Injection Flaws in Sails with Hmac Signatures

Injection flaws in Sails applications that rely on Hmac Signatures often stem from how signatures are generated and verified when untrusted input is incorporated into the signed payload. If the Hmac is computed over a string that includes user-controlled data without strict canonicalization, an attacker may be able to alter the signature or inject malicious content that the server still accepts.

Consider a Sails endpoint that accepts query parameters used to build a signed Hmac. If the signing process concatenates parameters directly into the string-to-sign without normalization, parameters that contain characters like &, =, or newline can change the semantic meaning of the signed data. An attacker might inject additional parameters or JSON fragments into the payload if the server deserializes the data loosely after verifying the Hmac. This can lead to Injection Flaws similar to those seen in the OWASP API Top 10 category Input Validation and the broader BOLA/IDOR attack surface when signature validation is bypassed through malformed input.

Another risk arises when the Hmac is used to select or influence internal behavior, such as determining which resource to fetch or which function to invoke. If the signed data includes identifiers that are later used in database queries or passed to eval-like mechanisms without proper validation, attackers may exploit this path to perform injection attacks. For example, an attacker could attempt to inject script or command fragments if the application logs or echoes data that was signed but not properly sanitized. In such contexts, SSRF risks may also emerge if the injected data causes the server to make unexpected network calls during signature processing.

Real-world attack patterns include attempts to manipulate the algorithm parameter in JWA-style headers or to exploit inconsistent handling of line endings and whitespace in the signature base string. These can map to findings such as Property Authorization weaknesses when the signature does not strictly bind all relevant input fields. Because middleBrick tests input validation and Authorization checks in parallel, such subtle mismatches between signature generation and runtime interpretation are surfaced as high-severity findings with remediation guidance to enforce strict canonical forms and avoid concatenating untrusted data into the signed string.

To prevent Injection Flaws when using Hmac Signatures in Sails, ensure that the data used to generate and verify the signature is canonical, strictly typed, and isolated from injection-prone contexts. Use a deterministic serialization method such as sorting keys, normalizing whitespace, and omitting optional parameters that are not essential to the signature scope.

Below is a concrete example of secure Hmac signing and verification in a Sails controller. The code uses Node.js built-in crypto to create a canonical string and a constant-time comparison to avoid timing attacks. It assumes the payload includes only selected, validated fields and excludes any user-controlled strings that could alter the signature semantics.

const crypto = require('crypto');

module.exports = {
  signData: function (data, secret) {
    // data is a plain object with known, validated keys
    const keys = Object.keys(data).sort();
    const canonical = keys.map(k => `${k}:${data[k]}`).join('|');
    return crypto.createHmac('sha256', secret).update(canonical, 'utf8').digest('hex');
  },

  verifyData: function (data, receivedSignature, secret) {
    const expected = this.signData(data, secret);
    // Use timing-safe compare
    return crypto.timingSafeEqual(
      Buffer.from(expected, 'utf8'),
      Buffer.from(receivedSignature, 'utf8')
    );
  },

  createSignedPayload: function (req, res) {
    const { amount, currency, orderId } = req.body;
    // Validate and cast types explicitly
    const validated = {
      amount: Number(amount).toString(),
      currency: String(currency).toUpperCase(),
      orderId: String(orderId)
    };
    const secret = sails.config.custom.hmacSecret;
    const signature = this.signData(validated, secret);
    return res.json({ data: validated, signature });
  },

  verifySignedPayload: function (req, res, next) {
    const { data, signature } = req.body;
    if (!data || !signature) {
      return res.badRequest('Missing data or signature');
    }
    const verified = this.verifyData(data, signature, sails.config.custom.hmacSecret);
    if (!verified) {
      return res.unauthorized('Invalid signature');
    }
    // Ensure data types match expected values before further use
    if (typeof data.amount !== 'string' || isNaN(Number(data.amount))) {
      return res.badRequest('Invalid amount');
    }
    return next();
  }
};

In this example, the canonical string is built by sorting keys and joining key-value pairs with a delimiter that is unlikely to appear in the data. This reduces the risk of injection through crafted separators. The verification step uses crypto.timingSafeEqual to mitigate timing side-channels, and input fields are explicitly cast to expected types before use in business logic.

When integrating with middleBrick, you can scan your Sails endpoints using the CLI with middlebrick scan <url> to detect whether your Hmac implementation exposes injection-prone behavior. The scan will highlight findings related to Input Validation and Property Authorization, and the dashboard allows you to track improvements over time. For CI/CD enforcement, the GitHub Action can fail builds if the risk score drops below your chosen threshold, ensuring that future changes do not reintroduce similar flaws.