What is CORS? [And How to fix a CORS error]

While you were developing a web application, have you ever run across a "CORS error"? If so, you are not by yourself. Understanding how Cross-Origin Resource Sharing (CORS) functions and how to correct any issues that may arise is crucial for modern web development.

We'll delve into CORS and examine its function in web development in this blog post. We'll define the Same-Origin Policy, discuss its drawbacks, and describe how CORS permits cross-origin requests. Also, we'll discuss the reasons behind CORS problems and look at alternative solutions, such as installing a CORS proxy, using JSONP, or enabling CORS on the server.

You'll have a comprehensive grasp of CORS by the end of this article and know how to handle it correctly in your online apps.

So let's get going!

So, what exactly is CORS then?

Cross-Origin Resource Sharing (CORS) is a mechanism that allows web servers to relax the Same-Origin Policy (SOP), which restricts web pages from making requests to a different domain than the one that served the web page.

But why is CORS crucial for the creation of websites? Simply put, it makes it possible for resources to be shared securely and responsibly between several origins. Web pages would only be able to make requests to the domain they were served from if CORS was in place, which would drastically limit the usefulness of many web applications.

Imagine, for instance, that you were unable to send an API call to a server other than the one that is hosting your website. This would restrict the kinds and quantities of data you could access, making it challenging to offer your users rich and dynamic content. By safely permitting cross-origin queries while yet applying the necessary security controls to prevent unwanted access to sensitive data, CORS enables us to get around these restrictions.

To put it briefly, CORS is a fundamental part of contemporary web development, and knowing how it operates is crucial for producing efficient and safe web applications.

Limitations of Same-Origin Policy

Despite being a crucial security feature of contemporary web browsers, the Same-Origin Policy (SOP) does have several restrictions that might make it difficult to develop intricate web applications. The following are some of the most important SOP limitations:

1. Cross-origin requests: As previously established, SOP forbids web pages from sending requests to domains other than the one that provided the web page in the first place. This means that the types and amounts of data that can be accessed are restricted to requests for resources coming from within the same domain.
2. Resource sharing is restricted: SOP prohibits the exchange of local storage and cookies between distinct domains. Maintaining a consistent user experience across various web application components might be difficult as a result.
3. Security for IFRAMEs: If an IFRAME has material loaded from a separate domain, SOP can block web pages from accessing it. Although this is a crucial security measure, some online applications that depend on material loaded within an IFRAME may not perform as intended.
4. The capabilities of AJAX queries, which are frequently used to build dynamic and interactive web applications, can likewise be restricted by SOP. It may occasionally render AJAX requests completely useless.

Although SOP is a crucial security feature, these restrictions can make it difficult to develop complicated online applications that need access to material loaded within an IFRAME or cross-domain data sharing. The solution to this problem is Cross-Origin Resource Sharing (CORS), which offers a way to enable cross-origin queries safely while still enforcing the necessary security controls to prevent unauthorised access to critical data.

How CORS enables cross-origin requests

Check out our article on Enabling CORS on your API

A method called Cross-Origin Resource Sharing (CORS) allows cross-origin queries while still upholding the necessary security precautions to prevent unwanted access to sensitive data. It functions by including a collection of HTTP headers in the server response that informs the browser if a specific request is permitted or not.

A browser will transmit a "Origin" header with the domain of the requesting website whenever it sends a cross-origin request. The server then determines whether to approve the request by comparing this header to a whitelist of permitted domains. If the request is approved, the server sends the browser a set of CORS headers informing it that accessing the requested resource is secure.

The "Access-Control-Allow-Origin" header, which outlines the domains that are permitted access to the resource, is the most significant of these headers. The browser will permit the answer to be processed and make the requested resource available to the client-side code if the asking domain is on this list.

In addition to the "Access-Control-Allow-Origin" header, CORS offers other headers that let servers regulate which HTTP methods, headers, and cookies can be sent in cross-origin requests, including "Access-Control-Allow-Methods," "Access-Control-Allow-Headers," and "Access-Control-Allow-Credentials."

CORS is a crucial tool for facilitating cross-origin requests and giving developers the flexibility they need to create complex online applications that need exchanging data across domains. Web developers may make sure that their online apps are secure and functional by employing the necessary CORS headers.

How to fix a CORS error

Let's explore how to resolve a CORS problem now that we are familiar with what CORS is and how it facilitates cross-origin requests. We'll first look into what causes a CORS error before covering some solutions to address the problem. Finally, we'll talk about situations in which fixing a CORS issue is not advised.

What causes a CORS error?

A CORS error could appear when client-side code like JavaScript sends a cross-origin request to a separate domain. Here are some of the most frequent causes of a CORS error, although there are other potential causes as well:

• Server-side CORS headers that are missing or incorrect: If the server does not send the right CORS headers in response, the browser will deny the request and a CORS error will ensue.
• Server-side CORS header configuration error: Even if the server is sending the right headers, the browser may still reject the request if the CORS headers are configured incorrectly.
• Missing or improper authentication: A CORS error may occur if the requested resource requires authentication but the authentication credentials are missing from the request.
• Browser add-ons: A few add-ons for certain browsers could interact with CORS and result in a CORS error.

Understanding the root cause of a CORS error is crucial to resolving the problem. In certain circumstances, the remedy might need changing the client-side code or configuration to get around the problem, while in others, it might entail changing the server-side code to include the proper CORS headers or credentials.

You may take the necessary actions to make sure that your online apps are secure and usable even while making cross-origin requests by being aware of the reasons why CORS errors occur.

Possible fixes for a CORS error to check out

The most effective strategy for fixing a CORS problem will vary depending on its precise root cause. You can try these three potential fixes:

Solution #1: Enabling CORS on the server-side

Missing or inaccurate CORS headers on the server-side are one of the most frequent causes of a CORS error. You must provide the proper headers in the server's response in order to activate CORS on the server-side. With the following code, you can enable CORS on a PHP server:

header("Access-Control-Allow-Origin: *");
header("Access-Control-Allow-Methods: GET, POST, PUT, DELETE");
header("Access-Control-Allow-Headers: Content-Type");

In this illustration, the necessary CORS headers are set using PHP's header() function. The domains that are permitted to access the resource are listed in the Access-Control-Allow-Origin header. The HTTP methods and headers that are permitted in the request are specified by the Access-Control-Allow-Methods and Access-Control-Allow-Headers headers.

Solution #2: Using JSONP (JSON with Padding)

JSONP is a workaround that involves using a script tag to load the requested data instead of making an AJAX request. Here's an example of how to use JSONP with jQuery:

$.ajax({
    url: "https://example.com/api/data",
    dataType: "jsonp",
    jsonpCallback: "myCallback",
    success: function(data) {
        // process the data
    }
});

function myCallback(data) {
    // handle the data
}

In this example, we're sending a JSONP request to the server using jQuery's ajax() function. The jsonpCallback parameter, which provides the name of the callback function to be called when the data is returned, is set to "jsonp" as the dataType parameter.

Solution #3: Implementing a CORS proxy

A CORS proxy can be used to forward requests to the destination server and add the appropriate CORS headers. Here's an example of how to implement a CORS proxy in Node.js:

const http = require("http");
const https = require("https");
const url = require("url");

http.createServer(function(req, res) {
    const reqUrl = url.parse(req.url, true);
    const headers = {
        "Access-Control-Allow-Origin": "*",
        "Access-Control-Allow-Methods": "GET,POST,PUT,DELETE",
        "Access-Control-Allow-Headers": "Content-Type"
    };
    const options = {
        headers: req.headers,
        hostname: reqUrl.hostname,
        path: reqUrl.path,
        method: req.method
    };
    const proxyReq = https.request(options, function(proxyRes) {
        Object.keys(proxyRes.headers).forEach(function(key) {
            res.setHeader(key, proxyRes.headers[key]);
        });
        proxyRes.pipe(res);
    });
    req.pipe(proxyReq);
}).listen(8080);

In this example, we're using Node.js to create a simple HTTP server that acts as a proxy for the request. The server adds the appropriate CORS headers to the response and forwards the request to the destination server.

You may take the necessary actions to make sure that your online apps are both functional and safe by being aware of the potential CORS fault remedies. There are numerous ways you can try to fix the problem, like using JSONP, activating CORS on the server, or putting in place a CORS proxy. The optimal course of action may, however, depend on the precise reason of the CORS problem, therefore it's crucial to keep in mind that. Care should be made to ensure that any solutions are done safely and responsibly.

When should I not fix a CORS error?

Even while fixing CORS problems is crucial, there are some situations in which you shouldn't even attempt to remedy the problem. For instance, you shouldn't try to get around constraints if the requested resource is secured with CORS headers to prevent unauthorised access. The requested data's security would be compromised if the CORS issue were to be fixed, and this could result in unauthorised access.

Also, you might not be able to resolve the CORS problem if you lack control over the server-side, such as when requesting data from a third-party API. You'll need to find an alternative solution or ask the API provider for assistance in these situations.

Understanding when it is permissible to try to correct a CORS problem and when it is not is crucial. By doing this, you can make sure that your web applications are safe and reliable while also adhering to the security protocols set up to safeguard private information.

Conclusion

Learn how to Enable CORS on your API

A crucial method for permitting cross-origin requests while preserving the necessary security precautions to prevent unwanted access to sensitive data is cross-origin resource sharing (CORS). Understanding CORS and how it functions, as well as potential causes of CORS issues and workarounds for them, is essential.

Your online applications will be secure and functional even while handling cross-origin requests if you use JSONP, enable CORS on the server side, or set up a CORS proxy. The optimal course of action may, however, depend on the precise reason of the CORS problem, therefore it's crucial to keep in mind that. Care should be made to ensure that any solutions are done safely and responsibly.

It's also crucial to understand the circumstances in which you shouldn't attempt to resolve a CORS error, such as when the requested resource is secured by CORS headers to prevent unauthorised access or when you have no control over the server-side.

CORS is a vital part of contemporary online development, and knowing how it operates is crucial for producing efficient and safe web applications. Web developers may make sure that their web apps are secure and functioning, offering the greatest user experience possible for their users, by being aware of the reasons of CORS problems and the solutions for them.