Broken Access Control in Fiber with Hmac Signatures

Broken Access Control in the Fiber web framework when paired with Hmac Signatures often stems from how signatures are generated, verified, and bound to authorization decisions. If the signature does not cover critical authorization inputs such as user identity, tenant or role, an attacker can reuse a valid signature across different users or resources. For example, signing only the request path and a static secret, while omitting the user ID or role, enables BOLA/IDOR-like scenarios where a valid signature for one record is applied to another.

Another common pattern is verifying the Hmac correctly but failing to enforce authorization checks afterward. A developer might confirm the signature is valid and then load a resource by ID directly, assuming the client is allowed to access it. This creates a BOLA/IDOR vector because the signature does not prove access rights for that specific resource. In multi-tenant setups, if the tenant context is not part of the signed payload, a client can substitute another tenant’s identifier and access data they should not see.

Input validation issues can also interact with Hmac verification. If the signature is computed over a query parameter or header that can be manipulated, and the application does not validate the format or scope of the associated data, attackers may inject values that bypass intended access boundaries. For instance, allowing the client to choose which key identifier is signed without ensuring it maps to an authorized relationship enables privilege escalation.

Additionally, missing or weak rate limiting on endpoints that verify Hmac signatures can allow brute-force or replay attempts against known valid signatures. Without proper inventory management controls, developers might inadvertently expose administrative endpoints with Hmac verification but without the corresponding authorization checks, increasing the risk of unauthorized operations.

To remediate Broken Access Control when using Hmac Signatures in Fiber, the signature must cover all data used for access decisions and the verification must be tightly coupled with authorization logic. The server should sign a canonical representation of the request that includes the user or client identifier, tenant or resource ID, intended action, and any other context required for authorization. Below are concrete code examples for a secure approach using Hmac Signatures in Fiber.

First, define a structure for the signed payload that includes authorization-relevant fields:

import { Hono } from 'hono';
import { createHmac, timingSafeEqual } from 'crypto';

const app = new Hono();

const SECRET = process.env.HMAC_SECRET;

interface SignedPayload {
  userId: string;
  resourceId: string;
  action: 'read' | 'write';
  ts: number; // Unix timestamp to prevent replay
}

function buildSignedPayload(userId: string, resourceId: string, action: 'read' | 'write'): { payload: SignedPayload, signature: string } {
  const payload: SignedPayload = {
    userId,
    resourceId,
    action,
    ts: Math.floor(Date.now() / 1000),
  };
  const data = JSON.stringify(payload);
  const hmac = createHmac('sha256', SECRET);
  const signature = hmac.update(data).digest('hex');
  return { payload, signature };
}

function verifySignedPayload(payload: SignedPayload, receivedSignature: string): boolean {
  const data = JSON.stringify(payload);
  const hmac = createHmac('sha256', SECRET);
  const expected = hmac.update(data).digest('hex');
  // Prevent timing attacks
  const left = Buffer.from(expected);
  const right = Buffer.from(receivedSignature);
  if (left.length !== right.length) {
    return false;
  }
  return timingSafeEqual(left, right);
}

When the client makes a request, include the payload and signature in headers. The server verifies the signature, reconstructs the payload, and then enforces authorization based on the contained fields:

app.get('/resource/:id', (c) => {
  const header = c.req.get('x-signed-payload');
  const sig = c.req.get('x-signature');
  if (!header || !sig) {
    return c.json({ error: 'missing_auth' }, 401);
  }
  let payload: SignedPayload;
  try {
    payload = JSON.parse(Buffer.from(header, 'base64').toString('utf-8'));
  } catch {
    return c.json({ error: 'invalid_payload' }, 400);
  }
  if (!verifySignedPayload(payload, sig)) {
    return c.json({ error: 'invalid_signature' }, 401);
  }
  // Enforce authorization using payload fields, not just the resource ID in the URL
  if (payload.userId !== c.get('userId')) {
    return c.json({ error: 'forbidden_user' }, 403);
  }
  if (payload.action !== 'read') {
    return c.json({ error: 'forbidden_action' }, 403);
  }
  // Ensure the resource ID in the payload matches the requested ID to prevent tampering
  if (payload.resourceId !== c.req.param('id')) {
    return c.json({ error: 'forbidden_resource' }, 403);
  }
  // Proceed with business logic, knowing the signature covers the critical authorization inputs
  return c.json({ data: 'secure resource' });
});

For multi-tenant scenarios, include the tenant identifier in the signed payload and verify it matches the authenticated user’s tenant before allowing access:

interface TenantPayload extends SignedPayload {
  tenantId: string;
}

// During signing
const tenantPayload: TenantPayload = {
  userId: 'u-123',
  resourceId: 'res-456',
  action: 'read',
  ts: Math.floor(Date.now() / 1000),
  tenantId: 'tenant-a',
};

// During verification, compare tenantId with the user’s resolved tenant
if (payload.tenantId !== resolvedTenantId) {
  return c.json({ error: 'tenant_mismatch' }, 403);
}

Finally, pair Hmac verification with server-side authorization checks and ensure endpoints that verify signatures are covered by appropriate rate limiting to reduce abuse risk. This approach aligns with findings that map to frameworks such as OWASP API Top 10 and supports compliance mapping for standards like PCI-DSS and SOC2.