Side Channel Attack in Feathersjs with Hmac Signatures

A side channel attack in a FeathersJS service that uses HMAC signatures exploits timing or behavioral differences introduced during signature verification rather than breaking the cryptographic algorithm itself. In FeathersJS, authentication layers often validate HMAC signatures by computing a signature over the request payload and comparing it with the value provided by the client. If this comparison is performed with a standard string equality check, an attacker can use timing differences to learn information about the correct signature.

Consider a FeathersJS service where the client sends a payload along with an HMAC-SHA256 signature in a header, for example X-Api-Signature. On the server, a common but vulnerable pattern is to compute the expected signature and use a strict equality check against the header value. In many JavaScript runtime environments, string comparison can short-circuit: as soon as a character mismatch is found, the comparison returns false. This means an attacker who can measure response times can iteratively guess the correct signature byte by byte, significantly reducing the search space from the full key length to a feasible number of requests.

The vulnerability is amplified when the server does not enforce constant-time comparison and when the service exposes slight behavioral differences for valid versus invalid signatures. For instance, a valid signature might lead to further processing such as deserialization or business logic execution, while an invalid signature results in an early, minimal response. These differences in latency and error handling create a side channel that an attacker can probe using carefully crafted requests and statistical analysis of response times.

Real-world attack patterns relevant to this scenario include timing-based oracle attacks, where the attacker sends modified requests and observes small variations in response time to infer correctness. In the context of FeathersJS, this often maps to the OWASP API Top 10 category of 'Broken Object Level Authorization' or more broadly to insecure implementation of cryptographic primitives. Systems that accept untrusted input for signature verification without mitigating timing differences effectively expose an unauthenticated attack surface that can be explored by black-box scanning methodologies similar to those used by tools that test API endpoints for authentication weaknesses.

To illustrate the problematic code, imagine a FeathersJS hook that manually verifies an HMAC signature using native Node.js APIs without constant-time comparison:

const crypto = require('crypto');

function verifySignature(requestBody, receivedSignature, secret) {
  const hmac = crypto.createHmac('sha256', secret);
  const computed = hmac.update(JSON.stringify(requestBody)).digest('hex');
  return computed === receivedSignature;
}

In this example, the === comparison is not guaranteed to run in constant time. An attacker interacting with the FeathersJS service can leverage repeated requests to infer the correct signature byte by byte, demonstrating a practical side channel. This highlights the need to replace such checks with constant-time comparison routines and to ensure that the overall authentication flow does not leak information through timing or error behavior.

Remediation focuses on replacing vulnerable string comparisons with a constant-time verification routine and ensuring that the HMAC validation logic does not introduce observable timing or behavioral differences. In Node.js, the crypto.timingSafeEqual method should be used to compare buffers of equal length. This ensures that the comparison always takes the same amount of time, removing the side channel.

Below is a secure example of HMAC signature verification in a FeathersJS hook. The code computes the HMAC over the request body using a shared secret and compares the result to the signature provided by the client using crypto.timingSafeEqual.

const crypto = require('crypto');

function verifySignatureConstantTime(requestBody, receivedSignatureHex, secret) {
  const hmac = crypto.createHmac('sha256', secret);
  const computed = hmac.update(JSON.stringify(requestBody)).digest();
  const received = Buffer.from(receivedSignatureHex, 'hex');

  // Ensure lengths match to avoid leaking information via length checks
  if (computed.length !== received.length) {
    return false;
  }

  return crypto.timingSafeEqual(computed, received);
}

// Example FeathersJS before hook
exports.hooks = {
  before: {
    all: [],
    find: [context => {
      const secret = process.env.HMAC_SECRET;
      const signature = context.headers['x-api-signature'];
      const body = context.data;

      if (!signature || !verifySignatureConstantTime(body, signature, secret)) {
        throw new Error('Invalid signature');
      }
      return context;
    }]
  }
};

This approach ensures that the comparison does not short-circuit and that the server's response time does not depend on how much of the signature is correct. It also normalizes error handling so that invalid signatures produce the same generic error response and similar processing overhead, reducing observable differences.

Additionally, it is important to ensure that the secret used for HMAC is stored securely and rotated periodically, and that the request body used for signing is canonicalized to avoid discrepancies caused by formatting differences. Using built-in, well-audited cryptographic functions and avoiding manual concatenation or ad-hoc encoding further reduces the risk of introducing new side channels.

When integrating such checks into a CI/CD pipeline, you can use the middleBrick CLI to scan your FeathersJS endpoints and validate that authentication patterns do not expose timing-related weaknesses. The tool reports findings aligned with frameworks like OWASP API Top 10 and provides remediation guidance without altering or blocking traffic.