Email Injection in Firestore

Email injection in Firestore environments typically occurs when user input flows through email generation systems without proper validation, allowing attackers to manipulate email headers or inject malicious content. The issue becomes particularly insidious in Firestore because of its flexible data model and common integration patterns with email services.

The most common Firestore-specific manifestation involves user-generated content stored in documents that later gets incorporated into transactional emails. Consider a Firestore document structure where users can input their name, email, and message:

// Vulnerable Firestore document structure
const userMessage = {
  name: "John Doe",
  email: "john@example.com",
  message: "Hello! I'd like to know more about your service."
};

When this data flows into email generation without sanitization, attackers can exploit newline characters to inject additional email headers. A malicious user might store:

// Malicious input with email header injection
const maliciousMessage = {
  name: "Attacker",
  email: "attacker@example.com\nCc: admin@example.com",
  message: "Hello!\nSubject: Urgent Security Issue\n\nThis is a test."
};

Firestore's flexible schema allows this data to be stored without validation, and when processed by email libraries, it can result in emails being sent to unintended recipients or with altered subjects. The attack works because many email libraries interpret newline sequences as header delimiters.

Another Firestore-specific pattern involves Cloud Functions triggered by document writes. Attackers can craft documents that, when processed by functions, trigger email generation with injected content:

// Cloud Function vulnerable to email injection
exports.sendNotification = functions.firestore
  .document('messages/{messageId}')
  .onCreate(async (snap, context) => {
    const data = snap.data();
    
    // Vulnerable: direct interpolation without sanitization
    const emailContent = `
      To: ${data.email}
      Subject: New Message from ${data.name}
      
      ${data.message}
    `;
    
    await sendEmail(emailContent); // sendEmail parses headers
  });

The Firestore security rules layer can also contribute to the problem. Rules that allow broad write access to collections where email data is stored increase the attack surface:

// Overly permissive Firestore security rules
match /messages/{messageId} {
  allow write: if request.auth != null; // Any authenticated user can write
}

Once an attacker gains write access, they can inject malicious content that persists in the database until processed by email generation systems.

Detecting email injection in Firestore environments requires a multi-layered approach combining static analysis, runtime monitoring, and specialized scanning tools.

Static code analysis should focus on identifying dangerous patterns where user input flows into email generation systems. Look for these specific Firestore patterns:

// Patterns to flag in code review
// 1. Direct interpolation of user data into email content
const emailContent = `To: ${userInput.email}\nSubject: ${userInput.subject}`;

// 2. Document triggers that process email data
exports.processEmail = functions.firestore
  .document('users/{userId}')
  .onUpdate(async (change, context) => { /* ... */ });

// 3. Security rules allowing broad write access to email-related collections
match /messages/{messageId} {
  allow write: if true; // Dangerous: no validation
}

Runtime monitoring should track document writes that contain suspicious patterns. Implement Cloud Function triggers that scan for newline characters in email fields:

// Runtime monitoring for email injection attempts
exports.monitorEmailInjection = functions.firestore
  .document('messages/{messageId}')
  .onCreate(async (snap, context) => {
    const data = snap.data();
    
    // Check for newline characters in email fields
    if (data.email?.includes('\n') || data.message?.includes('\n\n')) {
      console.warn('Potential email injection detected:', {
        document: context.params.messageId,
        data: data
      });
      
      // Alert security team or block the write
    }
  });

For comprehensive detection, middleBrick's API security scanner can identify email injection vulnerabilities by analyzing your Firestore endpoints. The scanner tests for header injection by sending payloads containing newline sequences and verifying if they're properly handled:

# Scan your Firestore API endpoints with middleBrick
middlebrick scan https://firestore.googleapis.com/

The scanner specifically tests for:

middleBrick's LLM security module also checks for AI-specific email injection scenarios where model outputs might be incorporated into email systems without proper validation.

Remediating email injection vulnerabilities in Firestore requires a defense-in-depth approach combining input validation, output encoding, and architectural safeguards.

Input validation should be implemented at the application layer before data reaches Firestore:

// Input validation middleware for email fields
function validateEmailInput(data) {
  const emailRegex = /^[^\s@]+@[^\s@]+\.[^\s@]+$/;
  
  if (!emailRegex.test(data.email)) {
    throw new Error('Invalid email format');
  }
  
  // Check for newline characters in critical fields
  if (data.email.includes('\n') || data.subject?.includes('\n')) {
    throw new Error('Email headers cannot contain newline characters');
  }
  
  return data;
}

// Apply validation before writing to Firestore
app.post('/messages', (req, res) => {
  try {
    const validatedData = validateEmailInput(req.body);
    await firestore.collection('messages').add(validatedData);
    res.status(200).send('Message saved');
  } catch (error) {
    res.status(400).send({ error: error.message });
  }
});

For email generation, use libraries that automatically handle header encoding and prevent injection:

// Secure email generation using Nodemailer
const nodemailer = require('nodemailer');

async function sendSecureEmail(to, subject, message) {
  const transporter = nodemailer.createTransporter({
    // configuration
  });
  
  // Use proper email library instead of string interpolation
  const mailOptions = {
    from: 'noreply@yourcompany.com',
    to: to,
    subject: subject,
    text: message
  };
  
  await transporter.sendMail(mailOptions);
}

// Alternative: manual sanitization
function sanitizeEmailInput(input) {
  return input
    .replace(/\r/g, '')
    .replace(/\n/g, '')
    .replace(/\t/g, ' ')
    .trim();
}

Firestore security rules should enforce field-level validation to prevent malicious data from being stored:

// Firestore security rules with validation
match /messages/{messageId} {
  allow read: if true;
  allow create: if isValidMessage(request.resource.data);
  allow update: if request.auth != null && isValidMessage(request.resource.data);
}

function isValidMessage(data) {
  // Check for newline characters in critical fields
  return !(data.email?.matches('.*\n.*') || 
           data.subject?.matches('.*\n.*') ||
           data.name?.matches('.*\n.*'));
}

Implement Cloud Function triggers that sanitize data before it's used in email generation:

// Data sanitization Cloud Function
exports.sanitizeEmailData = functions.firestore
  .document('messages/{messageId}')
  .onCreate(async (snap, context) => {
    const data = snap.data();
    const sanitizedData = {
      name: data.name?.replace(/[^\w\s]/g, ''),
      email: data.email?.replace(/[^\w@\.\-]/g, ''),
      message: data.message?.replace(/\n/g, ' ')
    };
    
    await snap.ref.update(sanitizedData);
  });

For enterprise deployments, middleBrick's Pro plan provides continuous monitoring that automatically scans your Firestore endpoints on a configurable schedule, alerting you when new email injection vulnerabilities are detected.