When you run a website, you’ll encounter many errors that are difficult to explain. Some of these errors involve Cross-Origin Resource Sharing (CORS), which is a standard that makes it possible for a server to ease up on the same-origin policy that browsers maintain. This standard allows you to obtain a specific resource from a different domain. While some cross-origin requests are allowed, others will be rejected.
Let’s say that your website provides visitors with a service that’s embedded directly into a webpage. In this scenario, you’ll likely need to relax some browser restrictions, which means that a CORS Configuration should be created. However, this process can be complicated and may lead to one of many different errors.
An example of a CORS configuration error is “CORS request external redirect not allowed”. This error can show up if an HTTP redirect sends the user to a different origin than the one that they requested. Each error can have a unique cause.
While some CORS errors result from mistakes during the original setup process, the user’s web application might also negatively impact the configuration. By understanding what CORS errors are and why they happen, you’ll be able to provide the right fix. Since the CORS mechanism is necessary for security, you must correct these errors immediately. In this guide, we’ll delve into CORS errors and how to resolve them.
What Is a CORS Error?
CORS is a tool that ensures only domains that you’ve authorized can gain access to sensitive resources or data. When you properly implement the CORS configuration, your web applications won’t be exposed to security vulnerabilities like cross-site scripting attacks. Your developers will have complete control over whether cross-origin requests are allowed or denied. With this configuration, a web page can embed scripts, stylesheets, images, videos, and iframes.
Before the CORS mechanism was available, webpages were only able to use the same-origin policy, which means that requests could only be made to the same domain as the page. The purpose of this policy was to ensure unauthorized requests from malicious websites didn’t get through and compromise the site’s security.
While CORS is a highly effective mechanism, there are many different errors that can occur once you integrate this technique into your website. When one of your web pages sends a request to another domain, you may receive an HTTP error if the “Origin” header in the request isn’t part of the CORS configuration. The types of CORS errors that you might encounter include the following:
- CORS disabled
- CORS request did not succeed
- Did not find method in CORS header Access-Control-Allow-Methods
- CORS request not HTTP
- CORS header origin can’t be added
- CORS preflight channel did not succeed
Causes of CORS Errors
There are many potential causes of CORS errors, including everything from misconfigured server heads to the use of third-party APIs and resources. Cross-origin request headers effectively show who is able to access site resources. If these headers aren’t configured correctly, you may receive a CORS error. A few of the more common types of CORS headers are:
- Access-control-allow
- Access-control-allow-origin
- Access-control-allow-methods
Restrictive browser security policies can also cause these errors to show up. By default, most browsers only allow API calls through the same IP or .com address. You need to set specific headers that loosen these restrictions by telling the browser which IP addresses and URLs are safe.
A CORS error can be displayed if a preflight request to your website fails. Two of the more common types of CORS requests that you can send include a simple request and a preflight one. A simple request can be a HEAD, POST, or GET request. These are basic headers that are sent to another server.
A preflight request can be sent before your site attempts to access data from a different server. It requests permission from the server. If this permission is denied, the request will fail. Configuration issues and incorrect server setup can lead to the failure of a preflight request.
Many websites and applications use third-party APIs and resources to implement additional features like payment processing or weather updates. CORS can enable these features by accessing the third-party APIs directly. If the connections between CORS and the APIs aren’t configured correctly, CORS errors will show up.
How to Diagnose CORS Errors
If you don’t have much experience with debugging CORS errors, this process might seem complicated at first. However, there are a few things you can do to troubleshoot and diagnose these errors to identify the right solutions. Before doing anything else, use the browser developer tools to identify CORS issues. Once you enter the browser console, you’ll be able to view all CORS-related errors, which include the line of code that caused the issue as well as the error message.
To interpret the error messages you receive, search for them online or check a database of the most common messages. Let’s say that you receive the following error message:
- The ‘Access-Control-Allow-Origin’ header has a value ‘http://example.com’ that’s not equal to the supplied origin.
This message indicates that you included the “Access-Control-Allow-Origin” header, but it had an incorrect value. When you’re diagnosing the specific CORS error that you’ve received, verify that your server is transmitting the right CORS headers.
Make sure that the request isn’t being blocked by a proxy or firewall. Some firewalls automatically block requests to different servers. Check the request URL to ensure it’s using HTTPS. There are some servers that don’t allow HTTP CORS requests. Your server should also be configured to accommodate different HTTP headers and methods. Otherwise, the request won’t be sent.
Strategies to Resolve CORS Errors
Once you diagnose the cause of the error, you can resolve it. The main reason that CORS errors are displayed is because of poorly configured headers. You can correct this issue by configuring all server-side headers correctly. The specific configuration you need to make depends on the type of header that’s being requested. For example, you may need to input the “Access-Control-Allow-Origin” designation in your header to specify which origins can access the resource.
CORS should also be enabled in different server environments, such as Apache, Nginx, and Node.js. This process differs with each server environment. In Apache, you can add the CORS authorization to your header with the following line:
- Header set Access-Control-Allow-Origin “*”
Use “apachectl -t” to check your configuration for errors. If a proxy or firewall blocks the CORS request, configure it to allow the specific request. You might need to use the allowlist technique to ensure the application’s domain is recognized in the proxy or firewall configuration. This change should bypass CORS restrictions.
You can also use JSONP as an alternative. While CORS is a more modern and reliable solution, the simplicity of JSONP allows it to properly retrieve data from an API when you’re receiving CORS errors.
Best Practices for Preventing CORS Errors
Improper configurations are behind most CORS vulnerabilities. You can keep CORS errors at bay by specifying the different origins that you’ll allow. If a resource consists of sensitive info, it’s important that you specify the allowed origins in full within the header. Setting up proper security policies is necessary to prevent these issues.
Regularly update and secure your APIs to ensure your digital assets and customer data are kept safe. During the development of your site, test cross-origin requests to identify if the headers have been properly configured. Use CORS middleware or libraries to streamline the CORS mechanism and avoid future errors. When you specify origins in the Access-Control-Allow-Origin header, only do so for sites you trust.
Common Pitfalls and Mistakes
To properly configure CORS, you must strike the right balance between fewer errors and robust security. A common mistake that site owners make involves overly permissive CORS settings. If you allow numerous origins and servers to access your resources, you’ll be at risk. As mentioned previously, you should only specify origins for websites that you trust completely.
It’s also important that you don’t make the mistake of relying solely on client-side solutions involving the browser. These errors often originate in servers. Adjust your server-side settings to make sure you include the correct CORS headers. Don’t ignore preflight request handling. You can avoid request denials by matching the preflight requirements or using simple requests.
Conclusion
CORS is an essential tool that allows you to integrate applications into your website. It accommodates cross-origin resource sharing and provides the client browser with the ability to check if a request is authorized before data is transferred. While this mechanism offers more freedom than same-origin requests, it’s also prone to errors if you don’t configure it correctly.
Approach CORS errors with a methodical troubleshooting mindset. Each error message has a specific meaning that’s easy to find online. While you learn how to diagnose and resolve CORS errors, implement additional web security techniques and safety measures to protect sensitive customer data.
