XHR (Cross-Origin Resource Sharing), often referred to as CORS, is a security feature implemented in modern web browsers that allows or restricts web applications running at one origin to make requests to resources located on a different origin (domain, protocol, or port). This is a critical feature in web development and ensures that websites can securely interact with external APIs or resources from different domains without compromising user data or security.
The term XHR refers to XMLHttpRequest, a JavaScript object that allows web pages to make asynchronous requests to servers. Initially designed to work with the same-origin policy, XHR restricts requests to only the domain from which the page was loaded. This ensures that a web page cannot request data from an unrelated website unless explicitly permitted. However, this strict policy posed challenges for applications needing data from multiple sources, such as APIs or third-party services.
CORS (Cross-Origin Resource Sharing) was developed as a way to relax the same-origin policy, enabling web developers to safely request resources across different domains by specifying which origins are allowed to access a server’s resources.
PUT or DELETE), browsers perform a “preflight” request to the server to verify whether the actual request is allowed. This preflight is an OPTIONS request that checks the CORS headers.*).GET, POST, PUT, etc.) that are allowed.Access-Control-Allow-Credentials header to true.The XHR (XMLHttpRequest) object is used to send asynchronous HTTP requests to servers. With the introduction of CORS, the browser includes special headers in these requests to communicate with the server. For example, when making a request to a different domain, the browser will send an Origin header that includes the domain of the page making the request. The server then responds with specific CORS headers to indicate whether the request is allowed.
Origin header in the request, indicating the domain from which the request originated.PUT or with custom headers), the browser sends an OPTIONS request to the server to check if the cross-origin request is allowed. The server must respond with the appropriate CORS headers to permit the actual request.Access-Control-Allow-Origin, which tells the browser whether or not the cross-origin request is allowed.Understanding the key terms related to Cross-Origin Resource Sharing (CORS) is essential for developers working with web applications, especially those that interact with APIs across different domains. CORS is a security feature implemented in browsers to control how web pages can make requests to a domain other than the one that served the page. Having a clear grasp of CORS-related terminology is crucial for handling security, API integration, and ensuring smooth communication between client and server in web development.
| Term | Definition |
|---|---|
| CORS (Cross-Origin Resource Sharing) | A security feature that allows or restricts web pages from making requests to a different domain than the one that served the web page. |
| Same-Origin Policy (SOP) | A security measure implemented by browsers to prevent scripts on one origin from interacting with resources from another origin. SOP is the basis for CORS. |
| Origin | Defined as a combination of the scheme (protocol), hostname (domain), and port. Two URLs have the same origin if these components match exactly. |
| Preflight Request | A CORS mechanism where the browser sends an HTTP OPTIONS request before the actual request to ensure that the server supports the intended cross-origin request. |
| Simple Request | A CORS request that does not trigger a preflight check. These requests use safe HTTP methods like GET, POST (with certain constraints), or HEAD. |
| Access-Control-Allow-Origin | A response header used by the server to specify which origins are allowed to access its resources. |
| Access-Control-Allow-Methods | A response header that specifies the HTTP methods (GET, POST, PUT, DELETE, etc.) that the server permits for cross-origin requests. |
| Access-Control-Allow-Headers | A response header that lists the HTTP headers the server allows in cross-origin requests. |
| Access-Control-Allow-Credentials | A response header that indicates whether credentials (cookies, authorization headers, etc.) are allowed in cross-origin requests. |
| Access-Control-Expose-Headers | A response header that specifies which headers the browser can expose to the client in a cross-origin response. |
| Access-Control-Max-Age | A response header that specifies how long the results of a preflight request can be cached by the browser. |
| OPTIONS Method | An HTTP method used to describe the communication options for the target resource. Often used in preflight CORS requests. |
| Wildcard (*) | In the context of CORS, using * allows access from any origin. It can be used in the Access-Control-Allow-Origin header to allow requests from all domains. |
| Custom Headers | Headers not part of the standardized set of HTTP headers. They often require explicit server permissions in CORS requests. |
| Credentials Mode | A flag that determines whether the browser includes credentials like cookies in cross-origin requests. By default, they are not included. |
| Fetch API | A modern JavaScript API used to make network requests, including cross-origin requests. It follows the CORS security model by default. |
| XMLHttpRequest | An older API used to interact with servers. It also adheres to the CORS security model when making cross-origin requests. |
| JSONP (JSON with Padding) | A technique used to bypass the same-origin policy by loading cross-origin requests as a <script> element. CORS is the modern alternative to JSONP. |
| CORS Error | An error message returned when a cross-origin request violates the server’s CORS policy, often leading to blocked requests by the browser. |
| Cross-Site Scripting (XSS) | A security vulnerability where malicious scripts are injected into web pages. Proper CORS implementation helps mitigate the risk of XSS. |
| Proxy Server | A server that acts as an intermediary for requests from clients. Proxies are often used to handle CORS requests by making same-origin requests on behalf of the client. |
| Whitelisting | A security strategy in which specific domains are explicitly allowed to access resources on a server, typically implemented in CORS policies. |
| Blacklisting | A strategy in which certain domains are explicitly denied access to resources, although less common in CORS policies compared to whitelisting. |
| HTTP Status Codes | Status codes that indicate the outcome of a CORS request, such as 200 (OK) or 403 (Forbidden). A 403 can indicate a CORS-related issue. |
| Client-Side Scripting | Scripts running on the user’s browser, which make HTTP requests to servers. CORS controls how these scripts can interact with servers on different domains. |
| Server-Side Scripting | Code that runs on the server side, handling requests and setting the necessary CORS headers to permit or deny cross-origin access. |
| Strict-Transport-Security (HSTS) | A security policy that forces browsers to use HTTPS to connect to a domain. Combined with CORS, it helps ensure secure cross-origin data exchanges. |
| Web Security | A broad term that includes CORS as one of many measures used to secure web applications and protect them from vulnerabilities like CSRF and XSS. |
| CSRF (Cross-Site Request Forgery) | A type of attack that tricks a user into executing unwanted actions on a web application in which they are authenticated. CORS helps mitigate CSRF risks. |
| Subresource Integrity (SRI) | A security feature that ensures that fetched resources (like scripts) are delivered without unintended changes. CORS can help enforce SRI policies. |
| Content Security Policy (CSP) | A security standard to prevent certain types of attacks, including XSS, by controlling which resources can be loaded. CSP often complements CORS policies. |
Understanding these key terms will help web developers properly implement and troubleshoot CORS-related issues, ensuring a secure and functional interaction between web pages and APIs across different domains.
XHR, or XMLHttpRequest, is a JavaScript object used to send asynchronous HTTP requests. In the context of Cross-Origin Resource Sharing (CORS), it allows web applications to make requests to different origins (domains, protocols, or ports), provided the server permits it using CORS headers.
CORS enhances security by allowing servers to specify which origins can access their resources. It prevents malicious sites from making unauthorized requests, ensuring that only trusted domains can retrieve sensitive data.
A preflight request is an OPTIONS HTTP request that the browser sends before making certain types of cross-origin requests. It checks with the server whether the actual request is allowed by examining CORS headers such as Access-Control-Allow-Origin.
Key CORS headers include Access-Control-Allow-Origin, which specifies which domains can access resources; Access-Control-Allow-Methods, which lists allowed HTTP methods; and Access-Control-Allow-Credentials, which indicates whether credentials like cookies can be shared across origins.
The Same-Origin Policy restricts how a webpage can make requests to another origin, preventing unauthorized access to sensitive data. CORS provides controlled flexibility, allowing specific origins to bypass this restriction under certain conditions.
Start for only $1. Unlock endless learning opportunities with over 2,600 hours of IT training at our lowest price ever. Plus, get all new and updated online courses for free while your subscription remains active.
Cancel at your convenience. This exceptional deal on IT training provides you access to high-quality IT education at the lowest monthly subscription rate in the market. Boost your IT skills and join our journey towards a smarter tomorrow.
October is Cybersecurity Month. Join us for this FREE course, Cybersecurity Essentials: Protecting Yourself in the Digital Age
