterns before deployment.