XSSI (Cross-Site Script Inclusion)

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Basic Information

Cross-Site Script Inclusion (XSSI) is a vulnerability that arises from the nature of the script tag in HTML. Unlike most resources, which are subject to the Same-Origin Policy (SOP), scripts can be included from different domains. This behavior is intended to facilitate the use of libraries and other resources hosted on different servers but also introduces a potential security risk.

Key Characteristics of XSSI:

  • Bypass of SOP: Scripts are exempt from the Same-Origin Policy, allowing them to be included across domains.

  • Data Exposure: An attacker can exploit this behavior to read data loaded via the script tag.

  • Impact on Dynamic JavaScript/JSONP: XSSI is particularly relevant for dynamic JavaScript or JSON with Padding (JSONP). These technologies often use "ambient-authority" information (like cookies) for authentication. When a script request is made to a different host, these credentials (e.g., cookies) are automatically included in the request.

  • Authentication Token Leakage: If an attacker can trick a user's browser into requesting a script from a server they control, they might be able to access sensitive information contained in these requests.


  1. Static JavaScript - This represents the conventional form of XSSI.

  2. Static JavaScript with Authentication - This type is distinct because it requires authentication to access.

  3. Dynamic JavaScript - Involves JavaScript that dynamically generates content.

  4. Non-JavaScript - Refers to vulnerabilities that do not involve JavaScript directly.

The following information is a sumary of https://www.scip.ch/en/?labs.20160414. Check it for further details.

Regular XSSI

In this approach, private information is embedded within a globally accessible JavaScript file. Attackers can identify these files using methods like file reading, keyword searches, or regular expressions. Once located, the script containing private information can be included in malicious content, allowing unauthorized access to sensitive data. An example exploitation technique is shown below:

<script src="https://www.vulnerable-domain.tld/script.js"></script>
<script> alert(JSON.stringify(confidential_keys[0])); </script>

Dynamic-JavaScript-based-XSSI and Authenticated-JavaScript-XSSI

These types of XSSI attacks involve confidential information being dynamically added to the script in response to a user's request. Detection can be performed by sending requests with and without cookies and comparing the responses. If the information differs, it may indicate the presence of confidential information. This process can be automated using tools like the DetectDynamicJS Burp extension.

If confidential data is stored in a global variable, it can be exploited using similar methods to those used in Regular XSSI. However, if the confidential data is included in a JSONP response, attackers can hijack the callback function to retrieve the information. This can be done by either manipulating global objects or setting up a function to be executed by the JSONP response, as demonstrated below:

      var angular = function () { return 1; };
      angular.callbacks = function () { return 1; };
      angular.callbacks._7 = function (leaked) {
<script src="https://site.tld/p?jsonp=angular.callbacks._7" type="text/javascript"></script>
      leak = function (leaked) {
<script src="https://site.tld/p?jsonp=leak" type="text/javascript"></script>

For variables not residing in the global namespace, prototype tampering can sometimes be exploited. This technique leverages JavaScript's design, where code interpretation involves traversing the prototype chain to locate the called property. By overriding certain functions, such as Array's slice, attackers can access and leak non-global variables:

Array.prototype.slice = function(){
  // leaks ["secret1", "secret2", "secret3"]

Further details on attack vectors can be found in the work of Security Researcher Sebastian Lekies, who maintains a list of vectors.


Takeshi Terada's research introduces another form of XSSI, where Non-Script files, such as CSV, are leaked cross-origin by being included as sources in a script tag. Historical instances of XSSI, such as Jeremiah Grossman’s 2006 attack to read a complete Google address book and Joe Walker’s 2007 JSON data leak, highlight the severity of these threats. Additionally, Gareth Heyes describes an attack variant involving UTF-7 encoded JSON to escape the JSON format and execute scripts, effective in certain browsers:

<script src="http://site.tld/json-utf7.json" type="text/javascript" charset="UTF-7"></script>
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