Padding Oracle in Fiber with Basic Auth

A padding oracle attack requires both a padding scheme (e.g., PKCS#7) and a way for an attacker to learn whether decrypted padding is valid. In Fiber, using HTTP Basic Auth in combination with encrypted request bodies or encrypted parameters can create conditions where padding errors are observable through authentication failures or distinct server responses. When credentials are passed in the Authorization header (e.g., Authorization: Basic base64(username:password)), an attacker can intercept or modify an encrypted request body or cookie and observe whether the server first validates authentication or first validates padding. If the server attempts to decrypt before checking credentials, padding errors may produce different status codes or timing differences that can be detected by an attacker.

Consider a scenario where a Fiber endpoint accepts an encrypted JSON payload in the body, and the route is protected with Basic Auth. The attacker can capture the ciphertext and modify it, then send it with valid or invalid credentials. If padding validation occurs after authentication, incorrect padding may still yield a 401 Unauthorized, but if the server decrypts first and returns 400 or 500 for bad padding before rejecting credentials, subtle timing differences or error messages can leak information about padding validity. This combination exposes a practical oracle: the attacker can infer padding correctness by observing responses when altering ciphertext and varying credentials, especially when error handling differs between padding failures and authentication failures.

In practice, this means a Fiber application that uses encrypted payloads (e.g., for tokens or sensitive fields) and relies on Basic Auth for route protection can inadvertently expose a padding oracle when decryption and authentication are not performed in a strict, atomic manner. For example, if decryption errors are logged or returned with distinguishable messages, and authentication is checked afterward, an attacker can mount an adaptive chosen-ciphertext attack. The risk is compounded when the server is behind a reverse proxy or load balancer that standardizes error responses, or when error handling is inconsistent across development and production environments.

To mitigate padding oracle risks when using Basic Auth in Fiber, ensure that decryption and authentication are performed in a way that does not reveal padding validity through timing or error messages. Always use constant-time comparison for any authentication-sensitive operations and avoid returning distinct errors for padding versus authentication failures. Below are concrete code examples that demonstrate secure handling of Basic Auth in Fiber.

Secure Basic Auth setup with encrypted payload handling

const { app } = require('@fibers/fiber');
const auth = require('basic-auth');

// Constant-time credential comparison helper
function safeCompare(a, b) {
  if (a.length !== b.length) {
    return false;
  }
  let result = 0;
  for (let i = 0; i < a.length; i++) {
    result |= a.charCodeAt(i) ^ b.charCodeAt(i);
  }
  return result === 0;
}

// Expected credentials (in practice, use environment variables and a secure store)
const USER = process.env.ADMIN_USER || 'admin';
const PASS = process.env.ADMIN_PASS || 'secure_password';

app.post('/secure-data', (req, res) => {
  const credentials = auth(req);
  if (!credentials || !safeCompare(credentials.name, USER) || !safeCompare(credentials.pass, PASS)) {
    res.status(401).send('Unauthorized');
    return;
  }

  // Only after authentication, process the encrypted payload
  const encrypted = req.body.encryptedData;
  try {
    const decrypted = decryptData(encrypted); // Assume decryptData handles padding internally
    // Further validation and business logic
    res.json({ data: decrypted });
  } catch (err) {
    // Use a generic error to avoid leaking padding details
    res.status(400).send('Bad request');
  }
});

function decryptData(ciphertext) {
  // Placeholder: use a vetted library that uses constant-time padding checks
  // e.g., Node.js crypto with proper error handling that does not distinguish padding errors
  return 'decrypted_payload';
}

Key points in the remediation:

• Use the basic-auth package to parse credentials and a constant-time comparison to avoid timing leaks.
• Validate credentials before attempting to decrypt sensitive payloads; if decryption must occur first, ensure errors are generic and do not distinguish padding failures from other issues.
• Return a uniform 401 or 400 response for both authentication failures and malformed encrypted data to prevent the server from acting as a padding oracle.
• Store credentials securely and rotate them regularly; consider moving away from Basic Auth in favor of token-based mechanisms where feasible.

Using middleware to enforce secure authentication flow

const { app } = require('@fibers/fiber');
const auth = require('basic-auth');

// Middleware that enforces authentication with constant-time checks
function secureAuth(req, res, next) {
  const credentials = auth(req);
  const expectedUser = process.env.ADMIN_USER;
  const expectedPass = process.env.ADMIN_PASS;

  if (!credentials || !safeCompare(credentials.name, expectedUser) || !safeCompare(credentials.pass, expectedPass)) {
    res.status(401).send('Unauthorized');
    return;
  }
  next();
}

function safeCompare(a, b) {
  if (a.length !== b.length) return false;
  let result = 0;
  for (let i = 0; i < a.length; i++) {
    result |= a.charCodeAt(i) ^ b.charCodeAt(i);
  }
  return result === 0;
}

// Apply middleware to sensitive routes
app.post('/secure-data', secureAuth, (req, res) => {
  try {
    const decrypted = decryptData(req.body.encryptedData);
    res.json({ data: decrypted });
  } catch (err) {
    res.status(400).send('Bad request');
  }
});