👉 Overview

👀 What ?

Blocking main page to steal postmessage is a cyber attack tactic in which an attacker interferes with the communication between websites and users' browsers by blocking the main page and stealing the postmessage. The postmessage is a method that enables secure cross-origin communication in web applications. It's used to send data between web pages without the need for a server-side intermediary.

🧐 Why ?

This topic is important because it's a tactic used in cyber attacks to steal sensitive information. By understanding this tactic, you can better protect your web applications from similar attacks. Furthermore, understanding how the postmessage method is exploited can help you develop more secure ways of implementing cross-origin communication in your web applications.

⛏️ How ?

To protect your web applications from this type of attack, you should always validate the origin of messages received through the postmessage method. This can be done by checking the origin property of the message event against a whitelist of trusted origins. Additionally, avoid sending sensitive information through postmessages whenever possible. If you must send sensitive information, ensure it's properly encrypted before sending.

⏳ When ?

The use of the postmessage method and the possibility of its exploitation became more prevalent with the advent of complex web applications that require secure cross-origin communication. As such, it's been a relevant cybersecurity topic for several years.

⚙️ Technical Explanations

At a technical level, the postmessage method is a part of the Web API provided by browsers. It allows for the sending of data from one window to another, regardless of the origin of the two windows. This is done by dispatching a MessageEvent which carries the data. The recipient window then listens for the MessageEvent and processes the data. An attacker can block the main page of a website and position their own page in such a way that it receives the postmessages intended for the main page. They can then read the data in the messages, which can be anything from non-sensitive information to user credentials or other sensitive data.