How to avoid receiving postMessages from attackers ?

PostMessage is a JavaScript API that enables cross-origin communication between web pages. While it is a useful feature, it can also be a security concern if not handled properly. Attackers can use postMessage to perform malicious actions, such as stealing sensitive information or injecting scripts into the page.

To avoid receiving postMessages from attackers, there are several best practices that can be followed:

1. Origin validation: Verify that the sender of the postMessage is from a trusted origin by checking the event.origin property. Only accept postMessages from known and trusted origins. This can be done by comparing the origin of the sender against a list of allowed origins.
2. Use event.source: Verify that the sender of the postMessage is a valid window object by checking the event.source property. Only accept postMessages from known and trusted window objects.
3. Use targetOrigin: When sending postMessages, use the targetOrigin parameter to specify the origin of the recipient. This will ensure that the postMessage is only delivered to the intended recipient.
4. Use a token-based authentication mechanism: In addition to origin validation, you can use a token-based authentication mechanism to ensure that the postMessage is coming from a trusted source. For example, you can generate a unique token for each valid sender and include it in the postMessage. The recipient can then verify the token before processing the postMessage.
5. Sanitize the data: Before you process the postMessage, sanitize the data to ensure that it is safe to use. This can include checking for and removing any potentially dangerous characters or scripts.
6. Use Content Security Policy (CSP): CSP is a security feature that helps to detect and mitigate certain types of attacks, including Cross-Site Scripting (XSS) and data injection attacks. CSP can be used to prevent the browser from executing any scripts that are not explicitly allowed by the policy.

It’s important to keep in mind that there is no 100% foolproof way to protect against postMessage attacks, but using a combination of these methods can significantly reduce the risk. Keep your system and software updated, and it’s also important to stay up-to-date with the latest security best practices and to test your application for vulnerabilities on a regular basis.

In summary, postMessage is a powerful JavaScript API that enables cross-origin communication between web pages. However, it is important to handle it properly to avoid security concerns. By using origin validation, event.source, targetOrigin, token-based authentication, data sanitization, and Content Security Policy, you can greatly reduce the risk of receiving postMessages from attackers.

Sure, here are some examples of how to implement some of the postMessage security best practices:

Origin validation:

window.addEventListener("message", function(event) {
 if (event.origin !== "https://example.com") {
   // The message is not from a trusted origin. Do not process it.
   return;
 }
 // The message is from a trusted origin. Process it.
});

Use event.source:

window.addEventListener("message", function(event) {
 if (event.source !== window.parent) {
   // The message is not from a trusted window object. Do not process it.
   return;
 }
 // The message is from a trusted window object. Process it.
});

Use targetOrigin:

// When sending a postMessage
otherWindow.postMessage("Hello!", "https://example.com");

Use a token-based authentication mechanism:

const token = "123456";
// When sending a postMessage
otherWindow.postMessage({token: token, message: "Hello!"}, "https://example.com");
// When receiving a postMessage
window.addEventListener("message", function(event) {
 if (event.data.token !== token) {
   // The token is not valid. Do not process the message.
   return;
 }
 // The token is valid. Process the message.
});

Sanitize the data:

// When receiving a postMessage
window.addEventListener("message", function(event) {
 const message = event.data.replace(/<\/?script>/gi, "");
 // Process the sanitized message
});

Use Content Security Policy (CSP)

// Specifying a policy in the HTTP header
Content-Security-Policy: default-src 'self'; script-src 'self' https://example.com;
// Specifying a policy in a meta tag
<meta http-equiv="Content-Security-Policy" content="default-src 'self'; script-src 'self' https://example.com;">