How can I dynamically set CORS in <ktor.io>? Unfor...

# server

[Jakub Neukirch]

How can I dynamically set CORS in ktor.io? Unfortunatelly call.request.origin.host returns localhost and after some reconfiguration of nginx it returns <http://backend.domain.com|backend.domain.com> instead of <http://frontend.domain.com|frontend.domain.com>. Setting up CORS for all endpoints with host("<http://frontend.domain.com|frontend.domain.com>") works properly, allows only frontend domain, and blocks all other

[Jakub Neukirch]

I can see that Referer header has proper address <http://frontend.domain.com|frontend.domain.com> - is it safe to use it to block using my backend on some origins?

[Holger Steinhauer [Mod]]

Not quite sure about the initial question but do not rely on the Referer header! It is easily spoofable.

[Jakub Neukirch]

Basically I need origin. But i dont get 'origin' header, and 'host' header points to my backend

[Jakub Neukirch]

I need something not spoofable, taht will give me real hostname of frontend using my backend in iframe

[Holger Steinhauer [Mod]]

Do you use a reverse proxy in front of your backend?

[Holger Steinhauer [Mod]]

You can usually configure it (nginx / Apache httpd) to send you the original hostname used in a custom header. This header is only used by your reverse proxy and it will override / ignore it, when someone outside tries to set the header

[Jakub Neukirch]

Yes

[Holger Steinhauer [Mod]]

A quick search brought up this article for NGINX: https://www.nginx.com/resources/wiki/start/topics/examples/forwarded/

[Jakub Neukirch]

I will take a look into it

[Holger Steinhauer [Mod]]

If you use httpd, search for http x-forward-for 😉

[Holger Steinhauer [Mod]]

Fingers crossed. Let us know how it goes

[Jakub Neukirch]

I will let you know tomorrow

[Jakub Neukirch]

So i took a look into this. It doesn't help because new headers do not appear. But as I read, CORS can be spoofed easily too, it is client side protection.

On backend i have one endpoint which returns HTML, it is intended to be displayed in iframe. But I want to block some users from putting in iframe on their public sites, it doesnt need to be protected from gathering this data at all.
In such case Referer would be good enough?
If i read well, Referer can be spoofed in postman, curl, custom browser etc. but majority users wont use such tools just to display iframe on someones site

[Jakub Neukirch]

So putting it in simple words and confirming: Can Referer header be set just in JS frontend app?

[Holger Steinhauer [Mod]]

Oh, this is nothing you would check on the server side.

[Holger Steinhauer [Mod]]

So you can send headers that define who is allowed to to use it in (i)frames

[Holger Steinhauer [Mod]]

https://www.w3.org/Security/wiki/Same_Origin_Policy#:~:text=For%20example%2C%20the%20same%2Dorigin,sending%20requests%20to%20other%20origins.

[Holger Steinhauer [Mod]]

Rule of thumb: Never trust user data. This includes the http headers

[Holger Steinhauer [Mod]]

In this case, you send header and because “legit” browsers respect them, an attacker would have to get a hacked browser to the victim’s computer before he can work around the headers. This is very unlikely and can’t be counteracted. There is only so much we can do 😉