Privilege Escalation in Feathersjs with Hmac Signatures

FeathersJS is a framework for real-time applications that often relies on service authentication and authorization. When HMAC-based signatures are used for verifying requests, incorrect implementation can lead to privilege escalation. A common misconfiguration is allowing the HMAC secret to be accessible to unauthenticated or low-privilege endpoints. If an endpoint that verifies HMAC signatures does not enforce proper role-based access control, an attacker may be able to call that endpoint with manipulated signature data to gain higher privileges.

Consider a FeathersJS service that uses HMAC signatures to validate webhook or internal requests. If the service does not validate the requester’s role or scope within the signature verification logic, an attacker could intercept or replay a signed payload and modify the payload’s subject (e.g., changing a user role from "user" to "admin") before the server verifies the signature. Because the server trusts the signature without re-evaluating authorization context, the modified payload may be accepted as valid, leading to unauthorized access or elevated permissions.

Another scenario involves improper handling of the signature verification key. If the HMAC secret is hardcoded or exposed via an unauthenticated endpoint, an attacker can retrieve the secret and forge valid signatures for any payload, including those that perform administrative actions. This becomes a privilege escalation vector when the signed data includes claims such as user ID, role, or scope, and the server does not re-check these claims after signature verification.

In FeathersJS, services often rely on hooks for authentication and authorization. If the HMAC verification is placed in a hook that runs before role-based authorization hooks, or if the verification step does not integrate with the application’s identity and access management layer, the system may incorrectly authorize actions based solely on signature validity. This gap allows an attacker to exploit the trust placed in the HMAC signature to escalate privileges across service boundaries.

Real-world attack patterns include tampering with signed tokens or webhook payloads where role or permission fields are embedded in the signed payload but not re-validated against a dynamic policy store. For example, an attacker could modify a signed JSON payload from {"userId":"123","role":"user"} to {"userId":"123","role":"admin"} and send it to a vulnerable FeathersJS service that only verifies the signature and not the role field post-verification.

To detect this class of issue during automated scanning, tools like middleBrick evaluate whether HMAC signature verification is coupled with proper authorization checks, whether secrets are exposed, and whether signed payloads are validated in the context of the requester’s permissions. The scanner does not modify the application but identifies gaps where trust in the signature bypasses necessary privilege checks.

Remediation requires ensuring that HMAC signature verification is followed by explicit role and scope checks, and that secrets are never exposed to unauthenticated endpoints. Below are concrete code examples for a secure implementation in FeathersJS.

Secure HMAC verification with role re-validation

const crypto = require('crypto');

function verifyHmac(payload, receivedSignature, secret) {
  const computedSignature = crypto.createHmac('sha256', secret)
    .update(JSON.stringify(payload))
    .digest('hex');
  return crypto.timingSafeEqual(
    Buffer.from(computedSignature),
    Buffer.from(receivedSignature)
  );
}

// FeathersJS hook example
const hmacAuthentication = context => {
  const { signature, data } = context.data;
  const secret = process.env.HMAC_SECRET; // stored securely, not exposed

  if (!secret) {
    throw new Error('HMAC secret not configured');
  }

  const isValid = verifyHmac(data, signature, secret);
  if (!isValid) {
    throw new Error('Invalid signature');
  }

  // Re-validate role and permissions from a trusted source
  const userRole = data.role;
  if (userRole !== 'admin') {
    throw new Error('Insufficient permissions');
  }

  // Ensure the payload user matches the requesting user if applicable
  // e.g., context.params.user.id === data.userId

  return context;
};

module.exports = {
  before: {
    all: [],
    find: [],
    get: [],
    create: [hmacAuthentication],
    update: [hmacAuthentication],
    patch: [hmacAuthentication],
    remove: [hmacAuthentication]
  }
};

Do not expose HMAC secret via unauthenticated service

Ensure that no service endpoint returns the HMAC secret or allows retrieval by unauthenticated users. The following incorrect pattern should be avoided:

// ❌ Insecure: exposing secret via an unauthenticated endpoint
app.service('config').create({
  secret: process.env.HMAC_SECRET // DO NOT DO THIS
});

Instead, keep the secret in environment variables and never include it in API responses:

// ✅ Secure: secret remains server-side
app.get('/health', (req, res) => {
  res.json({ status: 'ok' });
});

Use role-based access control after signature verification

After verifying the HMAC, enforce role-based access control using your application’s identity store. Example with a user service:

const userService = app.service('users');

userService.hooks({
  before: {
    get: [context => {
      const requesterRole = context.params.user.role; // from auth layer
      const targetUserId = context.id;
      if (requesterRole !== 'admin' && requesterUserId !== targetUserId) {
        throw new Error('Access denied');
      }
      return context;
    }]
  }
});

These practices reduce the risk of privilege escalation by ensuring that HMAC signatures are part of a broader authorization strategy rather than a standalone trust mechanism. middleBrick can evaluate whether your endpoints follow these patterns and highlight areas where signature verification does not align with least-privilege principles.