According to https://fetch.spec.whatwg.org/#origin-header :
The Origin header is a version of the Referer header that does not reveal a path. It is used for all HTTP fetches whose request’s response tainting is "cors", as well as those where request’s method is neither GET nor HEAD. Due to compatibility constraints it is not included in all fetches.
As I understand it, in particular, the Origin header should be sent with any same-origin request except GET and HEAD methods. How the Origin header can be useful in this same-origin case? And why the exception is made for the GET and HEAD methods?
It can help as an CSRF mitigation. So you know your own site made the POST. Using Origin as a CSRF mitigation was an afterthought and since it was already deployed for CORS usage, some servers got confused when it was added for GET (at least), which is why it's not included there.
I'm using Restangular. I would like to get full response so I set
Restangular.setFullResponse(true);
but then I discovered that my custom headers does not work. Documentation for setFullResponse() method says:
in order for Restangular to access custom HTTP headers, your server must respond having the Access-Control-Expose-Headers: set.
I would like to send my custom headers but I don't want to change server settings. Is it possible?
If I leave default settings i.e.setFullResponse(false) there is no issue with custom headers. Is there another solution except changing server settings?
The sentence you cite from the setFullResponse() docs is about what headers from the response your Restangular app will be able to access. And what that is saying is, it’s not possible to access most of the headers from the response unless the server’s already configured to send the right response-header names in the Access-Control-Expose-Headers header.
Without the server setting any value for that header, the only response headers that browsers will let you access from client-side JavaScript in your web app are the Cache-Control,
Content-Language,
Content-Type,
Expires,
Last-Modified
&
Pragma response headers.
See https://fetch.spec.whatwg.org/#cors-safelisted-response-header-name for the spec on that.
I would like to send my custom headers but I don't want to change server settings. Is it possible?
If you mean you want to send custom headers in a request from your client-side Restangular code to the server, please provide more details about exactly which custom headers you want to send.
There too though, there’s another CORS header your server must send in the response: the Access-Control-Allow-Headers response header. If the server isn’t configured to send that with the right header names listed, then your request will fail. The reason in that case is, you will hit https://developer.mozilla.org/en-US/docs/Web/HTTP/Access_control_CORS#Preflighted_requests.
I've been stuck on this for a few hours with no luck, so I figured that I would ask here.
I have a service with a bunch of endpoints, most of which accept GET and POST http methods. In that case, my service simply specifies Access-Control-Allow-Origin to be * in the response headers, in the event that one of my apps is on a different domain/port and wants to use the service.
I have one endpoint that uses the DELETE http method, and I can't seem to get it to work. When I call this endpoint from my client app, I get this message in my console:
Response to preflight request doesn't pass access control check:
No 'Access-Control-Allow-Origin' header is present on the requested resource.
I've been reading up on CORS for the past couple hours and have tried a few different things, all of which have not worked. I (think I) at least understand that when I'm using http methods other than GET and POST, the browser sends a pre-flighted request with OPTIONS as the http method.
What is the best way to handle this? Is there a way to disable this pre-flighted request? I specify in my client app that the http method to call this endpoint is DELETE. Should I be putting something specifying headers in my AJAX function that calls this endpoint (I'm using straight JavaScript)?
If I can't figure out a way around this, I'm just going to change my endpoint to use a GET or POST method, but I wanted to find a way to handle this issue before I took the easy way out.
I (think I) at least understand that when I'm using http methods other than GET and POST, the browser sends a pre-flighted request with OPTIONS as the http method.
Yes
Is there a way to disable this pre-flighted request?
No there isn’t. It’s initiated by browsers automatically, with no way to disable it from your JS. As long as you’re sending that DELETE request cross-origin in your JS, browsers will do that preflight.
Should I be putting something specifying headers in my AJAX function that calls this endpoint?
Given the “No 'Access-Control-Allow-Origin' header is present on the requested resource” message you’re getting, no changes you make in your client code will make any difference.
The place where more headers need to be sent to deal with this is instead on the server side.
If I can't figure out a way around this, I'm just going to change my endpoint to use a GET or POST method
You might want to experiment with trying that first. It seems even if you make that change you’re still gonna get that “No 'Access-Control-Allow-Origin' header is present on the requested resource”.
The SO question "No 'Access-Control-Allow-Origin' header is present on the requested resource" is a good place to read up on this to get a better idea of what’s happening.
Is there a way to enable CORS for REST endpoints, as stated here and here? It looks like Firebase no longer adds them by default. It would be helpful for apps that don't need realtime connectivity or cant use websockets.
Here's a REST request to Firebase. If you check the response, there are no cors headers:
https://samplechat.firebaseio-demo.com/users/jack/name.json
Found the issue.
All the REST endpoints need to end in '.json', including put and post requests. That triggers Firebase to set the cross-origin headers to the origin on the request. By not adding the '.json' to the end of my POST request, the origin headers were not added so it looked like a CORS issue.
The example doesn't have a CORS header, so it looks like Firebase must add them only when needed.
I am building a web API. I found whenever I use Chrome to POST, GET to my API, there is always an OPTIONS request sent before the real request, which is quite annoying. Currently, I get the server to ignore any OPTIONS requests. Now my question is what's good to send an OPTIONS request to double the server's load? Is there any way to completely stop the browser from sending OPTIONS requests?
edit 2018-09-13: added some precisions about this pre-flight request and how to avoid it at the end of this reponse.
OPTIONS requests are what we call pre-flight requests in Cross-origin resource sharing (CORS).
They are necessary when you're making requests across different origins in specific situations.
This pre-flight request is made by some browsers as a safety measure to ensure that the request being done is trusted by the server.
Meaning the server understands that the method, origin and headers being sent on the request are safe to act upon.
Your server should not ignore but handle these requests whenever you're attempting to do cross origin requests.
A good resource can be found here http://enable-cors.org/
A way to handle these to get comfortable is to ensure that for any path with OPTIONS method the server sends a response with this header
Access-Control-Allow-Origin: *
This will tell the browser that the server is willing to answer requests from any origin.
For more information on how to add CORS support to your server see the following flowchart
http://www.html5rocks.com/static/images/cors_server_flowchart.png
edit 2018-09-13
CORS OPTIONS request is triggered only in somes cases, as explained in MDN docs:
Some requests don’t trigger a CORS preflight. Those are called “simple requests” in this article, though the Fetch spec (which defines CORS) doesn’t use that term. A request that doesn’t trigger a CORS preflight—a so-called “simple request”—is one that meets all the following conditions:
The only allowed methods are:
GET
HEAD
POST
Apart from the headers set automatically by the user agent (for example, Connection, User-Agent, or any of the other headers with names defined in the Fetch spec as a “forbidden header name”), the only headers which are allowed to be manually set are those which the Fetch spec defines as being a “CORS-safelisted request-header”, which are:
Accept
Accept-Language
Content-Language
Content-Type (but note the additional requirements below)
DPR
Downlink
Save-Data
Viewport-Width
Width
The only allowed values for the Content-Type header are:
application/x-www-form-urlencoded
multipart/form-data
text/plain
No event listeners are registered on any XMLHttpRequestUpload object used in the request; these are accessed using the XMLHttpRequest.upload property.
No ReadableStream object is used in the request.
Have gone through this issue, below is my conclusion to this issue and my solution.
According to the CORS strategy (highly recommend you read about it) You can't just force the browser to stop sending OPTIONS request if it thinks it needs to.
There are two ways you can work around it:
Make sure your request is a "simple request"
Set Access-Control-Max-Age for the OPTIONS request
Simple request
A simple cross-site request is one that meets all the following conditions:
The only allowed methods are:
GET
HEAD
POST
Apart from the headers set automatically by the user agent (e.g. Connection, User-Agent, etc.), the only headers which are allowed to be manually set are:
Accept
Accept-Language
Content-Language
Content-Type
The only allowed values for the Content-Type header are:
application/x-www-form-urlencoded
multipart/form-data
text/plain
A simple request will not cause a pre-flight OPTIONS request.
Set a cache for the OPTIONS check
You can set a Access-Control-Max-Age for the OPTIONS request, so that it will not check the permission again until it is expired.
Access-Control-Max-Age gives the value in seconds for how long the response to the preflight request can be cached for without sending another preflight request.
Limitation Noted
For Chrome, the maximum seconds for Access-Control-Max-Age is 600 which is 10 minutes, according to chrome source code
Access-Control-Max-Age only works for one resource every time, for example, GET requests with same URL path but different queries will be treated as different resources. So the request to the second resource will still trigger a preflight request.
Please refer this answer on the actual need for pre-flighted OPTIONS request: CORS - What is the motivation behind introducing preflight requests?
To disable the OPTIONS request, below conditions must be satisfied for ajax request:
Request does not set custom HTTP headers like 'application/xml' or 'application/json' etc
The request method has to be one of GET, HEAD or POST. If POST, content type should be one of application/x-www-form-urlencoded, multipart/form-data, or text/plain
Reference:
https://developer.mozilla.org/en-US/docs/Web/HTTP/Access_control_CORS
When you have the debug console open and the Disable Cache option turned on, preflight requests will always be sent (i.e. before each and every request). if you don't disable the cache, a pre-flight request will be sent only once (per server)
Yes it's possible to avoid options request. Options request is a preflight request when you send (post) any data to another domain. It's a browser security issue. But we can use another technology: iframe transport layer. I strongly recommend you forget about any CORS configuration and use readymade solution and it will work anywhere.
Take a look here:
https://github.com/jpillora/xdomain
And working example:
http://jpillora.com/xdomain/
For a developer who understands the reason it exists but needs to access an API that doesn't handle OPTIONS calls without auth, I need a temporary answer so I can develop locally until the API owner adds proper SPA CORS support or I get a proxy API up and running.
I found you can disable CORS in Safari and Chrome on a Mac.
Disable same origin policy in Chrome
Chrome: Quit Chrome, open an terminal and paste this command: open /Applications/Google\ Chrome.app --args --disable-web-security --user-data-dir
Safari: Disabling same-origin policy in Safari
If you want to disable the same-origin policy on Safari (I have 9.1.1), then you only need to enable the developer menu, and select "Disable Cross-Origin Restrictions" from the develop menu.
As mentioned in previous posts already, OPTIONS requests are there for a reason. If you have an issue with large response times from your server (e.g. overseas connection) you can also have your browser cache the preflight requests.
Have your server reply with the Access-Control-Max-Age header and for requests that go to the same endpoint the preflight request will have been cached and not occur anymore.
I have solved this problem like.
if($_SERVER['REQUEST_METHOD'] == 'OPTIONS' && ENV == 'devel') {
header('Access-Control-Allow-Origin: *');
header('Access-Control-Allow-Headers: X-Requested-With');
header("HTTP/1.1 200 OK");
die();
}
It is only for development. With this I am waiting 9ms and 500ms and not 8s and 500ms. I can do that because production JS app will be on the same machine as production so there will be no OPTIONS but development is my local.
You can't but you could avoid CORS using JSONP.
you can also use a API Manager (like Open Sources Gravitee.io) to prevent CORS issues between frontend app and backend services by manipulating headers in preflight.
Header used in response to a preflight request to indicate which HTTP headers can be used when making the actual request :
content-type
access-control-allow-header
authorization
x-requested-with
and specify the "allow-origin" = localhost:4200 for example
After spending a whole day and a half trying to work through a similar problem I found it had to do with IIS.
My Web API project was set up as follows:
// WebApiConfig.cs
public static void Register(HttpConfiguration config)
{
var cors = new EnableCorsAttribute("*", "*", "*");
config.EnableCors(cors);
//...
}
I did not have CORS specific config options in the web.config > system.webServer node like I have seen in so many posts
No CORS specific code in the global.asax or in the controller as a decorator
The problem was the app pool settings.
The managed pipeline mode was set to classic (changed it to integrated) and the Identity was set to Network Service (changed it to ApplicationPoolIdentity)
Changing those settings (and refreshing the app pool) fixed it for me.
OPTIONS request is a feature of web browsers, so it's not easy to disable it. But I found a way to redirect it away with proxy. It's useful in case that the service endpoint just cannot handle CORS/OPTIONS yet, maybe still under development, or mal-configured.
Steps:
Setup a reverse proxy for such requests with tools of choice (nginx, YARP, ...)
Create an endpoint just to handle the OPTIONS request. It might be easier to create a normal empty endpoint, and make sure it handles CORS well.
Configure two sets of rules for the proxy. One is to route all OPTIONS requests to the dummy endpoint above. Another to route all other requests to actual endpoint in question.
Update the web site to use proxy instead.
Basically this approach is to cheat browser that OPTIONS request works. Considering CORS is not to enhance security, but to relax the same-origin policy, I hope this trick could work for a while. :)
One solution I have used in the past - lets say your site is on mydomain.com, and you need to make an ajax request to foreigndomain.com
Configure an IIS rewrite from your domain to the foreign domain - e.g.
<rewrite>
<rules>
<rule name="ForeignRewrite" stopProcessing="true">
<match url="^api/v1/(.*)$" />
<action type="Rewrite" url="https://foreigndomain.com/{R:1}" />
</rule>
</rules>
</rewrite>
on your mydomain.com site - you can then make a same origin request, and there's no need for any options request :)
It can be solved in case of use of a proxy that intercept the request and write the appropriate headers.
In the particular case of Varnish these would be the rules:
if (req.http.host == "CUSTOM_URL" ) {
set resp.http.Access-Control-Allow-Origin = "*";
if (req.method == "OPTIONS") {
set resp.http.Access-Control-Max-Age = "1728000";
set resp.http.Access-Control-Allow-Methods = "GET, POST, PUT, DELETE, PATCH, OPTIONS";
set resp.http.Access-Control-Allow-Headers = "Authorization,Content-Type,Accept,Origin,User-Agent,DNT,Cache-Control,X-Mx-ReqToken,Keep-Alive,X-Requested-With,If-Modified-Since";
set resp.http.Content-Length = "0";
set resp.http.Content-Type = "text/plain charset=UTF-8";
set resp.status = 204;
}
}
What worked for me was to import "github.com/gorilla/handlers" and then use it this way:
router := mux.NewRouter()
router.HandleFunc("/config", getConfig).Methods("GET")
router.HandleFunc("/config/emcServer", createEmcServers).Methods("POST")
headersOk := handlers.AllowedHeaders([]string{"X-Requested-With", "Content-Type"})
originsOk := handlers.AllowedOrigins([]string{"*"})
methodsOk := handlers.AllowedMethods([]string{"GET", "HEAD", "POST", "PUT", "OPTIONS"})
log.Fatal(http.ListenAndServe(":" + webServicePort, handlers.CORS(originsOk, headersOk, methodsOk)(router)))
As soon as I executed an Ajax POST request and attaching JSON data to it, Chrome would always add the Content-Type header which was not in my previous AllowedHeaders config.