Unicode Normalization in Fiber with Hmac Signatures

Unicode normalization inconsistencies become a security risk in Fiber when Hmac Signatures are used to validate the integrity of request data that may contain Unicode characters. If an API endpoint accepts user-supplied strings (e.g., usernames, identifiers, or headers) and incorporates them into the data that is signed, different Unicode representations of the same logical string can produce distinct byte sequences. Without normalization, these distinct byte sequences generate different Hmac Signatures, even though they are semantically equivalent. This mismatch can lead to signature validation bypasses or inconsistent enforcement of authentication/authorization checks.

In Fiber, routes that verify Hmac Signatures typically rely on a shared secret and a canonical representation of the request body or selected headers. If the body or header values are not normalized before signing and verification, an attacker can supply a crafted Unicode string that normalizes to the same logical value as a trusted input but yields a different signature. For example, the character “é” can be represented as a single code point U+00E9 or as a decomposed sequence “e” + U+0301. Without explicit normalization (e.g., NFC), these forms are treated as different inputs, and the Hmac verification may either incorrectly reject valid requests or, in some configurations, accept a manipulated representation if comparison logic is inconsistent.

This issue intersects with the broader category of BOLA/IDOR and Input Validation checks that middleBrick assesses. An API relying on Hmac Signatures for integrity must ensure that both client and server apply the same Unicode normalization form before computing or comparing signatures. Otherwise, attackers may exploit normalization discrepancies to forge requests that appear authentic. middleBrick’s checks for Input Validation and Data Exposure help surface such risks by correlating runtime behavior with spec definitions, including OpenAPI/Swagger 2.0/3.0/3.1 documents with full $ref resolution, to detect inconsistencies in how schemas and security schemes describe string handling.

To remediate Unicode normalization issues when using Hmac Signatures in Fiber, normalize all inputs that participate in signature generation before computing or verifying the Hmac. Use a consistent Unicode normalization form, such as NFC, across client and server. Below are concrete code examples for a Fiber-based API that signs requests with Hmac and validates them securely.

First, ensure normalization is applied to the payload or selected headers before signing. In Node.js, you can use the built-in unorm package or the normalize function from unicode-normalization to canonicalize strings. Here is an example that signs a JSON body using Hmac-SHA256 after normalizing string values recursively:

const express = require('express');
const crypto = require('crypto');
const unorm = require('unorm');

function normalizeObject(obj) {
  if (obj === null || typeof obj !== 'object') return obj;
  if (Array.isArray(obj)) return obj.map(normalizeObject);
  if (typeof obj === 'string') return unorm.nfkc(obj);
  const normalized = {};
  for (const [key, value] of Object.entries(obj)) {
    normalized[unorm.nfkc(key)] = normalizeObject(value);
  }
  return normalized;
}

const app = express();
app.use(express.json());

app.post('/webhook', (req, res) => {
  const normalizedBody = normalizeObject(req.body);
  const payload = JSON.stringify(normalizedBody);
  const signature = crypto.createHmac('sha256', process.env.WEBHOOK_SECRET)
    .update(payload)
    .digest('hex');
  // Compare signature with header, e.g., 'X-Hub-Signature-256'
  const expected = `sha256=${signature}`;
  const received = req.header('X-Hub-Signature-256');
  if (!received || !crypto.timingSafeEqual(Buffer.from(expected), Buffer.from(received))) {
    return res.status(401).send('Unauthorized');
  }
  res.json({ received: true });
});

On the client side, apply the same normalization before computing the Hmac. This ensures that the byte sequence used for signing matches the byte sequence used for verification, eliminating discrepancies caused by different Unicode representations. Also, prefer UTF-8 encoding explicitly when converting strings to buffers to avoid environment-dependent encoding differences:

const crypto = require('crypto');
const unorm = require('unorm');

function signPayload(data, secret) {
  const normalized = normalizeObject(data); // same normalizeObject as above
  const payload = JSON.stringify(normalized);
  return crypto.createHmac('sha256', secret)
    .update(payload, 'utf8')
    .digest('hex');
}

// Example usage:
const data = { user: 'café', meta: { name: 'naïve' } };
const token = signPayload(data, process.env.WEBHOOK_SECRET);

Additionally, validate and constrain acceptable Unicode ranges for identifiers where appropriate, and document the normalization form in API specifications (OpenAPI/Swagger) to inform consumers. middleBrick’s OpenAPI/Swagger analysis can help verify that string schemas describe normalization expectations, and its checks for Input Validation and Unsafe Consumption can identify endpoints where untrusted input is used directly in signed contexts. For production deployments, combine these code-level fixes with continuous monitoring (available in the Pro plan) to detect regressions in signature validation behavior.