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This error occurs when Rails' request forgery protection system does not accept a `POST`, `PUT` or `DELETE` request.
When Rails generates the HTML for a form or a XHR-powered link it adds a authenticity_token hidden field or parameter. Before letting the request through to the controller Rails checks that the parameter is present and correct.
The purpose of this protection mechanism is making it harder for bots do automated requests. They have to fetch the form as well and extract the authenticity_token from it.
There a number of things that can go wrong to cause this error:
This Rails helper adds two meta tags that can be used from JavaScript to add the right parameters. This helper should be called in your layout file. If it's missing, requests sent via JavaScript will result in this error on the server.
<head> <%= csrf_meta_tags %> </head>
If the unobtrusive scripting adapter for jQuery is not required in your assets the necessary parameters are not added. Make sure the `jquery-rails` gem is installed and that this is present in your main assets file. If it's missing, requests sent via JavaScript will result in this error.
//= require jquery //= require jquery_ujs
If you use page caching, Rails will keep the static HTML for a page in the cache. After a while the authenticity token for this page is stale and posting a form will result in this error. In this case either don't use page caching, or disable the forgery protection for that controller action:
skip_before_action :verify_authenticity_token
If you have controllers that are meant to be called by external parties you should disable forgery protection. This is relevant the case for APIs and web hooks:
skip_before_action :verify_authenticity_token
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