Enhancing PostMessage XSS Detection with Proxy Object Instrumentation

The article introduces a new method for detecting PostMessage Cross-Site Scripting (XSS) vulnerabilities using JavaScript Proxy objects, which enhances traditional dynamic fuzzing techniques.

Thu 04 April 2024


Cross-Site Scripting (XSS) vulnerabilities remain a persistent threat. It exploits the dynamic nature of web applications to execute malicious scripts. Among the myriad channels through which XSS attacks can be facilitated, the HTML5 postMessage API stands out due to its widespread use in enabling cross-origin communications.

However, this utility also opens a Pandora's box of vulnerabilities, particularly when not rigorously validated. While effective to a degree, traditional detection methods often fall short in accurately pinpointing and mitigating such vulnerabilities, especially in complex, dynamically generated web environments.

This article introduces a novel approach to enhancing the detection of PostMessage XSS vulnerabilities. It leverages the capabilities of JavaScript Proxy objects for instrumentation combined with a profiling phase preceding dynamic fuzzing.

By integrating Proxy object instrumentation into the preliminary profiling of web applications, we set the stage for a more targeted and efficient fuzzing process. This approach streamlines the detection of nuanced XSS vectors.

The following sections will delve into the technical details of this approach and its implementation. Through this exploration, we aim to arm developers, security professionals, and researchers with a robust toolset to fortify web applications against the ever-present threat of XSS attacks.


The postMessage API, integral to HTML5, revolutionized web communications by enabling cross-origin message passing. Designed to facilitate interaction between documents from different origins, it plays a crucial role in the modern web, powering everything from third-party widgets to complex single-page applications. However, its flexibility and power make it a target for exploitation, mainly through Cross-Site Scripting (XSS) attacks.

XSS attacks involve injecting malicious scripts into web pages viewed by other users, exploiting the trust a user has for a particular site. Traditional XSS vulnerabilities arise from improperly sanitized user input directly included in web page content. The postMessage API introduces a new vector for such attacks by sending messages cross-origin, potentially including malicious content that can be executed in the context of the receiving page.

Common Vulnerabilities and Pitfalls

Origin Validation

One of the main vulnerabilities associated with postMessage is the lack of origin validation. When a message is received, failing to check the message's origin or incorrectly implementing this check can lead to the acceptance of messages from malicious sources. The following example demonstrates a vulnerable implementation:

window.addEventListener('message', (event) => {

  // Dangerous: No check for `event.origin`



In this snippet, the message event listener indiscriminately executes code contained in any received message without verifying its origin. This approach opens the door open for executing arbitrary JavaScript code, making it a prime target for XSS attacks.

Example of code with origin validation.

window.addEventListener('message', (event) => {
  // Securely checking the origin of the message
  if (event.origin === '') {
    // Assuming the content is safe, further validation can also be implemented here
  } else {
    console.error('Untrusted origin:', event.origin);

Cross-Site Scripting

PostMessage XSS occurs when an application improperly handles data received through the postMessage API, leading to the execution of untrusted and potentially malicious code. This vulnerability often arises from two common oversights:

  • Lack of Origin Validation: Failing to validate the origin of the incoming message can allow attackers to send malicious messages from untrusted sources.
  • Improper Sanitization of Message Content: Treating the content of received messages as trusted input without adequate sanitization can lead to the execution of arbitrary JavaScript code.

Consider a web application that listens for messages to update content based on data received through postMessage dynamically:

window.addEventListener('message', (event) => {
  // Assume the content of the message is a URL to be navigated to
  if ( {
    window.location.href =; // Potential for exploitation

In this scenario, an attacker could craft a message containing a JavaScript URL (javascript:), leading to arbitrary code execution:

parent.postMessage({url: "javascript:alert('XSS')"}, "*");

Proxy Objects in Javascript

The JavaScript Proxy object is a powerful feature introduced in ECMAScript 2015 (ES6) that enables the creation of a proxy for another object. This allows you to intercept and redefine fundamental operations for that object, such as property lookup, assignment, enumeration, and function invocation. This capability is particularly useful in a variety of advanced scenarios, such as object virtualization, logging, profiling, and data validation.

Fundamentals of Proxy Object Instrumentation

A Proxy object is created with two parameters: the target object it encapsulates and a handler object defining traps for various operations. The handler object is where the interception logic is defined. Here's a basic example:

let target = {};
let handler = {
  get: function(obj, prop) {
    console.log(`Accessing property ${prop}`);
    return prop in obj ? obj[prop] : 37; // Default value

let proxy = new Proxy(target, handler);
console.log(proxy.a); // Output: Accessing property a
                      // 37 (since 'a' is not a property of target)

In this example, the get trap logs access to properties and returns a default value if the property does not exist on the target object.

Limitations of Proxy Object

While JavaScript Proxies provide powerful capabilities for security instrumentation and other advanced operations, they also come with certain limitations. Understanding these limitations is crucial for developers to effectively utilize Proxies in their applications.

  1. Inability to Intercept Actions on Native Types: JavaScript Proxies cannot directly intercept actions performed on native types such as strings, numbers, or Booleans. Since these types are not objects, operations on them cannot be intercepted by a Proxy. For example, you cannot use a Proxy to directly intercept string comparisons or transformations. This limitation can impact the ability to monitor and validate operations involving primitive values.
let proxy = new Proxy('example string', handler); // This will throw an error
  1. Not Able to Monitor Some Built-in Object Operations: Proxies cannot intercept certain operations on built-in objects, such as changing the length of an array directly or setting properties on functions. While you can intercept method calls and property accesses, changes to internal properties that don't trigger setter/getter access are beyond a Proxy's reach.
let numbers = new Proxy([1, 2, 3], handler);

numbers.length = 2; // This operation cannot be intercepted directly
  1. Non-transparent to Some Built-in Functions: Certain JavaScript built-in functions and methods can behave differently when their arguments are Proxy objects. For example, Array.isArray(proxyObject) will return false even if the proxy encapsulates an array. This non-transparent behavior can lead to unexpected code results that rely on type-checking or native behaviors.

Detection of PostMessage XSS with Proxy Object Instrumentation

Message Profiling

Incorporating a profiling phase before fuzzing enhances the effectiveness and efficiency of identifying vulnerabilities within a JavaScript application, particularly focusing on postMessage handlers and storage access patterns.

The profiling step aims to gather detailed insights into the application's behavior, focusing on capturing postMessage handlers and messages, along with interactions with web storage (like localStorage and sessionStorage). This information serves two primary purposes:

  • Retargeting for Fuzzing: By collecting detailed information about the application's interactions and data flows, it's possible to tailor the fuzzing inputs to exercise more code paths more effectively, leading to the discovery of vulnerabilities that might otherwise remain hidden.
  • Enhancing Performance: Profiling identifies which inputs are relevant and utilized by the application, allowing the subsequent fuzzing phase to focus on these areas. This targeted approach avoids wasting time on irrelevant inputs, thereby maximizing the efficiency of the fuzzing process.

To capture postMessage handlers, the addEventListener method is overridden to log all event listeners being registered. This is particularly useful for identifying message handlers that could be potential targets for fuzzing.

// Object to store event listeners
const registeredEventListeners = [];

// Save the original addEventListener method
const originalAddEventListener = EventTarget.prototype.addEventListener;

// Override the addEventListener method
EventTarget.prototype.addEventListener = function(type, listener, options) {
   // Store the event details
   registeredEventListeners.push({ element: this, type, listener, options });

   // Call the original addEventListener method, type, listener, options);

This code snippet stores each registered event listener in an array, making it easy to review and analyze the event listeners later, especially those associated with message events.

To monitor and log postMessage activities, a custom handler is added that logs messages being posted, using the console for efficiency and simplicity.

function handleMessage(event) {
   console.error("magic_post_message", JSON.stringify({data:, origin: event.origin}));

window.addEventListener('message', handleMessage, false);

This handler captures incoming messages and logs their content and origin, providing valuable insights into how postMessage is used within the application and highlighting potential areas for further investigation during fuzzing.

The profiling extends to tracking how the application interacts with objects, including nested property accesses, by using a Proxy to log accesses:

* This function creates a proxy around the provided obj that tracks access to its properties. If a property of the object
* (or a nested object) is accessed, the accessHandler function is called with the object, the property name, and the path
* to the property.
function createAccessTrackingProxy(obj, accessHandler) {
   // A recursive function to create a proxy for an object and its nested objects
   const createProxy = (target, path) => {
       return new Proxy(target, {
           get(target, property, receiver) {
               // Trigger the access handler function
               accessHandler(obj, property, path);

               // Check if the property accessed is an object and not null for recursion
               if (target[property] !== null && typeof target[property] === 'object') {
                   return createProxy(target[property], path.concat(property));

               // Return the actual property value
               const value = Reflect.get(target, property, receiver);
               if (isObject(value)) {
                   return createProxy(value, path.concat(property));
               } else {
                   if (coinFlip()) {
                       return createProxy({}, path.concat(property));
                   } else {
                       return value;


   return createProxy(obj, []);

* Logs path of object accesses
function accessHandler(obj, property, path) {
   try {
       console.log('magic_post_message', JSON.stringify({
           data: obj,
           origin: null,
           path: [...path, property],
   } catch (e) {
       // Pass.

This approach involves creating a Proxy around an object (or nested objects) and logging each access to its properties. The accessHandler function is invoked whenever a property is accessed, logging the path to the accessed property. This detailed tracking aids in understanding how the application interacts with its data, further informing the fuzzing process.

The use of a coin flip in this context is a simple way to introduce variability into the profiling phase, potentially uncovering hidden paths and behaviors by dynamically deciding whether to return the actual value or a proxy to a new object.

PostMessage Dynamic Fuzzing

The fuzzing step involves enriching the corpus of messages collected during the profiling phase with the discovered access paths. This augmentation process is key to uncovering more intricate vulnerabilities by ensuring that fuzzing exercises a broader range of code paths within the JavaScript application. Following augmentation, the message set is minified to remove redundant or irrelevant inputs, optimizing the efficiency of the fuzzing process.

Ostorlab, employs an insertion point generator that understands nested encoding schemes. This capability is crucial for testing applications that deal with complex objects, such as SAML messages, which might involve nested structures with base64, XML, HTTP query, and JSON encodings.

The provided code demonstrates how the fuzzer iterates over collected postMessage events, dynamically constructing fuzzed messages based on the collected access paths and generating insertions to probe the application:

if request.profile is not None:
    post_message_events = request.profile.post_messages or []
    for post_message_event in post_message_events:
        post_message = post_message_event["data"]

        # Traverse the message structure based on collected paths
        post_message_pointer = post_message
        for elm in post_message_event.get("path", []):
            if elm not in post_message_pointer:
                post_message_pointer[elm] = {}
            post_message_pointer = post_message_pointer[elm]

        # Generate and yield fuzzed messages
        for generated_insertions in self.generate(post_message):
            insertions = [WhatInsertionPoints.POST_MESSAGE, post_message]
            yield insertions

        # Repeat the process for JSON-encoded messages
        for generated_insertions in self.generate(json.dumps(post_message)):
            insertions = [WhatInsertionPoints.POST_MESSAGE, json.dumps(post_message)]
            yield insertions

Detection of Cross-Site Scripting (XSS) vulnerabilities occurs when a JavaScript method callback, triggered by a fuzzed message, executes malicious code. Ostorlab enhances the detection process by utilizing console.trace to collect stack traces whenever an XSS vulnerability is triggered. This approach allows for the precise identification of the execution path leading to the vulnerability, facilitating deeper analysis and more effective remediation.


The combination of profiling, Proxy-object Instrumentation, and traditional dynamic fuzzing has proven effective in detecting unseen PostMessage XSS vulnerabilities.

However, challenges such as string comparison highlight the need for further enhancements. Our next step is to incorporate static analysis to detect field comparisons to ensure even better coverage.