Auth Bypass in Fiber with Hmac Signatures

Hmac Signatures are commonly used in Fiber to verify the integrity and origin of requests. When a client sends a request, it includes an HMAC of the payload and selected headers using a shared secret. The server recalculates the HMAC and compares it to the value provided. Auth Bypass can occur when the comparison is implemented inconsistently or when the server accepts ambiguous or weakly constructed inputs before signature verification. For example, if the server parses and normalizes the request body differently than the client, the computed HMAC may not match even for a legitimate request, which can be abused to submit unsigned or tampered requests through alternative paths.

Another vector arises when endpoints that should be protected rely on the presence of the HMAC header but also accept requests without it, or when middleware incorrectly allows the request to proceed after partial validation. If the comparison logic uses a non-constant-time check, timing attacks can leak information about the signature, enabling an attacker to iteratively forge a valid HMAC. Additionally, if the server includes query parameters or specific headers in the HMAC scope inconsistently, an attacker may shift the signed context and bypass intended authorization checks. Misconfigured routes that handle both authenticated and unauthenticated flows can further weaken the protection, effectively creating an Auth Bypass in the expected authentication boundary.

Consider a scenario where the client computes the HMAC over the JSON body and the X-Date header, but the server mistakenly includes an extra whitespace or uses a different serialization order. The signature will not match, and if the server falls back to a default or unauthenticated handler, the request may be processed without proper authorization. This highlights the importance of aligning the exact data covered by the HMAC and ensuring strict validation before any business logic executes. The risk is especially pronounced when endpoints expose sensitive operations and the Hmac Signature mechanism is the sole line of defense without complementary checks.

To mitigate Auth Bypass when using Hmac Signatures in Fiber, ensure the server computes and compares signatures using a canonical, consistent representation of the request data. Use strict constant-time comparison and validate the signature before processing any request-specific logic. Below are concrete, working examples for a Fiber-based API.

Client-side HMAC generation

The client creates a canonical string from the selected parts of the request and signs it using HMAC-SHA256. Include the HTTP method, the request path, a timestamp, and the request body to bind the signature to the exact request context.

const crypto = require('crypto');

function generateHmac(method, path, timestamp, body, secret) {
  const payload = `${method}\n${path}\n${timestamp}\n${body}`;
  return crypto.createHmac('sha256', secret).update(payload, 'utf8').digest('hex');
}

// Example usage for a POST /transfer request
const method = 'POST';
const path = '/v1/transfer';
const timestamp = Date.now().toString();
const body = JSON.stringify({ from: 'alice', to: 'bob', amount: 100 });
const secret = process.env.HMAC_SECRET;
const signature = generateHmac(method, path, timestamp, body, secret);
// Include signature and timestamp in headers:
// Authorization: Hmac username="service", algorithm="hmac-sha256", headers="(request-target) x-date", signature="..."

Server-side verification in Fiber

On the server, reconstruct the canonical string using the same components and perform a constant-time comparison before allowing the request to proceed. Ensure the timestamp is within an acceptable window to prevent replay attacks.

const crypto = require('crypto');
const { request } = require('express'); // Fiber-compatible pattern

function verifyHmac(req, secret, tolerance = 300) {
  const signature = req.get('Authorization')?.match(/signature="([^"]+)"/)?.[1];
  const timestamp = req.get('X-Date');
  if (!signature || !timestamp) {
    return false;
  }
  // Reject old or far-future requests to mitigate replay
  const now = Date.now();
  const reqTime = Number(timestamp);
  if (Math.abs(now - reqTime) > tolerance * 1000) {
    return false;
  }
  const method = req.method.toUpperCase();
  const path = req.path;
  const body = JSON.stringify(req.body); // ensure body is parsed consistently
  const payload = `${method}\n${path}\n${timestamp}\n${body}`;
  const expected = crypto.createHmac('sha256', secret).update(payload, 'utf8').digest('hex');
  // Constant-time comparison to avoid timing leaks
  return crypto.timingSafeEqual(Buffer.from(signature), Buffer.from(expected));
}

// Middleware usage
app.post('/v1/transfer', (req, res, next) => {
  const valid = verifyHmac(req, process.env.HMAC_SECRET);
  if (!valid) {
    return res.status(401).json({ error: 'invalid signature' });
  }
  next();
  // proceed with handler logic
});

Additional remediation steps include explicitly defining the scope of signed headers (e.g., (request-target) and x-date) and rejecting requests with missing or extra unsigned headers that alter the intended context. Rotate the shared secret periodically and monitor for repeated signature failures, which may indicate probing or attack attempts. These measures reduce the likelihood of Auth Bypass by ensuring that Hmac Signatures are applied consistently and verified robustly in Fiber applications.