Injection Flaws in Fiber with Jwt Tokens

Injection flaws in API frameworks occur when untrusted data is passed to an interpreter as part of a command or query. In Fiber, a popular fast HTTP framework for Node.js, this commonly manifests through insecure handling of JWT (JSON Web Token) values used for authentication. When endpoints concatenate or interpolate JWT header, payload, or signature values into system commands, database queries, or dynamic code evaluation, attackers may inject malicious content.

Consider an endpoint that extracts a JWT from an Authorization header and then uses part of the token (e.g., the payload segment) in a shell command or database operation without validation:

// Risky: using JWT payload content directly in a system command
app.get('/user/:token', (req, res) => {
  const token = req.params.token;
  // Decode token (simplified example)
  const payload = JSON.parse(Buffer.from(token.split('.')[1], 'base64').toString());
  const username = payload.sub;
  // Dangerous: directly using untrusted data in a command
  exec(`greet-user ${username}`, (err, stdout) => {
    if (err) return res.status(500).send(err);
    res.send(stdout);
  });
});

In this pattern, an attacker-supplied JWT with a malicious sub claim can lead to command injection. Even if the JWT is signed, the server should not trust the decoded payload for constructing system commands. Similarly, if a JWT is used to parameterize database queries — for example, embedding the token or its claims directly into raw query strings — it can lead to NoSQL or SQL injection depending on the backend.

Another vector involves log injection via JWT values. If a server logs raw JWTs or decoded claims without sanitization, attackers can inject newline characters or structured payloads that corrupt log integrity and facilitate log forging or log injection attacks. These issues align with common OWASP API Top 10 categories such as Broken Object Level Authorization (BOLA/IDOR) and Injection when JWTs are mishandled.

middleBrick scanning a Fiber endpoint that processes JWTs in this manner would flag the injection risk, showing how untrusted token data can reach sensitive interpreters. The scanner evaluates the unauthenticated attack surface and does not rely on credentials, making it effective for identifying these design issues early.

To remediate injection flaws when using JWTs in Fiber, avoid passing any part of a JWT (header, payload, or signature) directly into interpreters that execute code, queries, or shell commands. Always treat JWT content as untrusted data and apply strict validation, encoding, and separation of concerns.

1. Use parameterized commands and avoid string interpolation

Replace dynamic command construction with safe patterns. For example, instead of using exec with token-derived input, use a controlled mapping or whitelist approach:

// Safer: map usernames to commands without direct interpolation
const allowedUsers = new Set(['alice', 'bob', 'charlie']);
app.get('/user/:token', (req, res) => {
  const token = req.params.token;
  try {
    const payload = JSON.parse(Buffer.from(token.split('.')[1], 'base64').toString());
    const username = payload.sub;
    if (!allowedUsers.has(username)) {
      return res.status(403).send('User not allowed');
    }
    // Safe: no direct use of untrusted input in commands
    res.send(`Hello, ${username}`);
  } catch (err) {
    res.status(400).send('Invalid token');
  }
});

2. Parameterize database queries

If JWT claims are used to query data, use parameterized queries or an ORM to prevent injection. Here is an example using a generic database client with placeholders:

// Safe: parameterized query with JWT-derived user identifier
app.get('/profile/:token', async (req, res) => {
  const token = req.params.token;
  const payload = JSON.parse(Buffer.from(token.split('.')[1], 'base64').toString());
  const userId = payload.sub;
  // Assuming a database client with parameterized query support
  const result = await db.query('SELECT * FROM profiles WHERE user_id = $1', [userId]);
  res.json(result.rows);
});

3. Sanitize and encode outputs

When logging or returning JWT-related data, ensure proper encoding to prevent log injection or cross-site scripting. For example, sanitize newlines and special characters before writing to logs:

// Safe logging: remove or replace control characters
const sanitizeLog = (str) => str.replace(/[\r\n]+/g, ' ');
app.get('/log/:token', (req, res) => {
  const token = req.params.token;
  const sanitized = sanitizeLog(token);
  console.log(`Token logged: ${sanitized}`);
  res.send('Logged safely');
});

Using the middleBrick CLI (middlebrick scan <url>) or the GitHub Action to add API security checks to your CI/CD pipeline can help detect remaining JWT-related injection risks in your Fiber services. The scanner reviews unauthenticated attack surfaces and provides prioritized findings with remediation guidance, complementing secure coding practices.