How to fix CORS errors: A comprehensive guide for web developers

Highlights

You’ll learn how to diagnose and fix CORS errors

CORS error causes: Misconfigured headers, wrong tokens, or protocol mismatches

Diagnosis steps:

• Check browser console: Identify error messages to know the cause of the error
• Use debugging tools: Use Postman, Fiddler, or browser extensions to debug 

CORS errors fixing methods:

• Adjust server configuration: Set the right CORS header content
• Configure frontend requests: Ensure proper request methods and headers
• Use proxy servers: Proxy servers allow you to bypass CORS restrictions

Cut through the process with a compliant headless CMS that offers serverless functions to solve CORS errors. Talk to us today!

Keep reading to learn more!

 Most developers are familiar with CORS errors as they have had to face them at least once. It is a security rule that prevents unauthorized access to data from requests made from a different domain. You get a CORS error in the browser when this rule is broken.

Considering that CORS errors are disruptive and take time to resolve, developers must plan for them, understand them and define clear policies to deal with them.

What is a CORS error?

If a web page tries to make an unauthorized API request to another website, the browser will get a CORS error because these are two different domains. A CORS error happens when a web application attempts to access resources in another server without having the right permissions.

When a front-end code running on a local server tries to make requests to an online server, wrongly set CORS headers on a server or incorrect authorization tokens can also lead to these errors. Also, using different protocols, such as HTTP versus HTTPS, between the server and client can trigger CORS issues.

CORS errors will seriously affect the website users' experience because important resources that depend on access will be blocked, leading to broken features on the website. Developers must understand CORS and configure the system correctly to handle it.

Understanding CORS headers

CORS headers control how web apps handle requests from other domains. They establish the rules for what kind of access is allowed and what is not. This is why it is important to know these headers- as some CORS errors come from misconfigured headers. Here are the main CORS headers and what they do:

• Access-Control-Allow-Origin: This header tells the web browser which domains can access the resources. It specifies the ‘allowed origins’.

• Access-Control-Allow-Methods: This specifies valid methods for accessing resources. They include GET, POST, PUT, etc.

• Access-Control-Allow-Headers: This response header indicates all headers allowed in a request. It specifies headers that can be used when making a cross-origin request.

• Access-Control-Allow-Credentials: The server sets this header to indicate that credentials can be included in requests. Credentials include client-side SSL certificates, cookies, or HTTP authentication. 

• Access-Control-Max-Age: This header states how long the browser can cache the results of a preflight request. It gives the maximum age in seconds.

How to diagnose CORS errors

CORS errors are at the web browser level and require developer tools to debug them. To deal with them, you must understand the error message. Here are a few ways to do it.

Debugging from your browser console

When a CORS error happens, the console tab displays the specific error messages. These messages contain information about what went wrong and the location of the error. That allows you to identify and troubleshoot the error at the source.

Common CORS error messages

When you get a CORS error, check the browser console for any of these messages. 

• No 'Access-Control-Allow-Origin' header is present: This error indicates that the server response does not contain an Access-Control-Allow-Origin header.

• The CORS policy does not allow access from this origin: This error message indicates that the server's CORS policy restricts access to resources from specific origins.

• Preflight request failed: A preflight request is like a test request before the main request. It fails when the browser does not receive a valid response from the server.

Extra debugging tools for CORS errors

Aside from the native methods explained, you can use several tools to debug CORS errors. Here are a few.

• Postman: You can use Postman to send requests to your server to see the response. This helps you check if the CORS headers are present and configured correctly.

• Fiddler: This web debugging tool allows you to monitor and change HTTP requests and responses. It also allows you to check headers to identify the source of CORS issues.

• Browser extensions: Browser extensions, like CORS Unblock, Moesif Origin, and CORS Changer, simulate CORS requests and can help you quickly identify problems.

How to fix CORS errors

CORS errors can be fixed, depending on the cause, by adjusting the server-side configuration, configuring frontend requests, and using proxy servers.

Adjusting server-side configuration

The most straightforward way to resolve CORS errors is to set the server to send the right CORS headers in its responses. Set your server to allow access to resources by permitting origins that have the Access-Control-Allow-Origin header. You can also use the “*” to permit all origins. However, that could leave you open to security issues.

Access-Control-Allow-Credentials is also useful because of cookies. For each HTTP request to a domain, the browser will attach the cookies associated with that domain. The cookies are for authentication purposes. By ensuring this header is set correctly, the server will realize that your website has the authentication for access.

Configuring frontend requests 

When CORS errors happen on the front end, you can adjust how your application sends requests to the server.

For example, using simple requests like GET and POST is good practice. But if you send a complex request—like PUT to a cross-origin site, your browser will send a preflight request to the server, asking if it is allowed to make this request. 

This preflight request will have Access-Control-Request-Method and Access-Control-Request-Headers headers. These headers contain the methods and headers needed for the request on the client side. The allowed methods and headers must match the requested methods and headers to get a valid response from the server.

Resolving CORS errors with proxy servers

You may not be able to change server settings when using third-party APIs. In such cases, you can defer to a proxy server. The proxy server requests the resource on behalf of the web application and adds the important headers to the response.

The proxy server will stay between your request and the target server. Therefore, because the request comes from an unknown source, the CORS proxy can be tricked into thinking it is from an allowed location.

Troubleshooting CORS errors

Here is a list of the most common CORS configuration mistakes and simple ways to troubleshoot them.

• Missing or incorrect Access-Control-Allow-Origin header: The server did not specify which origins are allowed to access its resources. Check the server-side code for this header and include the origin of your application.

• Incorrect Access-Control-Allow-Methods: You will need to verify that the methods used by your application are listed in this header.

• Missing Access-Control-Allow-Headers: Verify your server configuration and include necessary CORS headers. That ensures your server sends the Access-Control-Allow-Headers response with the headers list. 

• Wildcard problem: The wildcard "*" creates potential security issues. So, replace it with specific origins in Access-Control-Allow-Origin. Only allow trusted and set proper CORS headers for each request.

• Misconfigured preflight request settings: Preflight requests are sent before actual requests. They determine if the server accepts the method and headers. Misconfiguring these can cause CORS errors. Ensure Access-Control-Request-Method and Access-Control-Request-Headers are correctly set on the server.

• Server-side function errors: A server-side function error indicates the issue is from your end. Check your logs, debug code, validate inputs, and update dependencies.

Best practices for preventing CORS errors

With the right configurations on the server and client sides, the chances of you having errors will be minimal.

Use proper server and client-side configurations

• Only allow specific websites like your own by setting the "Access-Control-Allow-Origin" header.
• For the client side, use standard requests like GET or POST if you can.

Consider security in CORS Policies

• Implement fine-grained control over allowed methods and headers.
• Be clear about who can access your server instead of using a wildcard (`*`) origin.
• Use HTTPS for communication to keep things encrypted and safe.

Implement a robust CORS policy setting

• Allow the exact methods (like GET or POST requests) and any special instructions (headers) that are truly needed.
• Use serverless functions for complex setups.

Case studies

Welcome Tech

WelcomeTech wanted a future-proof technology for good security, and Contentstack’s headless CMS offered that, including SOC 3-compliant and serverless functions for handling CORS issues.

By switching to Contentstack, WelcomeTech improved security and publishing speed, and their productivity improved by 50%.

Hear from William Leborgne, the Director of Content. “I’ve looked at more than half a dozen CMSes, and Contentstack came to the top for all the right reasons. The product is incredibly user-friendly. The support team was excellent. The integration with our system was seamless. The engineering team is delighted. Integrations for personalization, AB testing, and all the other bells and whistles are there in spades, so it’s a no-brainer.”

Read more about how Welcome Technologies boosted productivity with Contentstack’s enterprise CMS.

MoneyHero Group

MoneyHero needed a secure solution to manage content and ease developer experience.

Contentstack offered that with its secure headless CMS that also provided serverless functions to manage CORS errors, easing developer experience. The changes improved product rollout time by 85%.

Hear from Andrew de Ridder, the Head of Application Engineering. “The technical tooling and APIs that Contentstack provides allows our engineering teams the power and flexibility to surface content wherever it's needed, from multiple websites in different regions, to email, social media, or whatever use case comes next.”

Read more about how MoneyHero delivered content agility by opting for Contentstack.

FAQ section

How do you resolve a CORS error in a web application?

The most logical way to resolve CORS issues is to find the root cause. You can do so by checking the browser log for the error message. Once you know the cause of the error, you can apply the right fix.

Can CORS errors be fixed at the browser level?

No, the server decides what can access its resources by setting CORS headers. Making changes at the browser level will not force a server to allow requests from unauthorized origins.

How to resolve the CORS error

Resolving the CORS error involves understanding the reason for the error and implementing a solution, usually on the server.

How do I enable CORS in a web application?

Set your web application's allowed origins, methods, and headers to enable CORS. That way, you outline the rules for who can access your server and cross-origin resources.

Can we fix a CORS error from the front end?

It is generally not possible as CORS is a browser-level policy. However, you can use CORS proxy services, proxy servers, and JSONP to find your way around it.

Learn more

Every developer knows about CORS errors and must have dealt with them at one point. They inhibit cross-origin resource sharing, API integration and web application functionalities. They also take time to fix, which increases the time spent on development work.

Working with a compliant headless CMS takes care of these issues. Contentstack gives you the extra option of using serverless functions to deal with CORS issues. That way, you can maintain business stability, integrate third-party services, and save time on web development. Contentstack’s headless CMS offers that and more. Talk to us today.