What is Same Origin Policy (SOP)? An easy-to-understand explanation of the basic concepts of web security

Explanation of IT Terms

What is the Same Origin Policy (SOP)? An Easy-to-Understand Explanation of the Basic Concepts of Web Security


In the world of web development and security, the Same Origin Policy (SOP) plays a significant role in protecting users’ sensitive information. It is a crucial concept that helps ensure the trustworthiness and security of web applications. In this blog post, we will provide a simplified explanation of SOP, its purpose, and its importance in maintaining the integrity of web browsing.

Understanding Same Origin Policy

The Same Origin Policy is a security measure implemented by web browsers to restrict interactions between different origins. An origin is defined by the combination of a protocol (such as HTTP or HTTPS), domain, and port number. Simply put, SOP mandates that web pages or scripts from one origin (a combination of protocol, domain, and port) cannot access or interact with resources from a different origin.

The main goal of SOP is to prevent undesirable actions, such as the unauthorized access or manipulation of data, which may occur due to malicious scripts or websites. By isolating different origins, the Same Origin Policy mitigates the risk of cross-site scripting (XSS) attacks and ensures the integrity of sensitive information.

How Same Origin Policy Works

Let’s consider an example to help illustrate how SOP works. Imagine you visit a trustworthy website, www.example.com, which contains an embedded HTML form for submitting personal information like your name and email address. You fill out the form and click the submit button.

If SOP is not in place, malicious websites or scripts could potentially access the form data without your knowledge or consent, putting your personal information at risk. However, thanks to the Same Origin Policy, the browser blocks any attempts made by scripts from other origins, preventing unauthorized access to the data.

To be more precise, SOP restricts the following interactions:

1. It prohibits reading properties (such as cookies, local storage, or DOM) of a document in another origin.
2. It prevents sending XMLHttpRequest or Fetch API requests to a different origin.
3. It discontinues executing JavaScript code of a different origin within the same document.

By enforcing these restrictions, SOP ensures that web browsing is secure and that sensitive information remains protected from unauthorized access.


In summary, the Same Origin Policy is a fundamental security measure that enhances the trustworthiness and integrity of web applications. It restricts interactions between different origins, preventing unauthorized access to sensitive information and protecting users from potential attacks.

As web developers, it is essential to understand and adhere to the principles of SOP to ensure the security of the applications we create. By implementing proper security measures, following best practices, and staying updated on evolving web security standards, we contribute to a safer and more secure web environment for everyone.

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