Owasp Testing Guide v4

Test Web Messaging (OTG-CLIENT-011)


Web Messaging (also known as Cross Document Messaging) allows applications running on different domains to communicate in a secure manner. Before the introduction of web messaging the communication of different origins (between iframes, tabs and windows) was restricted by the same origin policy and enforced by the browser, however developers used multiple hacks in order to accomplish these tasks, most of them were mainly insecure.

This restriction within the browser is in place to restrict a malicious website to read confidential data from other iframes, tabs, etc, however there are some legitimate cases where two trusted websites need to exchange data between each other. To meet this need Cross Document Messaging was introduced within he WHATWG HTML5 draft specification and implemented in all major browsers. It enables secure communication between multiple origins across iframes, tabs and windows.

The Messaging API introduced the postMessage() method, with which plain-text messages can be sent cross-origin. It consists of two parameters, message and domain.

There are some security concerns when using '*' as the domain that we discuss below. Then, in order to receive messages the receiving website needs to add a new event handler, and has the following attributes:

  • data: The content of the incoming message
  • origin: The origin of the sender document
  • source: source window

An example:

Send message:

iframe1.contentWindow.postMessage(“Hello world”,”http://www.example.com”);

Receive message:

window.addEventListener(“message”, handler, true);
function handler(event) {
if(event.origin === 'chat.example.com') {
     /* process message (event.data) */
} else {
    /* ignore messages from untrusted domains */

Origin Security Concept

The origin is made up of a scheme, host name and port and identifies uniquely the domain sending or receiving the message, it does not include the path or the fragment part of the url. For instance, https://example.com/ will be considered different from http://example.com because the schema in the first case is https and in the second http, same applies to web servers running in the same domain but different port.

From a security perspective we should check whether the code is filtering and processing messages from trusted domains only, normally the best way to accomplish this is using a whitelist. Also within the sending domain, we also want to make sure they are explicitly stating the receiving domain and not '*' as the second argument of postMessage() as this practice could introduce security concerns too, and could lead to, in the case of a redirection or if the origin changes by other means, the website sending data to unknown hosts, and therefore, leaking confidential data to malicious servers.

In the case the website failed to add security controls to restrict the domains or origins that can send messages to a website most likely will introduce a security risk so it is very interesting part of the code from a penetration testing point of view. We should scan the code for message event listeners, and get the callback function from the addEventListener method to further analysis as domains must be always be verified prior data manipulation.

event.data Input Validation

Input validation is also important, even though the website is accepting messages from trusted domains only, it needs to treat the data as external untrusted data and apply the same level of security controls to it. We should analyze the code and look for insecure methods, in particular if data is being evaluated via eval() or inserted into the DOM via the innerHTML property as that would create a DOM-based XSS vulnerability.

How to Test

Black Box testing

Black box testing for vulnerabilities on Web Messaging is not usually performed since access to the source code is always available as it needs to be sent to the client to be executed.

Gray Box testing

Manual testing needs to be conducted and the JavaScript code analyzed looking for how Web Messaging is implemented. In particular we should be interested in how the website is restricting messages from untrusted domain and how the data is handled even for trusted domains. Below are some examples:

Vulnerable code example:

In this example, access is needed for every subdomain (www, chat, forums, ...) within the owasp.org domain. The code is trying to accept any domain ending on .owasp.org:

window.addEventListener(“message”, callback, true);

function callback(e) {
     </b>if(e.origin.indexOf(".owasp.org")!=-1) {<b>
          /* process message (e.data) */

The intention is to allow subdomains in this form:


Insecure code. An attacker can easily bypass the filter as www.owasp.org.attacker.com will match.

Example of lack of origin check, very insecure as will accept input from any domain:

window.addEventListener(“message”, callback, true);

function callback(e) {
     /* process message (e.data) */

Input validation example: Lack of security controls lead to Cross-Site Scripting (XSS)

window.addEventListener(“message”, callback, true);

function callback(e) {
     if(e.origin === "trusted.domain.com") {
          element.innerHTML= e.data;

This code will lead to Cross-Site Scripting (XSS) vulnerabilities as data is not being treated properly, a more secure approach would be to use the property textContent instead of innetHTML.


  • OWASP Zed Attack Proxy (ZAP) - https://www.owasp.org/index.php/OWASP_Zed_Attack_Proxy_Project ZAP is an easy to use integrated penetration testing tool for finding vulnerabilities in web applications. It is designed to be used by people with a wide range of security experience and as such is ideal for developers and functional testers who are new to penetration testing. ZAP provides automated scanners as well as a set of tools that allow you to find security vulnerabilities manually.


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