A Breakdown of The Same-Origin Policy and How It Protects Your Sweet Treats

Introduction The Same-Origin Policy (SOP) is a fundamental browser security feature that prevents scripts from one website from tampering with content on another. It’s the reason your cookies, those sweet data packet treats, stay safe from nosy neighbors. While SOP is primarily enforced by web browsers, it works hand-in-hand with server-side mechanisms like CORS (Cross-Origin Resource Sharing), which lets the browser know when it’s okay to loosen the restrictions for specific cross-origin requests. Understanding Origins An origin consists of three parts: The Protocol - http://, https://, ftp://, ssh://, etc. The Host - example.com, example2.com, example3.com, etc. The Port - 80, 443, 999, 8080, 8081, etc. With this in mind, we can imagine an origin that looks like this: http://some-website.com:8081/ In our imagined example: The Origin is: https:// The Host is: some-website.com and The Port is: 8081 We can also imagine an origin like: https://some-website.com/ Now, you might have noticed that the port is missing—so is this still a valid origin? Yes, it is! This is because when a port isn’t explicitly stated, the browser assumes the default port for the scheme. In the case of https, the default port is 443, so the origin is still valid and complete. Below, I have a table showing how the Same-Origin Policy would be applied if content from a specific URL tries to access other origins. Our Origin: https://some-website.com/ Origin Accessed Access Permitted https://some-website.com/ Yes: Same protocol, domain, and port. https://some-website.com/admin/ Yes: Same protocol, domain, and port (The directory does not matter). https://admin.some-website.com/ No: Different domain (While directories do not matter, sub-domains do). https://some-other-website.com/ No: Different domain. http://some-website.com/ No: Different port (The http protocol has the default port of 80) NOTE: Due to legacy requirements, SOP applies more relaxed rules to cookies. For cookies, only the host is checked, not the protocol or port. This means cookies are typically accessible from all subdomains of a site, despite each subdomain being technically a different origin. Why is the Same-Origin Policy even necessary? When a browser sends an HTTP request from one origin to another, it automatically includes any cookies that are associated with the target origin, such as session tokens or authentication cookies. These cookies allow the server to recognize the user and respond accordingly, often delivering content specific to the authenticated session. For example, if you're logged into your bank’s website and your browser sends a request to that bank's domain, your session cookie is attached so the server knows it's you and returns your personal account information. Now, imagine what would happen if there were no Same-Origin Policy. If you visited a malicious site while still logged into another service, like Gmail, Facebook, or your bank, that site could silently send requests to those other domains on your behalf. Since your session cookies would be sent along with those requests, the malicious site could trick the browser into making authenticated requests without your knowledge, and potentially even read or manipulate sensitive data in the responses. This kind of attack is known as Cross-Site Request Forgery (CSRF). The Same-Origin Policy prevents this by ensuring that even though the browser may send a request to another origin, it blocks any access to the response from that other origin unless it explicitly allows it (via mechanisms like CORS). Conclusion The Same-Origin Policy may seem like just another behind-the-scenes browser rule, but it's one of the web's most critical security features. By enforcing boundaries between different origins, it protects users from a wide range of attacks, particularly those that attempt to steal sensitive data by exploiting trusted sessions, like CSRF or cross-site scripting. Understanding how the Same-Origin Policy works and how it interacts with mechanisms like cookies and CORS is essential for any web developer aiming to build secure applications. It's a powerful reminder that while the open nature of the web is what makes it great, it also demands thoughtful constraints to keep users safe. So next time you reach for your sweet treats (a.k.a. cookies), remember: it’s the Same-Origin Policy that’s guarding the jar. References Same-origin policy - Security on the web | MDN What is Same Origin Policy (SOP)? | GeeksforGeeks Same-origin policy (SOP) | Web Security Academy Same-origin policy | Articles | web.dev Same-Origin Policy (SOP)

May 13, 2025 - 06:27

A Breakdown of The Same-Origin Policy and How It Protects Your Sweet Treats

Introduction

The Same-Origin Policy (SOP) is a fundamental browser security feature that prevents scripts from one website from tampering with content on another. It’s the reason your cookies, those sweet data packet treats, stay safe from nosy neighbors. While SOP is primarily enforced by web browsers, it works hand-in-hand with server-side mechanisms like CORS (Cross-Origin Resource Sharing), which lets the browser know when it’s okay to loosen the restrictions for specific cross-origin requests.

Understanding Origins

An origin consists of three parts:

The Protocol - http://, https://, ftp://, ssh://, etc.
The Host - example.com, example2.com, example3.com, etc.
The Port - 80, 443, 999, 8080, 8081, etc.

With this in mind, we can imagine an origin that looks like this:

http://some-website.com:8081/

In our imagined example:

The Origin is: https://
The Host is: some-website.com and
The Port is: 8081

We can also imagine an origin like:

https://some-website.com/

Now, you might have noticed that the port is missing—so is this still a valid origin? Yes, it is! This is because when a port isn’t explicitly stated, the browser assumes the default port for the scheme. In the case of https, the default port is 443, so the origin is still valid and complete. Below, I have a table showing how the Same-Origin Policy would be applied if content from a specific URL tries to access other origins.

Our Origin: https://some-website.com/

Origin Accessed	Access Permitted
`https://some-website.com/`	Yes: Same protocol, domain, and port.
`https://some-website.com/admin/`	Yes: Same protocol, domain, and port (The directory does not matter).
`https://admin.some-website.com/`	No: Different domain (While directories do not matter, sub-domains do).
`https://some-other-website.com/`	No: Different domain.
`http://some-website.com/`	No: Different port (The `http` protocol has the default port of 80)

NOTE: Due to legacy requirements, SOP applies more relaxed rules to cookies. For cookies, only the host is checked, not the protocol or port. This means cookies are typically accessible from all subdomains of a site, despite each subdomain being technically a different origin.

Why is the Same-Origin Policy even necessary?

When a browser sends an HTTP request from one origin to another, it automatically includes any cookies that are associated with the target origin, such as session tokens or authentication cookies. These cookies allow the server to recognize the user and respond accordingly, often delivering content specific to the authenticated session. For example, if you're logged into your bank’s website and your browser sends a request to that bank's domain, your session cookie is attached so the server knows it's you and returns your personal account information.

Now, imagine what would happen if there were no Same-Origin Policy. If you visited a malicious site while still logged into another service, like Gmail, Facebook, or your bank, that site could silently send requests to those other domains on your behalf. Since your session cookies would be sent along with those requests, the malicious site could trick the browser into making authenticated requests without your knowledge, and potentially even read or manipulate sensitive data in the responses. This kind of attack is known as Cross-Site Request Forgery (CSRF).

The Same-Origin Policy prevents this by ensuring that even though the browser may send a request to another origin, it blocks any access to the response from that other origin unless it explicitly allows it (via mechanisms like CORS).

Conclusion

The Same-Origin Policy may seem like just another behind-the-scenes browser rule, but it's one of the web's most critical security features. By enforcing boundaries between different origins, it protects users from a wide range of attacks, particularly those that attempt to steal sensitive data by exploiting trusted sessions, like CSRF or cross-site scripting.

Understanding how the Same-Origin Policy works and how it interacts with mechanisms like cookies and CORS is essential for any web developer aiming to build secure applications. It's a powerful reminder that while the open nature of the web is what makes it great, it also demands thoughtful constraints to keep users safe.

So next time you reach for your sweet treats (a.k.a. cookies), remember: it’s the Same-Origin Policy that’s guarding the jar.