Timing Attack in Feathersjs with Hmac Signatures

A timing attack in a Feathersjs service that uses Hmac Signatures arises when signature verification does not run in constant time. Feathersjs is a framework that typically exposes business logic as services; if you add Hmac-based authentication at the service level (for example, by validating a signature you compute on the server against one provided by the client), the comparison method matters.

Consider a scenario where the client sends a request with an Hmac signature in a header (e.g., x-api-signature). If the server recomputes the expected signature and uses a standard equality check (e.g., JavaScript strict equality == or an implicit string comparison), the check can short-circuit: it returns false as soon as a character mismatches. This difference in execution time depending on how many leading characters match can be measured by an attacker making many requests and observing response times, potentially leaking information about the expected signature.

In Feathersjs, this often occurs in a hook or a custom authentication layer. For example, if you compute Hmac using Node.js's crypto module and then compare the digest with the client-supplied value using signature === expected, the comparison is not guaranteed to be constant-time. An attacker who can send requests and measure timing may iteratively guess the signature byte by byte, especially if they can cause the service to authenticate (or attempt authentication) without being blocked by rate limiting or if error messages differ between authentication failure and other errors.

The risk is compounded when the Hmac key is static or predictable, when the payload includes user-controlled data that influences the signature length, or when the service returns distinct error messages for invalid signatures versus other validation errors. These differences provide side-channel cues. Even if Feathersjs itself does not introduce the vulnerability, the developer’s implementation choices in service hooks or authentication plugins can expose the unauthenticated attack surface that middleBrick scans for, such as Authentication and Input Validation checks.

To mitigate timing attacks when using Hmac Signatures in Feathersjs, you must ensure signature comparisons run in constant time and that related service behavior does not leak side-channel information. Use Node.js’s crypto.timingSafeEqual (available in the crypto module) to compare buffers of equal length. This method runs in constant time and is the recommended approach for cryptographic comparisons.

Below is a concrete example of a Feathersjs hook that validates an Hmac signature safely. The hook computes the Hmac over relevant request data using a shared secret and compares it with the signature provided by the client using timingSafeEqual. It also ensures the buffers are the same length before comparison to avoid errors and returns a generic authentication error to prevent information leakage.

const crypto = require('node:crypto');

function verifyHmacSignature(secret, payload, providedSignature) {
  const hmac = crypto.createHmac('sha256', secret);
  hmac.update(payload);
  const expected = hmac.digest(); // returns Buffer
  const provided = Buffer.from(providedSignature, 'hex');

  // Ensure lengths match to avoid leaking info via timing differences
  if (expected.length !== provided.length) {
    return false;
  }

  return crypto.timingSafeEqual(expected, provided);
}

// Feathersjs hook example
function hmacAuthentication(hook) {
  const secret = process.env.HMAC_SECRET; // store securely
  const { data, headers } = hook;
  const payload = JSON.stringify(data); // or include method + URL + body as appropriate
  const clientSignature = headers['x-api-signature'];

  if (!clientSignature || !verifyHmacSignature(secret, payload, clientSignature)) {
    throw new Error('Not authenticated'); // generic message
  }

  // optionally attach identity to hook for downstream services
  return hook;
}  before: {
    all: [hmacAuthentication],
    // limit to specific services or methods if needed
  },
  after: {
    all: [],
  },
  error: {
    all: [],
  },
};

Additional remediation practices include:

• Use fixed-length Hmac digests (e.g., SHA256 produces consistent 32-byte output) to simplify safe comparisons.
• Ensure the secret is stored securely (environment variables, secret management systems) and rotated periodically.
• Apply rate limiting at the service or global hook level to reduce the feasibility of iterative attacks.
• Return uniform error responses for authentication failures and other validation errors to avoid leaking whether the failure was due to an invalid signature.
• If you use multiple services, apply the same verification logic consistently; consider centralizing authentication in a before hook for the API.

These changes address the timing attack surface by removing data-dependent branching in the critical comparison path. They complement broader security checks such as those performed by middleBrick’s Authentication and Input Validation scans, and they align with secure coding guidance for cryptographic comparisons in Node.js.