Vulnerable Components in Fiber with Hmac Signatures

When using Hmac Signatures for request authentication in a Fiber application, developers often combine static or shared secrets with predictable signing logic, which can expose the API to tampering and injection. A typical vulnerable pattern is computing the Hmac over only a subset of the request data (e.g., raw body or selected headers) while neglecting metadata such as timestamps, nonce values, or version identifiers. If an attacker can influence any part of the request that is excluded from the signature, they may modify those elements without invalidating the Hmac, leading to unauthorized actions or bypass of intended integrity checks.

Another common issue is the use of weak or reused secrets across multiple services. In Fiber, if the same secret is used for both signing and verification across different environments or microservices, a compromise of that secret effectively grants broad access. Additionally, if the application does not enforce strict signature validation — for example, by using constant-time comparison and rejecting requests with missing or malformed signature headers — it may be vulnerable to signature comparison downgrade attacks or selective acceptance of unsigned payloads.

Timing side channels also play a role. If the verification logic short-circuits on signature mismatch rather than processing the full Hmac computation, an attacker can infer validity through response-time differences. This can be exploited in a padding-oracle-like fashion to gradually recover information about the expected signature. Furthermore, if the API does not implement replay protections (e.g., via nonce or timestamp validation within the signed payload), intercepted signed requests may be replayed to perform unauthorized operations, even when Hmac Signatures are in place.

In the context of OpenAPI/Swagger spec analysis, middleBrick detects when Hmac-related security schemes are defined but not consistently applied across operations. For instance, a spec may declare a security requirement using a custom Hmac scheme for some endpoints while leaving others unauthenticated. This inconsistency expands the attack surface, as attackers target endpoints with weaker or missing signature validation. Runtime probes then attempt to send manipulated requests to these unprotected paths, validating whether the server enforces signature checks uniformly.

Consider a vulnerable Fiber route that signs only the JSON body:

const express = require('express');
const crypto = require('crypto');
const app = express();
app.use(express.json());

const SHARED_SECRET = process.env.HMAC_SECRET;

function verifySignature(req, res, next) {
  const signature = req.headers['x-api-signature'];
  if (!signature) return res.status(401).send('Missing signature');
  const computed = crypto.createHmac('sha256', SHARED_SECRET).update(JSON.stringify(req.body)).digest('hex');
  if (computed !== signature) return res.status(403).send('Invalid signature');
  next();
}

app.post('/transfer', verifySignature, (req, res) => {
  res.json({ ok: true });
});

app.listen(3000);

This example is vulnerable because it excludes headers such as x-request-id or a timestamp from the signed payload. An attacker could modify the request path or add headers without invalidating the Hmac. MiddleBrick identifies such gaps by correlating the spec definition of the security scheme with actual runtime behavior, flagging endpoints where the effective verification scope is narrower than declared.

To remediate Hmac-related issues in Fiber, ensure that the signature covers all components that must remain immutable, including selected headers, a canonicalized body, and critical metadata. Use a deterministic serialization method for the body (e.g., sorted keys for JSON) and include a timestamp and nonce to prevent replay attacks. Validate signatures with constant-time comparison functions to mitigate timing side channels.

Below is a hardened example that signs and verifies a canonical representation of the request:

const express = require('express');
const crypto = require('crypto');
const app = express();
app.use(express.json());

const SHARED_SECRET = process.env.HMAC_SECRET;

function canonicalizeBody(obj) {
  // Deterministic JSON serialization to avoid signature mismatch due to key ordering
  return JSON.stringify(obj, Object.keys(obj).sort());
}

function signPayload(body, headers, timestamp, nonce) {
  const data = `${timestamp}:${nonce}:${canonicalizeBody(body)}:${headers['x-custom-context'] || ''}`;
  return crypto.createHmac('sha256', SHARED_SECRET).update(data).digest('hex');
}

function verifySignature(req, res, next) {
  const signature = req.headers['x-api-signature'];
  const timestamp = req.headers['x-timestamp'];
  const nonce = req.headers['x-nonce'];

  if (!signature || !timestamp || !nonce) {
    return res.status(400).send('Missing authentication headers');
  }

  // Basic replay protection: reject requests with timestamp older than 2 minutes
  const now = Date.now();
  if (Math.abs(now - parseInt(timestamp, 10)) > 120000) {
    return res.status(400).send('Request expired');
  }

  const expected = signPayload(req.body, req.headers, timestamp, nonce);

  // Constant-time comparison to avoid timing attacks
  const isValid = crypto.timingSafeEqual(Buffer.from(signature), Buffer.from(expected));
  if (!isValid) return res.status(403).send('Invalid signature');

  next();
}

app.post('/transfer', verifySignature, (req, res) => {
  res.json({ ok: true });
});

app.listen(3000);

In this approach, the signature incorporates a timestamp, a nonce, a canonicalized body, and a selected header (x-custom-context) to bind the context of the request. The server checks the timestamp window to mitigate replay attacks and uses crypto.timingSafeEqual for comparison. MiddleBrick’s scans validate that such controls are present and that the effective verification scope aligns with the declared security requirements in the API specification.

Additionally, rotate the Hmac secret periodically and avoid sharing it across unrelated services. If your API specification defines multiple security schemes, ensure that each operation requiring protection explicitly references the Hmac scheme so that documentation and runtime enforcement remain consistent. middleBrick’s continuous monitoring and CI/CD integration can alert you when a new endpoint is added without the required Hmac validation, helping maintain coverage as the API evolves.