Email Injection on Aws

Email injection in Aws applications typically occurs when user input is concatenated into email headers or message bodies without proper validation. This vulnerability allows attackers to inject additional email headers, modify recipients, or alter message content by exploiting special characters like newline ( ) and carriage return ( ).

In Aws environments, email injection often appears in applications using the Aws SDK for JavaScript (v3) or the Aws CLI. A common pattern involves building email content from user-provided data:

const { SESClient, SendEmailCommand } = require('@aws-sdk/client-ses');

const sesClient = new SESClient({ region: 'us-east-1' });

const sendEmail = async (params) => {
  const { to, subject, body } = params;
  
  // Vulnerable: direct concatenation without validation
  const emailParams = {
    Destination: { ToAddresses: [to] },
    Message: {
      Subject: { Data: subject },
      Body: { Text: { Data: body } }
    },
    Source: '[email protected]'
  };
  
  await sesClient.send(new SendEmailCommand(emailParams));
};

An attacker could exploit this by submitting a to parameter like:

[email protected]%0ABcc: [email protected]%0A%0AThis is injected content

The %0A represents newline characters, allowing the attacker to add BCC recipients and inject content into the email body. This works because the application doesn't validate or sanitize the input before passing it to SES.

Another Aws-specific manifestation occurs when using Lambda functions with SES for transactional emails. Developers often concatenate form data directly into email templates:

export const handler = async (event) => {
  const formData = JSON.parse(event.body);
  
  const emailContent = `
    Name: ${formData.name}
    Email: ${formData.email}
    Message: ${formData.message}
  `;
  
  // Vulnerable to header injection via formData.email
  await sesClient.send(new SendEmailCommand({
    Destination: { ToAddresses: ['[email protected]'] },
    Message: {
      Subject: { Data: 'New Contact Form Submission' },
      Body: { Text: { Data: emailContent } }
    },
    Source: '[email protected]'
  }));
};

The vulnerability here is that formData.email could contain newline characters, allowing injection into the email headers or body.

Detecting email injection in Aws applications requires both static analysis and runtime scanning. The Aws SDK for JavaScript (v3) provides built-in validation for some parameters, but developers often bypass these protections.

Static analysis should look for these patterns in your codebase:

# Search for vulnerable SES usage
grep -r "SendEmailCommand" --include="*.js" --include="*.ts"
grep -r "@aws-sdk/client-ses" --include="*.json"

Look for code that directly interpolates user input into email parameters without validation. The middleBrick scanner can automatically detect these patterns by analyzing your API endpoints and identifying unvalidated email parameters.

For runtime detection, middleBrick's black-box scanning tests for email injection by submitting payloads with newline characters and observing the response. The scanner checks:

• Whether newline characters in email fields are properly rejected
• If the application validates email header syntax
• Whether the email sending function sanitizes input
• If rate limiting prevents automated injection attempts

middleBrick specifically tests the unauthenticated attack surface, which is critical for email injection since many contact forms and registration endpoints are publicly accessible. The scanner sends payloads like:

[email protected]%0ABcc: [email protected]%0A%0AInjected%20content

and analyzes whether the email is sent with the injected headers or if the application rejects the malformed input.

For Aws Lambda functions, CloudWatch Logs provide additional detection capabilities. Monitor for:

# Look for suspicious patterns in logs
aws logs filter-log-events --log-group-name /aws/lambda/your-function \
  --filter-pattern "newline|Bcc|subject|Content-Type"

CloudTrail can also track SES API calls to identify unusual email patterns that might indicate injection attempts.

Remediating email injection in Aws applications requires input validation and proper use of Aws SDK features. The most effective approach combines validation at the application layer with secure configuration of email services.

First, implement strict input validation using a whitelist approach:

const { SESClient, SendEmailCommand } = require('@aws-sdk/client-ses');
const validator = require('validator'); // npm install validator

const sendSecureEmail = async (params) => {
  const { to, subject, body } = params;
  
  // Validate email format
  if (!validator.isEmail(to)) {
    throw new Error('Invalid email address');
  }
  
  // Check for newline characters
  if (to.includes('\n') || to.includes('\r')) {
    throw new Error('Email address contains invalid characters');
  }
  
  // Validate subject and body length
  if (subject.length > 998 || body.length > 10000) {
    throw new Error('Message too long');
  }
  
  // Use SES built-in validation
  const emailParams = {
    Destination: { ToAddresses: [to] },
    Message: {
      Subject: { Data: subject },
      Body: { Text: { Data: body } }
    },
    Source: '[email protected]'
  };
  
  await sesClient.send(new SendEmailCommand(emailParams));
};

The Aws SDK for JavaScript (v3) provides some built-in validation, but it's not comprehensive for security. Always add application-layer validation before calling SES.

For Aws Lambda functions, use environment variables for email configuration and implement request validation:

export const handler = async (event) => {
  const formData = JSON.parse(event.body);
  
  // Validate all form fields
  if (!validator.isEmail(formData.email)) {
    return { statusCode: 400, body: 'Invalid email format' };
  }
  
  // Sanitize input by removing control characters
  const sanitize = (input) => input.replace(/[\n\r%0a%0d]/g, '');
  
  const safeEmail = sanitize(formData.email);
  const safeName = sanitize(formData.name).substring(0, 100);
 const safeMessage = sanitize(formData.message).substring(0, 5000);
  
  const emailContent = `
    Name: ${safeName}
    Email: ${safeEmail}
    Message: ${safeMessage}
  `;
  
  await sesClient.send(new SendEmailCommand({
    Destination: { ToAddresses: ['[email protected]'] },
    Message: {
      Subject: { Data: 'New Contact Form Submission' },
      Body: { Text: { Data: emailContent } }
    },
    Source: process.env.EMAIL_SENDER || '[email protected]'
  }));
  
  return { statusCode: 200, body: 'Message sent successfully' };
};

For enhanced security, implement SES email policies and IAM restrictions:

{

This IAM policy restricts SES access to specific verified identities, preventing unauthorized email sending even if injection occurs.

middleBrick's continuous monitoring in the Pro plan can help maintain these security controls by regularly scanning your email endpoints and alerting you to any regression in validation logic.