HTML Injection in Email Invitations: A Cybersecurity Perspective

Introduction

HTML injection is a technique where malicious HTML or JavaScript code is injected into a web application, often leading to unauthorized content manipulation. In this case, a bug bounty hunter discovered that an email invitation system was vulnerable to HTML injection, allowing them to alter the email’s appearance. While this didn’t escalate to cross-site scripting (XSS), it highlights a critical security oversight in input validation.

Learning Objectives

• Understand how HTML injection works in email systems
• Learn how to test for HTML injection vulnerabilities
• Explore mitigation techniques to prevent such exploits

You Should Know

1. Testing for HTML Injection in Email Systems

Command/Code Snippet:

<b>Hacked!</b> 
<img src="invalid" onerror="alert('XSS Attempt')"> 

Step-by-Step Guide:

1. Identify an input field that renders content in an email (e.g., invitation descriptions).
2. Inject basic HTML tags like <b>, <i>, or <img>.
3. Check if the email client renders the HTML.
4. If successful, escalate with JavaScript (though many email clients block script execution).

2. Preventing HTML Injection in Web Applications

Code Snippet (PHP Example):

$clean_input = htmlspecialchars($_POST['user_input'], ENT_QUOTES, 'UTF-8'); 

Step-by-Step Guide:

1. Always sanitize user input before rendering it in emails or web pages.
2. Use built-in functions like `htmlspecialchars()` (PHP) or `DOMPurify` (JavaScript).
3. Implement Content Security Policy (CSP) headers to restrict inline scripts.

3. Detecting HTML Injection via Burp Suite

Steps:

1. Intercept the HTTP request containing the email content using Burp Suite.

2. Modify the input field with HTML payloads.

1. Forward the request and observe the email output.
2. If HTML is rendered, the application is vulnerable.

4. Mitigating Email-Based HTML Injection

Code Snippet (Node.js Example):

const sanitizeHtml = require('sanitize-html'); 
const clean = sanitizeHtml(userInput, { allowedTags: [] }); 

Step-by-Step Guide:

1. Use libraries like `sanitize-html` to strip unwanted HTML.

2. Whitelist only necessary tags (e.g., ``, ``).

Vulnerability: HTML Injection in Email Invitations 
Steps to Reproduce: 
1. Send an invite with `<b>Test</b>` in the description. 
2. Observe bold text in the received email. 
Impact: Allows attackers to manipulate email content.