Explain an exception to the Same Origin Policy that allows sites to submit forms to each other.
The Same Origin Policy (SOP) is a fundamental security concept in web applications that restricts the interaction between different origins (combinations of scheme, hostname, and port). It aims to prevent malicious websites from accessing sensitive information or performing unauthorized actions on behalf of the user. However, there are certain exceptions to the SOP that allow sites to submit forms to each other. One such exception is the Cross-Origin Resource Sharing (CORS) mechanism.
CORS is a mechanism that enables controlled access to resources hosted on a different origin. It allows a web page to make cross-origin requests to another domain, even if they have different origins. This is achieved through a combination of browser-side enforcement and server-side configuration.
To understand how CORS works, let's consider an example scenario. Suppose we have two websites, "example.com" and "api.example.com". The web page hosted on "example.com" wants to submit a form to the server hosted on "api.example.com".
By default, the browser would block this request due to the SOP. However, if the server on "api.example.com" includes the appropriate CORS headers in its response, the browser will allow the cross-origin request. These headers include the "Access-Control-Allow-Origin" header, which specifies the allowed origins for cross-origin requests.
In our example, the server on "api.example.com" would include the following header in its response:
Access-Control-Allow-Origin: https://example.com
This header indicates that requests from "https://example.com" are allowed to access resources on "api.example.com". The value can also be set to "*" to allow access from any origin, although this should be used with caution as it may introduce security risks.
Additionally, there are other CORS headers that can be used to further control the behavior of cross-origin requests. For example, the "Access-Control-Allow-Methods" header specifies the allowed HTTP methods, and the "Access-Control-Allow-Headers" header defines the allowed request headers.
It's important to note that CORS is enforced by the browser, and it relies on the server to correctly configure the CORS headers. If the server does not include the necessary headers or includes incorrect values, the browser will block the cross-origin request.
The Same Origin Policy is a critical security concept in web applications that restricts cross-origin interactions. However, the CORS mechanism provides an exception to this policy, allowing sites to submit forms to each other by configuring the appropriate CORS headers on the server-side.
Other recent questions and answers regarding EITC/IS/WASF Web Applications Security Fundamentals:
- Does implementation of Do Not Track (DNT) in web browsers protect against fingerprinting?
- Does HTTP Strict Transport Security (HSTS) help to protect against protocol downgrade attacks?
- How does the DNS rebinding attack work?
- Do stored XSS attacks occur when a malicious script is included in a request to a web application and then sent back to the user?
- Is the SSL/TLS protocol used to establish an encrypted connection in HTTPS?
- What are fetch metadata request headers and how can they be used to differentiate between same origin and cross-site requests?
- How do trusted types reduce the attack surface of web applications and simplify security reviews?
- What is the purpose of the default policy in trusted types and how can it be used to identify insecure string assignments?
- What is the process for creating a trusted types object using the trusted types API?
- How does the trusted types directive in a content security policy help mitigate DOM-based cross-site scripting (XSS) vulnerabilities?
View more questions and answers in EITC/IS/WASF Web Applications Security Fundamentals