‘access-control-allow-credentials’). in ‘true’ expected (reason: Azure Functions: CORS: How to support “Access-Control-Allow-Credentials” header AND OPTIONS pre-flight?

0 Answers

azure access-control-allow-credentials


Since doesn't like you to delete things. This was my own sillyness.

What I've learned:

  • The OPTIONS method does not currently appear to work on the Azure Functions Cli when running Azure functions locally.
  • Azure Functions on Azure support OPTIONS requests just fine and dandy. They just don't understand it when you mess up and have the wrong header in HOST of the request when manually testing via a proxy :-P.

I've got an Azure Function which requires cookies to carry the session details. As a result I need the header "Access-Control-Allow-Credentials" set to true.

As is detailed here: https://github.com/Azure/azure-webjobs-sdk-script/issues/620

If you disable the default "App Services" CORS settings for the Function App by removing all the CORS urls it will then allow you to set CORS related headers such as "Access-Control-Allow-Credentials". This works beautifully.

However, with the CORS settings disabled it requires you to handle all the CORS functionality manually. The problem with this is that Azure functions do not appear to allow you to use the OPTIONS verb when defining a function. As a result there does not appear to be a way to handle CORS Pre-Flight requests.

Is there any recommended work-around?

FYI, this is my function.json:

      "name": "req",
      "type": "httpTrigger",
      "direction": "in",
      "authLevel": "anonymous",
      "methods": [ "OPTIONS" ],
      "route": "1.0/myFunction"
      "name": "res",
      "type": "http",
      "direction": "out"

This is the code:

'use strict';

module.exports = function(context, req){
    var shared = require('../auth-shared-libraries');
    var cors_url = shared.getCORSUrl(context);
    context.res = {
        status: 200,
        headers: {
            "Access-Control-Allow-Credentials" : "true",
            "Access-Control-Allow-Origin" : cors_url,
            "Access-Control-Allow-Methods" : "POST, OPTIONS",
            "Access-Control-Allow-Headers" : "Content-Type, Set-Cookie",
            "Access-Control-Max-Age" : "86400",
            "Vary" : "Accept-Encoding, Origin",
            "Content-Type" : "application/json"
        body : JSON.stringify(req)

This is the example request being sent to it:

OPTIONS /api/1.0/myFunction HTTP/1.1
Host: localhost:7071
User-Agent: Mozilla/5.0 (Windows NT 10.0; WOW64; rv:51.0) Gecko/20100101 Firefox/51.0
Accept: text/html,application/xhtml+xml,application/xml;q=0.9,*/*;q=0.8
Accept-Language: en-US,en;q=0.5
Access-Control-Request-Method: POST
Origin: http://localhost:9000
Connection: close

This is the response I get if I'm running it against Azure-Functions hosted on Azure:

HTTP/1.1 404 Site Not Found
Content-Type: text/html
Server: Microsoft-IIS/8.0
Date: Tue, 14 Feb 2017 01:57:14 GMT
Connection: close
Content-Length: 5147

<!DOCTYPE html>
    <title>Microsoft Azure Web App - Error 404</title>
    <style type="text/css">
        html {
            height: 100%;
            width: 100%;

        #feature {
            width: 960px;
            margin: 75px auto 0 auto;
            overflow: auto;

        #content {
            font-family: "Segoe UI";
            font-weight: normal;
            font-size: 22px;
            color: #ffffff;
            float: left;
            width: 460px;
            margin-top: 68px;
            margin-left: 0px;
            vertical-align: middle;

            #content h1 {
                font-family: "Segoe UI Light";
                color: #ffffff;
                font-weight: normal;
                font-size: 60px;
                line-height: 48pt;
                width: 800px;

        p a, p a:visited, p a:active, p a:hover {
            color: #ffffff;

        #content a.button {
            background: #0DBCF2;
            border: 1px solid #FFFFFF;
            color: #FFFFFF;
            display: inline-block;
            font-family: Segoe UI;
            font-size: 24px;
            line-height: 46px;
            margin-top: 10px;
            padding: 0 15px 3px;
            text-decoration: none;

            #content a.button img {
                float: right;
                padding: 10px 0 0 15px;

            #content a.button:hover {
                background: #1C75BC;
    <script type="text/javascript">
        function toggle_visibility(id) {
            var e = document.getElementById(id);
            if (e.style.display == 'block')
                e.style.display = 'none';
                e.style.display = 'block';
<body bgcolor="#00abec">
    <div id="feature">
        <div id="content">
            <h1>Error 404 - Web app not found.</h1>
            <p>The web app you have attempted to reach is not available in this Microsoft Azure App Service region. This could be due to one of several reasons:
                1. The web app owner has registered a custom domain to point to the Microsoft Azure App Service, but has not yet configured Azure to recognize it. <a href="#" onclick="toggle_visibility('moreone');">Click here to read more</a></abbr>.
                <div id="moreone" style="display:none">
                    <font size=2>
                      When an app owner wants to use a custom domain with a <a href="http://www.windowsazure.com/en-us/services/web-sites/">Microsoft Azure Web Apps</a> web app, Azure needs to be configured to recognize the custom domain name, so that it can route the request to the appropriate server in the region. After registering the domain with a domain provider and configuring a DNS CNAME record to point to the app&#39;s Azurewebsites.net address (for example, contoso.azurewebsites.net), the web app owner also needs to go to the Azure Portal and configure the app for the new domain. <a href="http://www.windowsazure.com/en-us/documentation/articles/web-sites-custom-domain-name/">Click here</a> to learn more about configuring the custom domains for a web app.
          2. The web app owner has moved the web app to a different region, but the DNS cache is still directing to the old IP Address that was used in the previous region. <a href="#" onclick="toggle_visibility('moretwo');">Click here to read more.</a>
            <div id="moretwo" style="display:none">
                <font size=2>
                    With <a href="http://www.windowsazure.com/en-us/services/web-sites/">Web Apps</a>, the app is stored in a datacenter located in the region that the web app owner has selected when creating the app, and Azure�s DNS server resolves the web app address that was chosen for it to that datacenter. DNS servers are in charge of resolving the name of the server the user is trying to reach into an IP address, but clients cache this information in order to be able to load the page as fast as possible. If this app was deleted and re-created in another region, the new app will have a different IP address, but the client might still be caching the old IP address.

                    First, try clearing the cache on your client <a href="http://technet.microsoft.com/en-us/library/cc781949(v=ws.10).aspx">as described here</a>. If this does not help, this is probably due to the caching done on an intermediate DNS server such as the one used by your Internet Service Provider. If so, this issue should clear up soon, once the DNS cache reaches its time-to-live period. Please try to visit the app again in approximately one hour. If you continue to receive this error page, please contact <a href="http://www.windowsazure.com/en-us/support/options/">Microsoft support</a>.

This is the response I get if I run the request against the Azure-Functions-Cli running locally:

NOTE: It complains about the Origin, but there is no config I know of for local development for setting up the CORS. If there was, I'm not sure how that would translate to Azure when this appears to need to be manual.

HTTP/1.1 400 Bad Request
Content-Type: application/json; charset=utf-8
Server: Microsoft-HTTPAPI/2.0
Date: Tue, 14 Feb 2017 01:28:12 GMT
Connection: close
Content-Length: 64

{"Message":"The origin 'http://localhost:9000' is not allowed."}