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Access and CORS

Cross-Origin Resource Sharing ( CORS) is a mechanism that uses HTTP headers to grant a web application running on one origin permission to reach selected resources in a different origin. The web application executes a cross-origin HTTP request when it requests a resource that has a different origin from its own, including domain, protocol, or port.

For a CORS request to reach a site protected by Access, the request must include a valid CF-Authorization cookie. This may require additional configuration depending on the type of request:

​​ Allow simple requests

If you make a simple CORS request to an Access-protected domain and have not yet logged in, the request will return a CORS error. There are two ways you can resolve this error:

​​ Authenticate manually

  1. Visit the target domain in your browser. You will see the Access login page.
  2. Log in to the target domain. This generates a CF-Authorization cookie.
  3. Refresh the page that made the CORS request. The refresh resends the request with the newly generated cookie.

​​ Allow preflighted requests

If you make a preflighted cross-origin request to an Access-protected domain, the OPTIONS request will return a 403 error. This error occurs regardless of whether you have logged in to the domain. This is because the browser never includes cookies with OPTIONS requests, by design. Cloudflare will therefore block the preflight request, causing the CORS exchange to fail.

There are two ways you can resolve this error:

​​ Configure response to preflight requests

You can configure Cloudflare to respond to the OPTIONS request on your behalf. The OPTIONS request never reaches your origin. After the preflight exchange resolves, the browser will then send the main request which does include the authentication cookie (assuming you have logged into the Access-protected domain).

To configure how Cloudflare responds to preflight requests:

  1. In Zero Trust, go to Access > Applications.

  2. Locate the origin that will be receiving OPTIONS requests and select Edit.

  3. In the Settings tab, scroll down to CORS settings.

  4. Configure the dashboard CORS settings to match the response headers sent by your origin.

    For example, if you have configured api.mysite.comto return the following headers:

    headers: {
    'Access-Control-Allow-Origin': '',
    'Access-Control-Allow-Credentials' : true,
    'Access-Control-Allow-Methods': 'GET, OPTIONS',
    'Access-Control-Allow-Headers': 'office',
    'Content-Type': 'application/json',

    then go to in Access and configure Access-Control-Allow-Origin, Access-Control-Allow-Credentials, Access-Control-Allow-Methods, and Access-Control-Allow-Headers.

    Example CORS settings configuration in Zero Trust

  5. Select Save application.

  6. (Optional) You can check your configuration by sending an OPTIONS request to the origin with curl. For example,

    curl -I -XOPTIONS \
    -H 'origin:' \
    -H 'access-control-request-method: GET'

    should return a response similar to:

    HTTP/2 200
    date: Tue, 24 May 2022 21:51:21 GMT
    vary: Origin, Access-Control-Request-Method, Access-Control-Request-Headers
    access-control-allow-methods: GET
    access-control-allow-credentials: true
    expect-ct: max-age=604800, report-uri=""
    report-to: {"endpoints":[{"url":"https:\/\/\/report\/v3?s=A%2FbOOWJio%2B%2FjuJv5NC%2FE3%2Bo1zBl2UdjzJssw8gJLC4lE1lzIUPQKqJoLRTaVtFd21JK1d4g%2BnlEGNpx0mGtsR6jerNfr2H5mlQdO6u2RdOaJ6n%2F%2BS%2BF9%2Fa12UromVLcHsSA5Y%2Fj72tM%3D"}],"group":"cf-nel","max_age":604800}
    nel: {"success_fraction":0.01,"report_to":"cf-nel","max_age":604800}
    server: cloudflare
    cf-ray: 7109408e6b84efe4-EWR

​​ Send authentication token with Cloudflare Worker

If you have two sites protected by Cloudflare Access, and, requests made between the two will be subject to CORS checks. Users who log in to will be issued a cookie for When the user’s browser requests, Cloudflare Access looks for a cookie specific to The request will fail if the user has not already logged in to

To avoid having to log in twice, you can create a Cloudflare Worker that automatically sends authentication credentials to

​​ Prerequisites

​​ 1. Generate a service token

Follow these instructions to generate a new Access service token. Copy the Client ID and Client Secret to a safe place, as you will use them in a later step.

​​ 2. Add a Service Auth policy

  1. In Zero Trust, go to Access > Applications.

  2. Find your application and select Edit.

  3. Select the Policies tab.

  4. Add the following policy:

    Action Rule type Selector
    Service Auth Include Service Token

​​ 3. Create a new Worker

  1. Open a terminal and create a new Workers project.

    $ wrangler generate redirect-worker
  2. Go to the project directory.

    $ cd redirect-worker
  3. Open wrangler.toml in a text editor and insert your account ID.

    name = "redirect-worker"
    type = "javascript"
    account_id = "123abc456654abc123"
    workers_dev = true
    route = ""
    zone_id = ""
    compatibility_date = "2022-05-16"
  4. Open index.js and copy in the following example code.

    // The hostname where your API lives
    const originalAPIHostname = ''
    async function handleRequest(request) {
    /** Change just the host.
    If the request comes in on, the new URL is
    const url = new URL(request.url)
    url.hostname = originalAPIHostname
    /** If your API is located on (without "api/" in the path),
    remove the "api/" part of
    // url.pathname = url.pathname.substring(4)
    /** Best practice is to always use the original request to construct the new request
    to clone all the attributes. Applying the URL also requires a constructor
    since once a Request has been constructed, its URL is immutable.
    const newRequest = new Request(
    newRequest.headers.set('cf-access-client-id', CF_ACCESS_CLIENT_ID)
    newRequest.headers.set('cf-access-client-secret', CF_ACCESS_CLIENT_SECRET)
    try {
    const response = await fetch(newRequest);
    // Copy over the response
    const modifiedResponse = new Response(response.body, response);
    // Delete the set-cookie from the response so it doesn't override existing cookies
    return modifiedResponse;
    } catch (e) {
    return new Response(JSON.stringify({ error: e.message }), { status: 500 })
    addEventListener('fetch', event => {
  5. Deploy the Worker to your account.

    $ npx wrangler deploy

​​ 4. Configure the Worker

  1. Log in to the Cloudflare dashboard, select your account and go to Workers & Pages.

  2. Select your newly created Worker. In this example, the Worker is called redirect-worker.

  3. In the Triggers tab, scroll down to Routes and add*. The Worker is placed on a subpath of to avoid making a cross-origin request.

  4. In the Settings tab, select Variables.

  5. Under Environment Variables, add the following secret variables:

    • CF_ACCESS_CLIENT_ID = <service token Client ID>
    • CF_ACCESS_CLIENT_SECRET = <service token Client Secret>

The Client ID and Client Secret are copied from your service token.

  1. Enable the Encrypt option for each variable and select Save.

​​ 5. Update HTTP request URLs

Modify your application to send all requests to instead of

HTTP requests should now work seamlessly between two different Access-protected domains. When a user logs in to, the browser makes a request to the Worker instead of to The Worker adds the Access service token to the request headers and then forwards the request to Since the service token matches a Service Auth policy, the user no longer needs to log in to

​​ Troubleshooting

In general, we recommend the following steps when troubleshooting CORS issues:

  1. Capture a HAR file with the issue described, as well as the JS console log output recorded simultaneously. This is because the HAR file alone will not give full visibility on the reason behind cross-origin issues.
  2. Ensure that the application has set credentials: 'same-origin' in all fetch or XHR requests.
  3. If you are using the cross-origin setting on script tags, these must be set to “use-credentials”.