Email Injection in Fiber with Jwt Tokens

Email Injection in the Fiber web framework typically occurs when user-controlled data, such as query parameters, form fields, or headers, is directly interpolated into email-related operations like sending a confirmation link or a password reset email. When JWT tokens are used for authentication and contain or influence email values, the risk shifts from simple header injection to token-driven logic abuse. If a token is accepted without strict validation and the decoded payload is used to construct email headers or message bodies, an attacker can supply crafted input that manipulates the email destination or format.

Consider a scenario where a JWT includes an email claim that is not rigorously validated against the authenticated user’s record. An attacker who can influence the token (via weak signing keys, algorithm confusion, or token leakage) can change the email claim to include newline characters or commas, which are then passed to email-sending functions. In Fiber, routes might decode the token and use the claim to set the recipient or the From address. Without strict allowlisting and encoding, newline characters (\r or \n) can break the email header structure, enabling header injection or forcing the email to be sent to an unintended address. This becomes a stored or reflected injection depending on whether the token is issued by an attacker or obtained through another vector.

The interaction with JWT tokens also exposes business logic flaws. For example, if token validation does not verify issuer, audience, or expiration with strict checks, an attacker can submit a token that claims a different email and trigger email actions in the context of another user. The scan checks such flows by observing whether untrusted claims are used to parameterize email recipients or headers. Because JWTs often carry identity information, improper handling here can lead to email redirection, phishing, or account takeover support actions. The scan flags these patterns as high-severity findings because they combine authentication mechanisms with direct email-path inputs, increasing the potential impact.

To remediate Email Injection in Fiber when JWT tokens are involved, enforce strict validation and encoding at every boundary where token claims influence email operations. Never trust the email claim in a JWT; instead, derive the recipient from your server-side user record after verifying the token. Use allowlists for email domains when possible, and encode all dynamic values before inserting them into email headers or bodies.

Example: secure token verification and email construction in Fiber.

// Secure pattern: verify JWT, then fetch user email from your data store
const jwt = require('jwt-simple');
const crypto = require('crypto');
const nodemailer = require('nodemailer');

const transporter = nodemailer.createTransport({
  host: 'smtp.example.com',
  port: 587,
  auth: {
    user: process.env.SMTP_USER,
    pass: process.env.SMTP_PASS,
  },
});

app.post('/request-password-reset', async (req, res) => {
  const token = req.headers.authorization?.split(' ')[1];
  if (!token) {
    return res.status(401).send({ error: 'missing_token' });
  }

  let payload;
  try {
    // Use proper secret or public key; avoid algorithm confusion
    payload = jwt.decode(token, process.env.JWT_SECRET, true, 'HS256');
  } catch (err) {
    return res.status(401).send({ error: 'invalid_token' });
  }

  // Do not use payload.email directly for recipient; validate and map to your user record
  if (!payload || !payload.sub) {
    return res.status(400).send({ error: 'invalid_claims' });
  }

  // Example: look up the user in your database by subject (sub), not by email in the token
  const user = await db.users.findBy({ where: { sub: payload.sub } });
  if (!user) {
    return res.status(404).send({ error: 'user_not_found' });
  }

  // Encode dynamic values to prevent injection in headers
  const subject = encodeText(`Password reset for ${user.email}`);
  const resetLink = `https://app.example.com/reset?token=${encodeURIComponent(generateResetToken(user.id))}`;

  const mailOptions = {
    from: `"App" `,
    to: user.email, // Use server-side email, not token claim
    subject: subject,
    text: `Your reset link: ${resetLink}`,
  };

  try {
    await transporter.sendMail(mailOptions);
    res.status(202).send({ message: 'reset_email_sent' });
  } catch (err) {
    res.status(500).send({ error: 'email_failed' });
  }
});

function encodeText(value) {
  // Basic header-safe encoding: remove or replace control chars and dangerous delimiters
  return value.replace(/[\r\n,;:]/g, '_');
}

function generateResetToken(userId) {
  return crypto.randomBytes(48).toString('hex');
}

Key practices: verify JWT with strong secret and explicit algorithm; use the token’s subject (sub) to look up the user; never directly use claims for email recipients or headers; encode dynamic strings before embedding them in email headers; enforce short token lifetimes and rotate signing keys. The Dashboard can help track scans over time to ensure these patterns remain consistent across versions, and the CLI allows you to integrate checks into your build pipeline with middlebrick scan .