If the browser should allow the script to make authenticated requests (carrying credentials such as ` Authorization` headers or Cookies)Īny request headers that should be allowed with the requestĪny response headers to expect and pass on to the script What ` Method`s scripts from that origin are trusted to use If the validation passes, then we want to reply with our CORS headers providing confirmation to the browser that: The minimum validation we need to support here is ensuring that the ` Origin` header to ensure matches an expected value (access lists and regex rules are common solutions here). Therefore the bare minimum we need is being able to handle the ` OPTIONS` method and perform some validation on the headers received there. The ` OPTIONS` request is how the browser asks the resource if scripts from a given origin are safe, and what they should be allowed to do. In that mindset, browsers assume that any scripts that come from (sub)domains other than the resource they're trying to contact should not be able to contact that resource or make use of any of the credentials to identify the user to that resource. We have a more detailed explainer on that here, but to summarize: The goal of the browser is to ensure that it's not leaking user data to potentially malicious scripts. In order to implement the bare minimum support for Cross-Origin Resource Sharing, we need to understand what the browser is trying to validate and why. So, which solution you go with really comes down to your needs. The second method provides much more flexibility and the ability to change the response dynamically, but at the expense of greater initial complexity. The first method is ideal if what you're looking for is a quick fix to supporting a small number of sources of Cross-Origin requests, but gets very complex if you need the flexibility of supporting multiple sources with varying constraints.
Use a reverse proxy server or WSGI server(such as Nginx or Apache) to proxy requests to your resource and handle the OPTIONS method in the proxy.Īdd support for handling the OPTIONS method in the resource's code.īoth of these methods are equally valid but have different use-cases. There Are Two Approaches to Getting It Right. If the requested resource isn't configured to answer the OPTIONS request method or isn't configured to handle it correctly, then you'll see this error. If the constraints set by the resource are met by the script's request then the browser's access control check will pass, allowing the actual request to proceed. This allows the resource to define the policy that the browser should enforce on all scripts that wish to contact it. Let's take a look at what's actually going on under the hood of the browser when this occurs.Īs you can see from the sequence diagram, before making the script's actual request to the requested resource, the browser first makes a preflight request for the resource's OPTIONS.
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This error occurs when a script on your website/web app attempts to make a request to a resource that isn't configured to accept requests coming from code that doesn't come from the same (sub)domain, thus violating the Same-Origin policy. We've already written an explainer on what CORS headers are and what they do ( which you can find here), but to summarize: CORS is a mechanism for relaxing the "Same-Origin" policy of modern browsers to allow things like serving your static content from and your dynamic content from. In short, the 'access-control-allow-origin' header is a Cross-Origin Resource Sharing (CORS) header. Sure, it tells you that there's a header missing, but from where is it missing, and what should it be? Searching for it on the internet is likely to bring up a popular forum where the most common answer is worse than wrong – it's dangerous. This error is up there as one of the least helpful error messages.
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How to Fix "No 'Access-Control-Allow-Origin' Header Present"
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