Email Injection with Api Keys

Email injection in API keys typically occurs when user-supplied email addresses are incorporated into API requests without proper validation. Attackers exploit this by crafting malicious email headers that can trigger unintended API calls, exfiltrate data, or manipulate authentication flows.

The most common pattern involves API endpoints that accept email parameters for password resets, account verification, or notification delivery. Without sanitization, an attacker can inject newline characters (%0A or %0D) to add arbitrary headers or modify the email body structure.

// Vulnerable API key usage
const resetPassword = async (email) => {
  const apiKey = process.env.API_KEY;
  const url = `https://api.example.com/v1/reset-password?key=${apiKey}`;
  
  // No validation of email format or newline characters
  const payload = { email: email };
  
  return fetch(url, {
    method: 'POST',
    headers: { 'Content-Type': 'application/json' },
    body: JSON.stringify(payload)
  });
};

// Malicious input:
// "[email protected]%0A%0DBCC%[email protected]"
// This injects a BCC header into the email

Another manifestation occurs in API keys that handle email-based authentication. When the API processes login tokens sent via email, attackers can manipulate the email content to include script tags or redirect URLs that compromise the authentication flow.

# API endpoint vulnerable to email injection
@app.route('/api/verify-email', methods=['POST'])
def verify_email():
    data = request.get_json()
    email = data.get('email')
    
    # No validation - allows header injection
    send_verification_email(email)
    
    return {'status': 'verification_sent'}

Rate limiting bypass is another attack vector. Attackers can craft emails with multiple recipients in the CC/BCC fields, causing the API to process multiple verification requests in a single call, potentially exceeding rate limits or triggering bulk operations.

Detecting email injection in API keys requires a multi-layered approach. Start with input validation at the API boundary using regex patterns that strictly validate email formats before processing.

// Secure email validation using regex
const validateEmail = (email) => {
  const emailRegex = /^[a-zA-Z0-9._%+-]+@[a-zA-Z0-9.-]+\.[a-zA-Z]{2,}$/;
  
  // Check for newline characters
  if (email.includes('%0A') || email.includes('%0D')) {
    throw new Error('Email contains invalid characters');
  }
  
  return emailRegex.test(email);
};

// Usage in API endpoint
app.post('/api/send-email', (req, res) => {
  const { email } = req.body;
  
  if (!validateEmail(email)) {
    return res.status(400).json({ error: 'Invalid email format' });
  }
  
  // Proceed with safe email processing
});

For comprehensive detection, implement runtime scanning with tools like middleBrick that specifically test for email injection vulnerabilities. middleBrick's black-box scanning approach tests unauthenticated endpoints by sending crafted payloads containing newline characters and malicious email headers.

middleBrick scans for 12 security categories including input validation and data exposure. For email injection specifically, it tests:

• Newline character injection (%0A, %0D) in email parameters
• Header manipulation attempts (BCC, CC, Reply-To injection)
• Multiple recipient injection in single API calls
• Malformed email address formats designed to bypass validation

The scanner provides severity ratings and remediation guidance specific to your API's implementation. For API keys, middleBrick also checks if keys are exposed in email responses or logs, which could compound email injection risks.

Remediation starts with strict input validation using whitelist approaches rather than blacklist filtering. Implement email validation at multiple layers: API gateway, application code, and email service integration.

# Comprehensive email validation and sanitization
import re
from email_validator import validate_email, EmailNotValidError
from html import escape

def sanitize_email_input(email: str) -> str:
    """Validate and sanitize email input for API processing"""
    
    # Step 1: Validate email format
    try:
        valid = validate_email(email)
        email = valid.email  # Normalized form
    except EmailNotValidError:
        raise ValueError('Invalid email format')
    
    # Step 2: Check for malicious characters
    if any(char in email for char in ['\n', '\r', '%0A', '%0D']):
        raise ValueError('Email contains invalid characters')
    
    # Step 3: Sanitize for HTML output if needed
    return escape(email)

# Usage in API endpoint
@app.route('/api/verify', methods=['POST'])
def verify_email():
    data = request.get_json()
    email = data.get('email', '')
    
    try:
        safe_email = sanitize_email_input(email)
        send_verification(safe_email)
        return {'status': 'verification_sent'}
    except ValueError as e:
        return {'error': str(e)}, 400

For API keys that interact with email services, implement rate limiting and recipient validation. Limit the number of recipients per API call and validate domain ownership for verification emails.

// Rate-limited email API with recipient validation
const MAX_RECIPIENTS = 1;
const RATE_LIMIT_WINDOW = 60000; // 1 minute
const rateLimitMap = new Map();

const sendVerificationEmail = async (email) => {
  // Rate limiting check
  const now = Date.now();
  const windowStart = now - RATE_LIMIT_WINDOW;
  
  const requests = rateLimitMap.get(email) || [];
  const recentRequests = requests.filter(timestamp => timestamp > windowStart);
  
  if (recentRequests.length >= 5) {
    throw new Error('Rate limit exceeded');
  }
  
  rateLimitMap.set(email, [...recentRequests, now]);
  
  // Validate recipient count
  const recipients = email.split(',').map(e => e.trim());
  if (recipients.length > MAX_RECIPIENTS) {
    throw new Error('Too many recipients');
  }
  
  // Send email through secure service
  return await emailService.send({
    to: recipients[0],
    subject: 'Verification Code',
    html: generateVerificationTemplate()
  });
};

Implement logging and monitoring for suspicious email patterns. Track failed validation attempts, rate limit violations, and unusual email formats. Use middleBrick's continuous monitoring in Pro plans to automatically scan your APIs on a schedule and alert on new vulnerabilities.