Missing Authentication in Fiber with Hmac Signatures

Missing Authentication in a Fiber endpoint protected only by Hmac Signatures occurs when the application validates the signature but does not verify the identity of the caller or enforce a per-request nonce/context binding. Hmac Signatures typically consist of a shared secret, a canonical string-to-sign (often including method, path, timestamp, and body), and a hex-encoded MAC. If the server only checks that the MAC is valid without ensuring the request is tied to an authenticated principal or a one-time context, an attacker can replay a captured, properly signed request or substitute an unsigned but validly formed request when other protections (like route-level auth) are absent.

In a typical Fiber route, developers may use middleware that computes an Hmac using a shared secret and places the signature in a header such as x-signature. The route then verifies the MAC before processing. However, if the verification step does not include checks like a monotonic timestamp window, a nonce, or a binding to an authenticated session, the verification effectively becomes a no-op from an authentication perspective. The endpoint treats the request as authenticated solely because the signature matches, but the signature was never tied to a user or client identity. This enables BOLA/IDOR-like access when the endpoint exposes resources keyed by an identifier that the attacker can guess or iterate over (e.g., /api/users/{id}), because the server never confirms that the signer is the owner of that resource.

Consider a vulnerable Fiber handler where the Hmac is verified but no additional authentication context is enforced. An attacker who observes a single valid signed request (e.g., fetched from a victim’s browser) can replay it against the same endpoint with a different resource identifier, provided the server does not enforce replay protection. The signature remains valid because the string-to-sign and secret are unchanged, but the server skips any requirement for an API key, token, or session cookie. This gap is especially dangerous when combined with mass assignment or overly permissive route parameters, as the attacker may escalate from reading to modifying resources. The vulnerability is not in the Hmac algorithm itself but in the incomplete authorization logic around it, which fails to assert who is making the request beyond cryptographic integrity.

Real-world examples include endpoints that expose user data by ID without confirming the requester’s ownership, where a valid Hmac on a URL like /api/profile/123 is accepted even though the signer is user 456. OWASP API Top 10 category Broken Object Level Authorization (BOLA) maps directly to this pattern, and findings often reference missing contextual authentication alongside valid signatures. middleBrick’s LLM/AI Security checks and Authorization tests can surface such design weaknesses by probing endpoints with valid signatures but unverified subject linkage. By reporting per-category breakdowns tied to frameworks like OWASP and compliance regimes such as SOC2 and GDPR, it helps teams understand that cryptographic integrity alone does not equal authentication.

Remediation focuses on ensuring that Hmac verification is coupled with explicit authentication and anti-replay controls. You must bind the signature to a principal (e.g., an API key or user ID) and enforce freshness using timestamps and nonces. Below are concrete, working Fiber examples that demonstrate a secure pattern.

1. Shared secret and canonical string-to-sign with timestamp and nonce

Define a helper to build the string-to-sign from method, path, timestamp, nonce, and body. Verify the timestamp window (e.g., 5 minutes) and maintain a nonce cache to prevent replays.

import { Router } from 'https://deno.land/x/[email protected]/mod.ts';
import crypto from 'node:crypto';

const router = new Router();
const SHARED_SECRET = Deno.env.get('HMAC_SHARED_SECRET')!;
const nonceStore = new Set(); // in production, use a TTL cache
const ALLOWED_CLOCK_SKEW = 5 * 60 * 1000; // 5 minutes

function buildStringToSign(method: string, path: string, timestamp: string, nonce: string, body: string): string {
  return `${method}\n${path}\n${timestamp}\n${nonce}\n${body}`;
}

function verifyHmacSignature(req: any): boolean {
  const signature = req.headers.get('x-signature');
  const timestamp = req.headers.get('x-timestamp');
  const nonce = req.headers.get('x-nonce');
  if (!signature || !timestamp || !nonce) return false;

  const now = Date.now();
  if (Math.abs(now - parseInt(timestamp, 10)) > ALLOWED_CLOCK_SKEW) {
    return false;
  }
  if (nonceStore.has(nonce)) {
    return false; // replay
  }
  nonceStore.add(nonce);
  // prune nonce store periodically in real code

  const stringToSign = buildStringToSign(req.method, req.url.pathname, timestamp, nonce, req.rawBody || '');
  const expected = crypto.createHmac('sha256', SHARED_SECRET)
                         .update(stringToSign)
                         .digest('hex');
  return crypto.timingSafeEqual(Buffer.from(signature), Buffer.from(expected));
}

router.post('/api/profile/:id', (req, res) => {
  if (!verifyHmacSignature(req)) {
    res.status(401).send({ error: 'Invalid signature' });
    return;
  }
  // At this point, authentication is tied to the signer via nonce/timestamp and per-request context
  const id = req.params.id;
  // Ensure the authenticated principal (derived from nonce/timestamp or an API key) matches id ownership
  // For example, map a principal to the user id and compare
  res.send({ ok: true });
});

2. Binding Hmac to an authenticated principal (API key mapping)

Maintain a mapping between the signer’s key material (derived from the shared secret or an associated API key) and the user identity. Include the principal identifier in the string-to-sign so the signature cannot be reused across users.

import { Router } from 'https://deno.land/x/[email protected]/mod.ts';
import crypto from 'node:crypto';

const router = new Router();
const SHARED_SECRET = Deno.env.get('HMAC_SHARED_SECRET')!;
const principalMap: Record = { 
  /* keyed by hmacKeyId or derived fingerprint — in practice, a secure store */ 
  'user-a-hmac-id': 'user-a-id' 
};

function buildStringToSignPrincipal(method: string, path: string, timestamp: string, nonce: string, body: string, principalId: string): string {
  return `${method}\n${path}\n${timestamp}\n${nonce}\n${principalId}\n${body}`;
}

function verifyAndExtractPrincipal(req: any): string | null {
  const signature = req.headers.get('x-signature');
  const timestamp = req.headers.get('x-timestamp');
  const nonce = req.headers.get('x-nonce');
  const principalId = req.headers.get('x-principal-id');
  if (!signature || !timestamp || !nonce || !principalId) return null;

  const now = Date.now();
  if (Math.abs(now - parseInt(timestamp, 10)) > 5 * 60 * 1000) return null;
  if (/* check nonce cache */ false) return null;

  const stringToSign = buildStringToSignPrincipal(req.method, req.url.pathname, timestamp, nonce, req.rawBody || '', principalId);
  const expected = crypto.createHmac('sha256', SHARED_SECRET)
                         .update(stringToSign)
                         .digest('hex');
  const valid = crypto.timingSafeEqual(Buffer.from(signature), Buffer.from(expected));
  if (!valid) return null;
  return principalMap[principalId] || null;
}

router.put('/api/profile/:id', (req, res) => {
  const userId = verifyAndExtractPrincipal(req);
  if (!userId) {
    res.status(401).send({ error: 'Unauthorized' });
    return;
  }
  const id = req.params.id;
  if (userId !== id) {
    res.status(403).send({ error: 'Forbidden: cannot modify other profiles' });
    return;
  }
  res.send({ ok: true });
});

These examples emphasize that Hmac Signatures must be part of a broader authorization strategy that ties cryptographic integrity to identity and context. middleBrick’s scans can highlight when signature verification exists without these binding checks, providing prioritized findings and remediation guidance mapped to frameworks such as OWASP API Top 10.

• middleBrick CLI: scan from terminal with middlebrick scan <url>
• middleBrick GitHub Action: add API security checks to your CI/CD pipeline
• middleBrick MCP Server: scan APIs directly from your AI coding assistant

By combining strict Hmac construction with per-request nonces, timestamps, and principal binding, you mitigate replay and BOLA risks while maintaining the integrity benefits of Hmac Signatures.