Email Injection in Feathersjs with Jwt Tokens

Email Injection in a Feathersjs service occurs when user-controlled input is used to construct email headers or message content without proper validation or sanitization. This vulnerability is often related to the underlying transport configuration (e.g., SMTP) and the way Feathersjs passes data to mail plugins. When Jwt Tokens are used for authentication and authorization, the security boundary can be misunderstood: JWTs protect endpoints and identify users, but they do not inherently prevent malicious input from being passed into email-related service logic.

In Feathersjs, services are typically defined with a create method that accepts data from the request. If an email-related service (or a hook) uses fields such as to, from, or replyTo directly from the request payload or query parameters, an attacker may inject additional email headers or addresses. Common techniques include adding extra recipients via newline characters or injecting custom headers like Cc: or Bcc:. Because Jwt Tokens are often used to authorize access to the service, developers might assume that the authenticated identity fully constrains the operation, but the email injection risk exists in the data handling layer, not the auth layer.

The combination of Jwt Tokens and Feathersjs can inadvertently encourage unsafe patterns. For example, a service may trust the authenticated user’s identity (verified by JWT) but still use unchecked user-supplied fields for email composition. Consider a scenario where a user profile update includes an email field that is later used in a notification email. If the email field is not validated for format and is passed directly to a mailer, an attacker could supply a payload like [email protected]\r\nCc: [email protected], causing the email to be sent to an unintended recipient. JWTs do not mitigate this because the request is still authenticated and authorized; the vulnerability lies in the unchecked data used downstream.

Another angle involves hooks that send emails, such as after a user is created. If the hook uses data from the JWT payload (e.g., user ID or email) in combination with uncontrolled input, injection may still occur if the email service library does not strictly validate recipient lists. For instance, a hook might read context.data.email and use it as the to field while also including headers derived from the JWT’s payload. Attackers may attempt to exploit log injection or header injection techniques to manipulate logs or redirect email delivery, especially if newline or control characters are not stripped.

It is important to distinguish that Jwt Tokens in Feathersjs primarily secure API routes and service method access. They do not cleanse or validate the content of the data being processed. Therefore, email injection risks must be addressed at the point where email content and headers are assembled. This includes validating and sanitizing all email-related fields, using parameterized mail APIs, and avoiding direct concatenation of user input into email headers. MiddleBrick scans can detect such input validation and data exposure issues across the unauthenticated attack surface, helping identify whether email injection vectors exist in Feathersjs services that rely on JWT-based authentication.

To remediate Email Injection in Feathersjs when Jwt Tokens are used, focus on strict input validation, safe mail composition, and minimizing data exposure in hooks. Below are concrete code examples demonstrating secure practices.

First, validate and sanitize email fields in your service schema and hooks. Use a library such as validator to ensure email addresses conform to a safe format and do not contain control characters.

const validator = require('validator');

app.use('messages', {
  async create(data, params) {
    const { to, subject, body } = data;
    if (!validator.isEmail(to)) {
      throw new Error('Invalid email address');
    }
    // Further processing with a trusted mailer
    return mailer.send({
      to: validator.normalizeEmail(to),
      subject: subject,
      text: body
    });
  }
});

Second, avoid using user-controlled data directly in email headers. Instead, construct headers explicitly and do not concatenate raw input. If you need to set custom headers, define them server-side and ensure no newline characters are present in user input.

const sanitizeHeader = (value) => {
  return value.replace(/\r|\n/g, '').substring(0, 100);
};

app.hooks({
  before: {
    async create(context) {
      const userEmail = context.params.user.email; // from JWT identity
      const safeHeader = sanitizeHeader(context.data.customHeader || '');
      context.data.internalMeta = { senderName: safeHeader };
      return context;
    }
  }
});

Third, configure your mail transport to use parameterized APIs or SDKs that do not allow header injection. For example, using Nodemailer with explicitly defined options prevents injection through the message object.

const nodemailer = require('nodemailer');
const transporter = nodemailer.createTransport({
  host: 'smtp.example.com',
  port: 587,
  secure: false
});

app.service('notifications').hooks({
  after: {
    async create(context) {
      const message = {
        from: '[email protected]',
        to: context.data.recipient, // already validated
        subject: context.data.subject,
        text: context.data.text
      };
      await transporter.sendMail(message);
      return context;
    }
  }
});

Finally, review how JWT payload data is used in hooks and services. Do not assume that claims such as email are safe for use in headers without validation. Use middleware to verify and normalize data before it reaches email logic. The combination of Jwt Tokens for access control and rigorous input handling ensures that authenticated requests do not become injection channels.