Email Injection with Bearer Tokens

Email injection in Bearer Tokens occurs when user-supplied input containing newline characters is incorporated into email headers without proper sanitization. This vulnerability allows attackers to manipulate email headers by injecting additional headers or modifying existing ones, potentially leading to email spoofing, spam distribution, or information disclosure.

In the context of Bearer Tokens, email injection typically manifests in these specific scenarios:

• Password reset emails where the token is embedded in a URL that's included in the email body
• Account verification emails that contain dynamic content from user input
• Notification emails triggered by API endpoints that accept user data

The attack vector works by exploiting how email clients parse headers. When an attacker injects newline characters (%0A or %0D%0A in URL encoding), they can break out of the intended header context and inject malicious content.

// Vulnerable code pattern in Bearer Tokens context
const token = generateToken();
const emailBody = `
  Click here to reset your password:
  https://api.example.com/reset?token=${token}&email=${userInput}`;

mailer.send({
  to: user.email,
  subject: 'Password Reset',
  text: emailBody
});

If userInput contains newline characters followed by additional headers like CC: attacker@example.com, the email will be sent to unintended recipients. More sophisticated attacks might inject Bcc: headers or even attempt to add Content-Type: headers to change how the email is processed.

Another manifestation occurs when tokens are logged or stored in email templates without proper encoding. Consider this pattern:

// Template injection vulnerability
const template = `
  <div>
    <p>Reset your password: ${token}</p>
    <p>Email: ${userInput}</p>
  </div>
`;

// If userInput contains HTML or JavaScript, it executes in the recipient's client

This becomes particularly dangerous when combined with Bearer Tokens' token generation mechanism, as attackers might try to extract tokens by manipulating the email content structure or triggering unintended email routing through header injection.

Detecting email injection in Bearer Tokens requires a multi-layered approach that combines static analysis, dynamic testing, and runtime monitoring. Here are Bearer Tokens-specific detection strategies:

Static Code Analysis should flag these patterns:

// Search for these anti-patterns in your Bearer Tokens codebase
const vulnerablePatterns = [
  /\$\{[^}]*userInput[^}]*\}/, // Direct interpolation without sanitization
  /\n.*:/, // Newline followed by colon (header injection attempt)
  /CC:|BCC:|Content-Type:/i // Header injection keywords
];

Dynamic Testing involves sending payloads with newline characters through all email-sending endpoints:

// Test payload for email injection
describe('Email Injection Tests', () => {
  it('should reject newline characters in email content', async () => {
    const maliciousInput = 'test@example.com%0A%0DContent-Type: text/html%0A%0DSubject: Hacked!';
    
    const response = await api.post('/reset')
      .send({ email: maliciousInput })
      .expect(400); // Should reject or sanitize
  });
});

Runtime Monitoring can detect injection attempts by monitoring email sending patterns:

// Middleware to detect suspicious email patterns
const emailInjectionDetector = (req, res, next) => {
  const suspiciousPatterns = [
    /\n.*:/, // Newline with colon
    /CC:|BCC:/i, // Email header injection
    /Content-Type:/i
  ];
  
  const userInput = req.body?.email || req.body?.message;
  
  if (userInput && suspiciousPatterns.some(pattern => pattern.test(userInput))) {
    console.warn('Potential email injection detected:', userInput);
    // Log, alert, or reject the request
    return res.status(400).json({ error: 'Invalid email content' });
  }
  
  next();
};

Automated Scanning with middleBrick provides comprehensive email injection detection across your Bearer Tokens API:

// Using middleBrick CLI to scan for email injection vulnerabilities
npm install -g middlebrick

// Scan your Bearer Tokens API endpoints
middlebrick scan https://api.bearer-tokens.com \
  --category email-security \
  --output json > email-injection-report.json

middleBrick's email injection detection specifically looks for:

• Newline character injection in email-related parameters
• Header manipulation attempts in API responses
• Template injection vulnerabilities in email generation
• Cross-site scripting (XSS) in email content that could lead to token theft

The scanner tests 27 regex patterns for system prompt leakage and injection attempts, which includes email header manipulation patterns specific to modern web applications using Bearer Tokens.

Remediating email injection in Bearer Tokens requires implementing proper input validation, output encoding, and secure email handling practices. Here are Bearer Tokens-specific remediation strategies:

Input Sanitization is the first line of defense:

// Sanitize email input to prevent injection
const sanitizeEmailInput = (input) => {
  // Remove newline characters and other dangerous patterns
  const sanitized = input
    .replace(/\r/g, '')
    .replace(/\n/g, '')
    .replace(/CC:/gi, '')
    .replace(/BCC:/gi, '')
    .replace(/Content-Type:/gi, '')
    .trim();
  
  return sanitized;
};

// Use in your Bearer Tokens endpoints
app.post('/reset', (req, res) => {
  const email = sanitizeEmailInput(req.body.email);
  const token = generateToken();
  
  sendResetEmail(email, token);
  res.json({ success: true });
});

Output Encoding ensures that dynamic content in emails is safely rendered:

// Safe email template rendering
const renderSafeEmail = (template, data) => {
  // HTML escape all dynamic content
  const escapeHtml = (str) => {
    return str
      .replace(/&/g, '&')
      .replace(//g, '>')
      .replace(/"/g, '"')
      .replace(/'/g, ''')
      .replace(/\//g, '/');
  };
  
  return template.replace(/

/g, (match, key) => escapeHtml(data[key]));
};

// Usage in Bearer Tokens context
const emailTemplate = `
  <p>Click to reset: <a href="${resetUrl}">Reset Password</a></p>
  <p>If you didn't request this, ignore this email.</p>
`;

const safeEmail = renderSafeEmail(emailTemplate, { resetUrl });

Secure Email Libraries provide built-in protection against injection:

// Using Nodemailer with validation
const nodemailer = require('nodemailer');
const { validate } = require('email-validator');

const sendSecureEmail = async (to, subject, text) => {
  // Validate email format
  if (!validate(to)) {
    throw new Error('Invalid email address');
  }
  
  // Check for suspicious content
  const injectionPatterns = [
    /\n.*:/,
    /CC:/i,
    /BCC:/i,
    /Content-Type:/i
  ];
  
  if (injectionPatterns.some(pattern => pattern.test(text))) {
    throw new Error('Suspicious content detected');
  }
  
  const transporter = nodemailer.createTransport({
    host: 'smtp.example.com',
    secure: true
  });
  
  await transporter.sendMail({
    from: 'noreply@bearer-tokens.com',
    to,
    subject,
    text
  });
};

Rate Limiting and Monitoring helps detect abuse patterns:

// Rate limiting for email endpoints
const rateLimit = require('express-rate-limit');

const emailLimiter = rateLimit({
  windowMs: 15 * 60 * 1000, // 15 minutes
  max: 5, // Limit each IP to 5 requests per window
  message: 'Too many password reset requests',
  keyGenerator: (req) => {
    // Use email + IP for more granular limiting
    return req.ip + req.body.email;
  }
});

// Apply to Bearer Tokens email endpoints
app.post('/reset', emailLimiter, resetPasswordHandler);
app.post('/verify', emailLimiter, verifyEmailHandler);

Content Security Policy (CSP) for email clients that support HTML emails:

// CSP headers for email content
const cspPolicy = `
  default-src 'none';
  img-src https: data:;
  style-src 'none';
  font-src 'none';
  connect-src 'none';
`;

// Apply CSP to email headers
const applyCspToEmail = (headers) => {
  headers['Content-Security-Policy'] = cspPolicy;
  return headers;
};

For comprehensive protection, integrate middleBrick's continuous monitoring to automatically scan your Bearer Tokens API for email injection vulnerabilities. The Pro plan ($499/mo) includes continuous monitoring that scans your APIs on a configurable schedule and alerts you when new vulnerabilities are detected.