A quick CORS proxy in a few lines of Clojure

[2024-03-08 Fri] on Yann Esposito's blog

When I open a new tab in my browser I see my hand-made starter homepage. This is a single HTML file on my computer. Not a hosted website. This homepage is really useful and along the years I added some functionalities:

One of the section of links in this homepage contain a few websites I host. And I wanted to query these websites to make a health check from my file. It turns out that you cannot easily make a HTTP call to any external website from a file:// in your Browser as your are almost immediately blocked by CORS.

I don't want to explain how CORS are working. The important point is that it is a security measure that is very easy to circumvent.

Here is how to do it:

  1. Create a webservice that will play a role of "proxy". For example, the webservice will read the ?url=DESTINATION_URL.
  2. When receiving a request, the server will make a similar HTTP call to DESTINATION_URL and keep track of the Origin HTTP header of the request.
  3. Take the response from DESTINATION_URL and add a few headers, in particular add the Access-Control-Allow-Origin header that will contain the value in the Origin header of the request.

That's it.

So I wrote that in few minutes and use it now. So I can make these call and detect when I see my homepage if one of my hosted website is not reachable.

The Clojure code

Here is the code in a few lines of Clojure:

(ns fuck-cors-app.core
    [clj-http.client :as client]
    [ring.adapter.jetty :as jetty]
    [ring.middleware.params :refer [wrap-params]]
    [fuck-cors.core :refer [wrap-open-cors]])

(defn handler
  (if-let [url (get-in request [:query-params "url"])]
    (client/request {:request-method (:request-method request)
                     :url url})
    {:status 200
     :headers {"Content-Type" "text/plain; charset=utf-8"}
     :body "Let's bypass CORS ok?"}))

(defn -main
  [& _args]
    (-> handler
    {:port 1977
     :host ""}))

And that's it, this is a whole web application that will proxy any call to a website that do not allow you to call from some origin (like my file://) and will make it work anyway.

If you feel that using too many libraries is cheating, here is the actual almost full content of the lib taking care of handling CORS:

(defn- host-from-req
  (str (-> request :scheme name)
       (get-in request [:headers "host"])))

(defn- get-header
  [request header-name]
  (let [rawref (get-in request [:headers header-name])]
    (if rawref
        (clojure.string/replace rawref #"(http://[^/]*).*$" "$1")

(defn wrap-open-cors
  "Open your Origin Policy to Everybody, no limit"
  (fn [request]
    (let [origin (get-header request "origin")
          referer (get-header request "referer")
          host (host-from-req request)
          origins (if origin
                    (if referer
          headers {"Access-Control-Allow-Origin" origins
                   "Access-Control-Allow-Headers" "Origin, X-Requested-With, Content-Type, Accept, Cache-Control, Accept-Language, Accept-Encoding, Authorization"
                   "Access-Control-Allow-Methods" "HEAD, GET, POST, PUT, DELETE, OPTIONS, TRACE"
                   "Access-Control-Allow-Credentials" "true"
                   "Access-Control-Expose-Headers" "content-length"
                   "Vary" "Accept-Encoding, Origin, Accept-Language"}]
      (-> (handler request)
          (update-in [:headers] #(into % headers))))))

(defn wrap-preflight
  "Add a preflight answer. Will break any OPTIONS handler, beware.
  To put AFTER wrap-open-cors"
  (fn [request]
    (if (= (request :request-method) :options)
      (into request {:status 200 :body "preflight complete"})
      (handler request))))

I wrote it a long time ago, and I think I just found a potential bug related to the headers. I should probably retrieve all headers returned by the response, and add these header name to the Access-Control-Allow-Headers. But this list of allowed headers will work most of the time.

Bonus frontend code to check the availability of a website

As a bonus here is the code I use in my homepage to see if the website I am looking for are reachable or not.

Imagine you have an HTML block like this:

<div class="healthcheck">
  <a href="SOME_URL">website 1</a>
  <a href="SOME_URL">website 2</a>

I have a CSS rule that change the background of these link to green or red if I add the class ok or error to the <a>. And here is the javascript code:

// You can replace corsproxy.org (which is a public one)
// by the one you are hoting.
const corsproxyurl='https://corsproxy.org/?';
async function healthchecklink(a) {
  var linkurl=a.href;
  if (linkurl != undefined) {
    var url = corsproxyurl + encodeURIComponent(linkurl);
    try {
      var response = await fetch(url, {method: 'GET',
                                       redirect: 'manual',
                                       signal: AbortSignal.timeout(3000)
      if (response.ok || response.redirected || response.status === 0 ) {
      } else {
    } catch (err) {

function checkhealth() {
  var links = document.querySelectorAll('.healthcheck a');
  for (l in links) {


Notice the response.status == 0, this is due to one of my website returning a 303 redirection but some complication make it returns a status 0. If there were an error it would return an error status.

That's it. Another tool I created myself to prevent me using a service checking for the status of my website and sending me notifications about it. None of my website is crucial enough not be ok to wait a few hours to be re-enabled.