Implementing CORS policy: Best practices to prevent common CORS errors

Highlights

You’ll learn how to prevent CORS errors with an effective CORS policy.

Cross-origin resource sharing (CORS) manages cross-domain requests and is essential for secure web interactions and better user experiences.

Key strategies:

• Configure HTTP headers: Set the proper Access-Control-Allow-Origin
• Server settings: Define methods and headers that can access resources
• Security measures: Limit access to trusted domains and enforce HTTPS

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Keep reading to learn more!

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A CORS policy is a security measure that web browsers implement to manage how external domains can interact with resources on a web page in another domain. It allows servers to determine who can access stored resources and how such resources can be accessed. It controls cross-origin interactions and improves web security.

When things stray outside what is allowed in the CORS policy, you get a CORS error. Every developer tries to avoid that because it inhibits resource sharing and API integrations and needs ample time to be resolved.

CORS and its significance

The CORS policy supports cross-origin requests and enforces the same-origin policy (SOP). It offers a protective shield for websites and is essential for web security. It eases API interaction with web applications and services. There are two main types of CORS requests:

• Simple requests
• Pre-flighted requests

A simple request is a cross-origin HTTP request that uses simple HTTP methods like HEAD, GET, or POST. They also include certain specific headers and do not require preflighted approval. 

Pre-flighted requests are different. Think of them as a pre-cursor request. They go in first as an initial CORS options call to verify the safety of the main request. That way, the system verifies that the server supports the custom headers and requests HTTP methods before sending the main request. 

How does CORS work?

When a browser sends a request, it includes an origin header. That header has the domain (Origin of the request), protocol, and port of the page that makes the request. The server can allow or deny the request. 

If allowed, the server includes the Access-control-allow-origin header in its response. The header specifies the Origins that can access its resources. If the server rejects the request, it includes the Access-control-allow-origin header with a value of "*." This shows that it does not allow any origin to access resources. 

Common CORS issues

There are some common issues that you can identify with CORS, and here are a few to note.

• No 'Access-Control-Allow-Origin' header is present: You get this CORS error if no headers in a preflight request notify the browser of whether the origin has permission to access resources.

• CORS preflight request fails: An incorrect or missing CORS header in a preflight request response triggers this error. It is like a security alert that stops the browser from proceeding with the actual request.

• CORS request not HTTP: CORS only works with the HTTP or HTTPS URL protocols. So, you will get an error when you attempt to view a site via the local file system in your browser rather than as an HTTP(S) asset served via a web server.

Impact of CORS error on web applications

CORS errors prevent web applications from assessing resources and disrupt their functionality. They also prevent API integrations. Here are other impacts of CORS errors on web apps.

• Increased debugging time: Finding the source of a CORS error and applying the solution takes time and slows down development work.

• Blocked data access: CORS errors prevent you from accessing data that may be essential to building or maintaining web applications.

• Security risks: Wrong server or browser configurations can expose you to unauthorized access from illegal websites.

Best practices to avoid CORS errors

Considering the negative impact of CORS errors, here are the best practices to avoid them.

Use proper HTTP headers

Using proper HTTP headers allows you to comply with the CORS browser policies and avoid errors. Here are some of the headers to use.

• Access-control-allow-origin: This header specifies the origins of HTTP requests and access resources on a server.

• Access-control-allow-methods: This header outlines the allowed methods for cross-origin resource requests. The methods include GET, POST, PUT, and DELETE.

• Access-control-allow-headers: This mechanism ensures that only specified headers are included in the main cross-origin HTTP request.

Configure the API server correctly

• Set up API server for CORS: Configure server settings to include essential CORS headers in its responses. You may also use a middleware to do that.

• Enable CORS in the backend: Set up back-end logic to generate the right CORS headers based on the request’s origin. 

Handle preflight requests

A preflight request is an automatic and extra step in the cross-origin HTTP request. By including the right CORS headers, you can configure the server to handle OPTIONS requests before the main request. 

Implement proxy server solutions

Configure a proxy server to forward requests and modify headers appropriately. Proxy servers go between servers and clients, bypassing CORS restrictions. 

• Choose a server to host your proxy—it could be the cloud or a local machine.
• Install the required software and configure proxy settings.
• Set up authentication and access rules.
• Test the proxy server to ensure it works correctly.
• Monitor and maintain the server for performance and security.

Secure CORS implementation

Consider security, and do not use the (*) wildcard. Instead, Limit access to trusted domains only, validate inputs, and audit your CORS configuration regularly. 

Also, enforce HTTPS and validate requests with server-side checks, such as rate limiting, origin validation and token authentication. You can also use a CORS npm package to simplify CORS implementation in Node.js applications.

Troubleshooting and solving CORS problems

You can follow these steps to troubleshoot and fix common CORS problems.

• Check server logs for errors: From the server, go to your log files, open relevant files and look for error messages. 

• Verify CORS headers: Check the Access-Control-Allow-Origin response header and ensure it matches the request origin. Check the ‘Access-Control-Allow-Methods’ to ensure it includes the request method and validate the custom headers in "Access-Control-Allow-Headers."

• Use browser developer tools to diagnose issues: Inspect elements in browser development tools such as Chrome DevTools, Firefox developer tools, and cURL. You can also use a third-party tool like Postman. Also, check the console for errors.

• Use a serverless function: Serverless functions like AWS lambda, Azure functions, and Google Cloud functions allow you to resolve CORS errors. It is more robust than using proxy solutions. These serverless functions allow you to develop your micro-infrastructure to call web services and feed data to API endpoints.

Tools and resources for resolving CORS errors 

You can rely on several tools to resolve CORS errors, such as:

• Postman
• AWS API gateway
• Browser developer tools 
• Server logs
• Web security scanners

 

Case studies

MoneyHero Group

MoneyHero needed a secure solution to ease developer experience, and Contentstack’s headless CMS offered that with its robust security and serverless function support to manage CORS issues. 

It improved the developer experience for MoneyHero Group and its 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.

Sky

Sky needed a more secure and resilient system as part of its rebrand. They contacted Contentstack and opted for the headless CMS. 

Making the switch allowed them to leverage modular blocks and a custom stack management tool, which improved code maintenance and reduced development time.

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

Read about Sky's success story since switching to a headless CMS.

FAQ section

What is a CORS error, and how can it be fixed?

A CORS error is the response from a browser when it blocks an external web page from accessing its resources. To fix it, find the error message in the browser console to understand the cause. However, for most issues, you must configure the server "Access-Control-Allow-Origin" to allow specific methods and headers and properly handle preflight requests.

Why am I getting a CORS error in my application?

You may be getting a CORS error because your server does not properly handle preflight requests or it lacks the right header in its CORS response. It may also be due to localhost or browser security issues.

How do I configure my server to avoid CORS errors?

Set the "Access-Control-Allow-Origin," "Access-Control-Allow-Methods," and "Access-Control-Allow-Headers" headers, and handle preflight requests with proper responses.

Can a browser ignore CORS rules, and how?

No. By design, browsers enforce CORS rules. However, during testing, developers can use proxy servers to bypass them.

Learn more

CORS supports the same-origin policy to simplify resource sharing across domain boundaries. It offers essential browser-side protection, and all modern browsers and most enterprise applications support it. Creating a proper CORS policy eases developer work, supports API integration, and improves developer and user experience.

A headless CMS holds content that other systems access via APIs. That is a cross-origin scenario, and CORS settings are required to handle it securely. Contentstack's headless CMS adheres to CORS policy, allowing you to avoid errors, save time, and build resilient applications. Talk to us today.