How to identify and resolve CORS errors in API development

Highlights

You’ll learn how to identify and fix CORS errors in API development

1. Identify the problem:
   • Use browser consoles like Chrome and Firefox developer tools
   • Inspect the Network tab for failed requests and check headers for CORS entries
2. Fix CORS headers:
   • Correctly set Access-Control-Allow-Origin and Access-Control-Allow-Methods
   • Consult the API documentation or provider, if required
3. Configure the server:
   • Ensure servers support cross-origin requests
   • Validate security settings, including HTTPS and user inputs
4. Advanced techniques:
   • Consider proxy, middleware solutions or serverless functions

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 APIs are the backbone of software development as they facilitate seamless connection and data exchange between systems. Nearly over 90% of developers use APIs.

CORS error poses setbacks during API usage. You will get these errors when your web application needs resources on another server, but the requests are blocked. Solving CORS errors will help web developers to maximize their API development.

Understanding CORS and its importance

CORS is a web application security feature that allows browsers to manage how external systems interact with data in their servers. When a web application makes an API request, the browser checks the server to see if it allows such requests. This is known as a cross-origin request. 

It adds specific headers to its response, showing which origins can access its resources if allowed. CORS allows APIs to specify which web applications can access their resources. So, it controls cross-origin HTTP requests. Without a CORS policy, websites can become exposed to authorized requests and possible access.

What is the same origin policy?

Same-origin policy (SOP) is a security measure web browsers use to restrict web pages from making requests to a different origin than the page itself. So, imagine there are two origins—Origin 1 and 2. The same origin policy ensures that a web page script in origin one cannot access data from origin two.

The exact origin policy helps prevent actions like cross-site scripting (XSS) and cross-site request forgery (CSRF), but it limits API integration. CORS is different and addresses the limitations of the same-origin policy.

Common CORS errors

Here are some of the most common CORS errors:

• Missing Access-Control-Allow-Origin header: This error shows that the response from the web server does not have this header.

• Custom headers not allowed by API settings: APIs may restrict specific headers for web application security. The browser will block the request if a custom header is not listed in Access-Control-Allow-Headers.

• Cache issues with preflight requests: Preflight requests are cached to improve system performance. Incorrect or outdated cache settings may lead to repeated CORS errors as the server misinterprets subsequent requests.

• Unspecified Content-Type in requests: This happens due to a missing or incorrect Content-Type header. Common Content-Type are application/JSON, text/html, text/plain, Etc.

Causes of common CORS errors

Many factors could be causing CORS errors, and understanding them helps you apply the right fix. Misconfigured HTTP headers top the list of common CORS errors. For example, if headers are missing in the API’s response, the browser blocks the request as a security measure. 

Aside from server issues, client-side mistakes can lead to CORS errors. Some of them include:

• Incorrect AJAX requests: Not setting the correct headers or methods in AJAX requests will cause CORS errors. The necessary headers, Origin and Content-Type have to be correctly configured.

• Omitting credentials in requests: Forgetting to include cookies can lead to CORS errors. Do not forget to include withCredentials: true in your AJAX setup.

• Unsupported HTTP methods: Requests made with prohibited HTTP methods, such as PUT or DELETE, when only GET and POST are allowed on the server will trigger CORS errors. Verify the methods allowed on the server side.

• Wrong handling of preflight requests: Ensure your client-side source code sends a preflight request if needed and that the server is configured to respond correctly to such requests.

Diagnosing and troubleshooting CORS errors

To successfully troubleshoot or diagnose CORS errors in APIs, you will need to follow a straightforward approach.

Identify the problem

Use browser consoles, like Chrome DevTools or Firefox developer tools, to identify CORS errors. These tools allow you to diagnose CORS-related issues.

Right-click on the web page. Then, click on 'inspect' to open the Chrome DevTools. You will find all network requests the page makes on the 'Network' tab. From there, open the failed request.

In the request, open the 'headers' section. You will find the request headers sent by the client and the server's header response. Look for CORS-specific headers like Access-Control-Allow-Origin, Access-Control-Allow-Methods, Access-Control-Allow-Headers, and Access-Control-Allow-Credentials.

Fix CORS headers

Set the “Access-Control-Allow-Origin” and “Access-Control-Allow-Methods” headers correctly. The API documentation usually has a detailed guide on how to do this. If you are unsure of what headers to use, you may also reach out to the API provider.

Configure the server

You may also look at server configuration and set the APIs to support cross-origin requests and manage requests from permitted servers. Use HTTP methods to establish connections for better security.

Best practices to avoid CORS errors

Knowing how to troubleshoot CORS errors is important. But prevention is better than cure. Here are tips that will help you drastically reduce the chances of having CORS errors. They will also help you maintain smooth API connections.

Ensuring web application security

To secure web applications, set the right CORS headers. Also, use HTTPS and validate user inputs. You may also apply OWASP security recommendations, such as multi-factor authentication (MFA), prepared statements and stored procedures, and regular security audits, among others.

Optimizing API requests

Host the API and client on the same domain, manage OPTIONS requests for preflight checks, and implement API gateways as a central mechanism to manage CORS. You may also reduce custom headers and HTTP methods and use CDNs for static resources. That way, you minimize overhead and latency while optimizing the API requests for your web applications.

Proper server configuration

Review the server-side configuration and ensure it properly handles CORS requests. Make sure that the web server sends the necessary CORS headers, including Access-Control-Allow-Origin, Access-Control-Allow-Methods, Access-Control-Allow-Headers, and Access-Control-Allow-Credentials. Also, verify that the responses are correctly configured.

Using proxies

Using proxy solutions allows you to bypass CORS restrictions. These proxies send requests on behalf of the client and help bypass CORS barriers. However, it is preferable to use them only in testing environments.

Serverless functions

Serverless functions are an alternative to proxy solutions. They allow you to build micro-infrastructures to call web services and feed data to an API endpoint. Cloud services like AWS Lambda and Azure Functions offer adequate server space to run these functions.

Middleware solutions

Middleware solutions intercept requests and add appropriate headers for each one. They streamline the process, ensuring consistent handling of cross-origin requests and mitigating potential CORS issues across various endpoints.

Advanced techniques and tools

If the already-stated solutions do not work, you may consider advanced techniques like path rewriting and other security considerations. 

Path rewriting

Path rewriting simplifies handling cross-origin requests by redirecting and changing request paths, usually using a reverse proxy. Path rewriting consolidates API endpoints from different servers and simplifies CORS management.

Case studies

Sky

As part of their rebrand, Sky wanted to simplify content management and ease developer workload. They reached out to Contentstack and opted for the headless CMS. 

The switch allowed them to leverage the custom stack management tool and modular blocks, leading to better code maintenance and faster development cycles.

Hear from Richard Mace. "We've had complicated pages with a large range of components, and at a glance, editors can understand exactly what's happening." 

Read more about Sky's success with Contentstack's enterprise headless CMS.

Welcome Tech

WelcomeTech went from struggling with security to implementing Contentstack’s SOC 3-compliant headless CMS with serverless functions for CORS issue handling.

It addressed their security risks, and productivity went up 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 Tech boosted productivity with Contentstack’s enterprise CMS.

FAQ section

How do I fix a CORS error in API?

To fix a CORS error in an API, configure the server and include the relevant CORS headers in its responses.

What triggers a CORS error?

A CORS error is triggered when a web app in a browser tries to make a cross-origin HTTP request to a different server, but the server lacks the relevant CORS headers in its response to allow the request.

What is the root cause of CORS error?

The root cause of errors is the same-origin policy, as it restricts web pages from making requests to different origins unless the server allows it through CORS headers.

How do you check whether CORS is enabled or not in APIs?

Use developer tools to inspect the response headers of a network request to the API to see if headers are present. If these headers are present, CORS is enabled.

Does CORS apply to APIs?

Yes, CORS applies to APIs. The role of CORS is to restrict or guide web applications running in browsers when trying to make requests to APIs on another server.

Learn more

APIs promote content reuse, integration, and data sharing, making them essential in modern software development. Troubleshooting CORS errors is straightforward, provided you understand the error message.

Use the right headers, manage preflight requests and configure the server correction. Better still, opt for a headless content management system that takes care of those processes in the background. Talk to us today.