Identification of Email Injection

Email injection is a type of vulnerability that occurs when an attacker manipulates an application's email sending functionality by injecting additional email headers or content into an email message. This is typically achieved by inserting newline characters followed by extra SMTP (Simple Mail Transfer Protocol) headers or body content into fields that the application will include in an outgoing email. If the application fails to properly sanitize user input used in email headers or content, an attacker can exploit this to send unauthorized emails, potentially leading to spam, phishing attacks, or disclosure of sensitive information.

Common Indicators of Email Injection:

  1. Unsanitized User Input: If user-supplied data is directly included in email headers or body without proper sanitization, it might be vulnerable to injection.
  2. Unexpected Email Content: Receipt of emails with unexpected headers or content, especially when related to user input, can indicate a vulnerability.
  3. Injection Payloads in Logs: Presence of typical email injection payloads in application or email server logs.

How to Identify Email Injection:

  1. Input Manipulation: Test the application's email functionality by injecting newline characters followed by SMTP headers or additional content into email-related input fields. For example, inserting "\r\nBcc:" in a message or name field.
  2. Code Review: Analyze the source code for functions that send emails and check how user input is incorporated into email headers and body. Look for lack of sanitization or validation.
  3. Automated Scanning: Use automated web vulnerability scanners that can identify potential injection points by testing various payloads.
  4. Email Log Monitoring: Monitor email logs for unusual activity, such as emails containing unexpected headers or being sent to unusual recipients, which may indicate an exploitation attempt.

Mitigation Strategies:

  1. Input Sanitization: Implement rigorous input sanitization to remove or encode special characters, particularly newline characters, that can be used to inject additional email headers.
  2. Use Email Libraries: Utilize well-established email libraries or frameworks that inherently manage the risks of email injection by properly encoding and handling email content and headers.
  3. Validate Input: Ensure that all user input used in email content or headers is strictly validated against expected formats and values.
  4. Testing: Regularly test email functionality with various inputs to ensure that injection vulnerabilities are not present.
  5. Monitor and Log: Monitor and log the application's email sending activity to detect and respond to potential abuse or unusual patterns indicative of an attack.

By identifying and addressing email injection vulnerabilities, organizations can protect their email functionality from being exploited for spam, phishing, or other malicious activities, thereby safeguarding their reputation and the security of their communications.