“Access-Control-Allow-Origin header contains multiple values” error - .htaccess

Question

I'm getting mirriad of errors trying to do an ajax call in a reactjs app using axios. I have an api that lives at a subdomain and making calls from the main domain.
.htaccess:
Header add Access-Control-Allow-Origin: "*"
Header add Access-Control-Allow-Credentials: "true"
Header add Access-Control-Allow-Methods: "GET,HEAD,OPTIONS,POST,PUT"
Header add Access-Control-Allow-Headers: "Access-Control-Allow-Headers, Origin,Accept, X-Requested-With, Content-Type, Access-Control-Request-Method, Access-Control-Request-Headers"
ajax headers in reactjs (using axios):
var options = {
method: 'GET',
url: 'http://admin.mysite.com/menus/5',
headers: {
'Access-Control-Allow-Origin': '*',
'Access-Control-Allow-Methods': 'GET,HEAD,OPTIONS,POST,PUT',
'Access-Control-Allow-Headers': 'Access-Control-Allow-Headers, Origin,Accept, X-Requested-With, Content-Type, Access-Control-Request-Method, Access-Control-Request-Headers'
}
}
I've tried making changes to each of these but get different errors. If I have the Header add Access-Control-Allow-Origin: "*" it complains about double origins. If I remove it I I get an error about Access-Control-Allow-Origin is not allowed by Access-Control-Allow-Headers in preflight response other changes has responded with Access-Control-Allow-Headers is not allowed by Access-Control-Allow-Headers in preflight response
I'm using wordpress as a headless CMS and tapping into the restful api to pull the data I need. I have noticed if I removed all of this I can get my data fine but I can't post without fixing the cross domain issues.

Answer 1

If I remove it I I get an error about Access-Control-Allow-Origin is not allowed by Access-Control-Allow-Headers in preflight response other changes has responded with Access-Control-Allow-Headers is not allowed by Access-Control-Allow-Headers in preflight response
Both those error messages are happening because in your frontend JavaScript code you have this:
headers: {
'Access-Control-Allow-Origin': '*',
'Access-Control-Allow-Methods': 'GET,HEAD,OPTIONS,POST,PUT',
'Access-Control-Allow-Headers': 'Access-Control-Allow-Headers, Origin,Accept, X-Requested-With, Content-Type, Access-Control-Request-Method, Access-Control-Request-Headers'
}
You need to remove that entire headers option from your request code.
The reason is, all the Access-Control-Allow-* headers are response headers that servers must return. The only effect you’ll have by sending them as request headers is to cause exactly the kinds of errors cited in the question.
If the reason you’re adding them is that your .htaccess settings on the server side for the API endpoint you’re sending the request to aren’t making the server send the right response headers, then you need to figure that out and fix that on the server side. Sending additional request headers from the client side isn’t going to fix that problem.
One suggestion you might try for your .htaccess: instead of Header add, use Header set:
Header set Access-Control-Allow-Origin: "*"
Header set Access-Control-Allow-Credentials: "true"
Header set Access-Control-Allow-Methods: "GET,HEAD,OPTIONS,POST,PUT"
Header set Access-Control-Allow-Headers: "Access-Control-Allow-Headers, Origin,Accept, X-Requested-With, Content-Type, Access-Control-Request-Method, Access-Control-Request-Headers"
Header set tells Apache to overwrite/clobber any existing value that might already be a set for a particular header, whereas Header add tells it to just add a header with the name and value given, without overwriting any header that might already be set with that name.
So Header add can cause multiple headers with the same name to be sent in a response, in which case the browser treats it as a single header with multiple values. You don’t want that.