Fixing Common Problems with CORS and JavaScript

Many websites have JavaScript functions that make network requests to a server, such as a REST API. The web pages and APIs are often in different domains. This introduces security issues in that any website can request data from an API. Cross-Origin Resource Sharing (CORS) provides a solution to these issues. It became a W3C recommendation in 2014. It makes it the responsibility of the web browser to prevent unauthorized access to APIs. All modern web browsers enforce CORS. They prevent JavaScript from obtaining data from a server in a domain different than the domain the website was loaded from, unless the REST API server gives permission.

From a developer’s perspective, CORS is often a cause of much grief when it blocks network requests. CORS provides a number of different mechanisms for limiting JavaScript access to APIs. It is often not obvious which mechanism is blocking the request. We are going to build a simple web application that makes REST calls to a server in a different domain. We will deliberately make requests that the browser will block because of CORS policies and then show how to fix the issues. Let’s get started!

NOTE: The code for this project can be found on GitHub.

Table of Contents

Prerequisites to Building a Go Application

First things first, if you don’t already have Go installed on your computer you will need to download and install the Go Programming Language.

Now, create a directory where all of our future code will live.

mkdir cors
cd cors

Finally, we will make our directory a Go module and install the Gin package (a Go web framework) to implement a web server.

go mod init cors
go get github.com/gin-gonic/gin
go get github.com/gin-contrib/static

A file called go.mod will get created containing the dependencies.

How to Build a Simple Web Front End

We are going to build a simple HTML and JavaScript front end and serve it from a web server written using Gin.

First of all, create a directory called frontend and create a file called frontend/index.html with the following content:

<html>
    <head>
        <meta charset="UTF-8" />
        <title>Fixing Common Issues with CORS</title>
    </head>
    <body>
        <h1>Fixing Common Issues with CORS</h1>
        <div>
            <textarea id="messages" name="messages" rows="10" cols="50">Messages</textarea><br/>
            <form id="form1">
                <input type="button" value="Get v1" onclick="onGet('v1')"/>
                <input type="button" value="Get v2" onclick="onGet('v2')"/>
            </form>
        </div>
    </body>
</html>

The web page has a text area to display messages and a simple form with two buttons. When a button is clicked it calls the JavaScript function onGet() passing it a version number. The idea being that v1 requests will always fail due to CORS issues, and v2 will fix the issue.

Next, create a file called frontend/control.js containing the following JavaScript:

function onGet(version) {
    const url = "http://localhost:8000/api/" + version + "/messages";
    var headers = {}
    
    fetch(url, {
        method : "GET",
        mode: 'cors',
        headers: headers
    })
    .then((response) => {
        if (!response.ok) {
            throw new Error(response.error)
        }
        return response.json();
    })
    .then(data => {
        document.getElementById('messages').value = data.messages;
    })
    .catch(function(error) {
        document.getElementById('messages').value = error;
    });
}

The onGet() function inserts the version number into the URL and then makes a fetch request to the API server. A successful request will return a list of messages. The messages are displayed in the text area.

Finally, create a file called frontend/.go containing the following Go code:

package main

import (
	"github.com/gin-contrib/static"
	"github.com/gin-gonic/gin"
)

func main() {
	r := gin.Default()
	r.Use(static.Serve("/", static.LocalFile("./frontend", false)))
	r.Run(":8080")
}

This code simply serves the contents of the frontend directory on requests on port 8080. Note that JavaScript makes a call to port http://localhost:8000 which is a separate service.

Start the server and point a web browser at http://localhost:8080 to see the static content.

go run frontend.go

How to Build a Simple REST API in Go

Create a directory called rest to contain the code for a basic REST API.

NOTE: A separate directory is required as Go doesn’t allow two program entry points in the same directory.

Create a file called rest/server.go containing the following Go code:

package main

import (
	"fmt"
	"strconv"
	"net/http"

	"github.com/gin-gonic/gin"
)

var messages []string

func GetMessages(c *gin.Context) {
	version := c.Param("version")
	fmt.Println("Version", version)
	c.JSON(http.StatusOK, gin.H{"messages": messages})
}

func main() {
	messages = append(messages, "Hello CORS!")
	r := gin.Default()
	r.GET("/api/:version/messages", GetMessages)
	r.Run(":8000")
}

A list called messages is created to hold message objects.

The function GetMessages() is called whenever a GET request is made to the specified URL. It returns a JSON string containing the messages. The URL contains a path parameter which will be v1 or v2. The server listens on port 8000.

The server can be run using:

go run rest/server.go

How to Solve a Simple CORS Issue

We now have two servers—the front-end one on http://localhost:8080, and the REST API server on http://localhost:8000. Even though the two servers have the same hostname, the fact that they are listening on different port numbers puts them in different domains from the CORS perspective. The domain of the web content is referred to as the origin. If the JavaScript fetch request specifies cors a request header will be added identifying the origin.

Origin: http://localhost:8080

Make sure both the frontend and REST servers are running.

Next, point a web browser at http://localhost:8080 to display the web page. We are going to get JavaScript errors, so open your browser’s developer console so that we can see what is going on. In Chrome it is *View** > Developer > Developer Tools.

Next, click on the Send v1 button. You will get a JavaScript error displayed in the console:

Access to fetch at ‘http://localhost:8000/api/v1/messages’ from origin ‘http://localhost:8080’ has been blocked by CORS policy: No ‘Access-Control-Allow-Origin’ header is present on the requested resource. If an opaque response serves your needs, set the request’s mode to ‘no-cors’ to fetch the resource with CORS disabled.

The message says that the browser has blocked the request because of a CORS policy. It suggests two solutions. The second suggestion is to change the mode from cors to no-cors in the JavaScript fetch request. This is not an option as the browser always deletes the response data when in no-cors mode to prevent data from being read by an unauthorized client.

The solution to the issue is for the server to set a response header that allows the browser to make cross-domain requests to it.

Access-Control-Allow-Origin: http://localhost:8080

This tells the web browser that the cross-origin requests are to be allowed for the specified domain. If the domain specified in the response header matches the domain of the web page, specified in the Origin request header, then the browser will not block the response being received by JavaScript.

We are going to set the header when the URL contains v2. Change the GetMessages() function in cors/server.go to the following:

func GetMessages(c *gin.Context) {
	version := c.Param("version")
	fmt.Println("Version", version)
	if version == "v2" {
		c.Header("Access-Control-Allow-Origin", "http://localhost:8080")
	}
	c.JSON(http.StatusOK, gin.H{"messages": messages})
}

This sets a header to allow cross-origin requests for the v2 URI.

Restart the server and go to the web page. If you click on Get v1 you will get blocked by CORS. If you click on Get v2, the request will be allowed.

A response can only have at most one Access-Control-Allow-Origin header. The header can only specify only one domain. If the server needs to allow requests from multiple origin domains, it needs to generate an Access-Control-Allow-Origin response header with the same value as the Origin request header.

Allowing Access from Any Origin Domain

There is an option to prevent CORS from blocking any domain. This is very popular with developers!

Access-Control-Allow-Origin: *

Be careful when using this option. It will get flagged in a security audit. It may also be in violation of an information security policy, which could have serious consequences!

CORS in Flight

Although we have fixed the main CORS issue, there are some limitations. One of the limitations is that only the HTTP GET, and OPTIONS methods are allowed. The GET and OPTIONS methods are read-only and are considered safe as they don’t modify existing content. The POST, PUT, and DELETE methods can add or change existing content. These are considered unsafe. Let’s see what happens when we make a PUT request.

First of all, add a new form to client/index.html:

<form id="form2">
    <input type="text" id="puttext" name="puttext"/>
    <input type="button" value="Put v1" onclick="onPut('v1')"/>
    <input type="button" value="Put v2" onclick="onPut('v2')"/>
</form>

This form has a text input and the two send buttons as before that call a new JavaScript function.

Next, add the JavaScript funtion to client/control.js:

function onPut(version) {
    const url = "http://localhost:8000/api/" + version + "/messages/0";
    var headers = {}

    fetch(url, {
        method : "PUT",
        mode: 'cors',
        headers: headers,
        body: new URLSearchParams(new FormData(document.getElementById("form2"))),
    })
    .then((response) => {
        if (!response.ok) {
            throw new Error(response.error)
        }
        return response.json();
    })
    .then(data => {
        document.getElementById('messages').value = data.messages;
    })
    .catch(function(error) {
        document.getElementById('messages').value = error;
    });
}

This makes a PUT request sending the form parameters in the request body. Note that the URI ends in /0. This means that the request is to create or change the message with the identifier 0.

Next, define a PUT handler in the main() function of rest/server.go:

r.PUT("/api/:version/messages/:id", PutMessage)

The message identifier is extracted as a path parameter.

Finally, add the request handler function to rest/server.go:

func PutMessage(c *gin.Context) {
	version := c.Param("version")
	id, _ := strconv.Atoi(c.Param("id"))
	text := c.PostForm("puttext")
	messages[id] = text
	if version == "v2" {
	    c.Header("Access-Control-Allow-Origin", "http://localhost:8080")
	}
	c.JSON(http.StatusOK, gin.H{"messages": messages})
}

This updates the message from the form data and sends the new list of messages. The function also always sets the allow origin header, as we know that it is required.

Now, restart the servers and load the web page. Make sure that the developer console is open. Add some text to the text input and hit the Send v1 button.

You will see a slightly different CORS error in the console:

Access to fetch at ‘http://localhost:8000/api/v1/messages/0’ from origin ‘http://localhost:8080’ has been blocked by CORS policy: Response to preflight request doesn’t pass access control check: No ‘Access-Control-Allow-Origin’ header is present on the requested resource. If an opaque response serves your needs, set the request’s mode to ‘no-cors’ to fetch the resource with CORS disabled.

This is saying that a preflight check was made, and that it didn’t set the Access-Control-Allow-Origin header.

Now, look at the console output from the API server:

[GIN] 2020/12/01 - 11:10:09 | 404 |  447ns |  ::1 | OPTIONS  "/api/v1/messages/0"

So, what is happening here? JavaScript is trying to make a PUT request. This is not allowed by CORS policy. In the GET example, the browser made the request and blocked the response. In this case, the browser refuses to make the PUT request. Instead, it sent an OPTIONS request to the same URI. It will only send the PUT if the OPTIONS request returns the correct CORS header. This is called a preflight request. As the server doesn’t know what method the OPTIONS preflight request is for, it specifies the method in a request header:

Access-Control-Request-Method: PUT

Let’s fix this by adding a handler for the OPTIONS request that sets the allow origin header in cors/server.go:

func OptionMessage(c *gin.Context) {
	c.Header("Access-Control-Allow-Origin", "http://localhost:8080")
}

func main() {
	messages = append(messages, "Hello CORS!")
	r := gin.Default()
	r.GET("/api/:version/messages", GetMessages)
	r.PUT("/api/:version/messages/:id", PutMessage)
	r.OPTIONS("/api/v2/messages/:id", OptionMessage)
	r.Run(":8000")
}

Notice that the OPTIONS handler is only set for the v2 URI as we don’t want to fix v1.

Now, restart the server and send a message using the Put v2 button. We get yet another CORS error!

Access to fetch at ‘http://localhost:8000/api/v2/messages/0’ from origin ‘http://localhost:8080’ has been blocked by CORS policy: Method PUT is not allowed by Access-Control-Allow-Methods in preflight response.

This is saying that the preflight check needs another header to stop the PUT request from being blocked.

Add the response header to cors/server.go:

func OptionMessage(c *gin.Context) {
	c.Header("Access-Control-Allow-Origin", "http://localhost:8080")
	c.Header("Access-Control-Allow-Methods", "GET, OPTIONS, POST, PUT")
}

Restart the server and resend the message. The CORS issues are resolved.

What Else Does CORS Block?

CORS has a very restrictive policy regarding which HTTP request headers are allowed. It only allows safe listed request headers. These are Accept, Accept-Language, Content-Language, and Content-Type. They can only contain printable characters and some punctuation characters are not allowed. Header values can’t have more than 128 characters.

There are further restrictions on the Content-Type header. It can only be one of application/x-www-form-urlencoded, multipart/form-data, and text/plain. It is interesting to note that application/json is not allowed.

Let’s see what happens if we send a custom request header. Modify the onPut() function in frontend/control.jsto set a header:

var headers = { "X-Token": "abcd1234" }

Now, make sure that the servers are running and load or reload the web page. Type in a message and click the Put v1 button. You will see a CORS error in the developer console.

Access to fetch at ‘http://localhost:8000/api/v2/messages/0’ from origin ‘http://localhost:8080’ has been blocked by CORS policy: Request header field x-token is not allowed by Access-Control-Allow-Headers in preflight response.

Any header which is not CORS safe-listed causes a preflight request. This also contains a request header that specifies the header that needs to be allowed:

Access-Control-Request-Headers: x-token

Note that the header name x-token is specified in lowercase.

The preflight response can allow non-safe-listed headers and can remove restrictions on safe listed headers:

Access-Control-Allow-Headers: X_Token, Content-Type

Next, change the function in cors/server.go to allow the custom header:

func OptionMessage(c *gin.Context) {
	c.Header("Access-Control-Allow-Origin", "http://localhost:8080")
	c.Header("Access-Control-Allow-Methods", "GET, POST, OPTIONS, PUT")
	c.Header("Access-Control-Allow-Headers", "X-Token")
}

Restart the server and resend the message by clicking the Put v2 button. The request should now be successful.

Restrictions on Response Headers

CORS also places restrictions on response headers. There are seven whitelisted response headers: Cache-Control, Content-Language, Content-Length, Content-Type, Expires, Last-Modified, and Pragma. These are the only response headers that can be accessed from JavaScript. Let’s see this in action.

First of all, add a text area to frontend/index.html to display the headers:

<textarea id="headers" name="headers" rows="10" cols="50">Headers</textarea><br/>

Next, replace the first then block in the onPut function in frontend/control.js to display the response headers:

.then((response) => {
    if (!response.ok) {
        throw new Error(response.error)
    }
    response.headers.forEach(function(val, key) {
        document.getElementById('headers').value += '\n' + key + ': ' + val; 
    });
    return response.json();
})

Finally, set a custom header in rest/server.go:

func PutMessage(c *gin.Context) {
	version := c.Param("version")
	id, _ := strconv.Atoi(c.Param("id"))
	text := c.PostForm("puttext")
	messages[id] = text
	if version == "v2" {
	    c.Header("Access-Control-Allow-Origin", "http://localhost:8080")
	}
	c.Header("X-Custom", "123456789")
	c.JSON(http.StatusOK, gin.H{"messages": messages})
}

Now, restart the server and reload the web page. Type in a message and hit Put v2. You should see some headers displayed, but not the custom header. CORS has blocked it.

This can be fixed by setting another response header to expose the custom header in server/server.go:

func PutMessage(c *gin.Context) {
	version := c.Param("version")
	id, _ := strconv.Atoi(c.Param("id"))
	text := c.PostForm("puttext")
	messages[id] = text
	if version == "v2" {
        c.Header("Access-Control-Expose-Headers", "X-Custom")
	    c.Header("Access-Control-Allow-Origin", "http://localhost:8080")
	}
	c.Header("X-Custom", "123456789")
	c.JSON(http.StatusOK, gin.H{"messages": messages})
}

Restart the server and reload the web page. Type in a message and hit Put v2. You should see some headers displayed, including the custom header. CORS has now allowed it.

Credentials Are a Special Case

There is yet another CORS blocking scenario. JavaScript has a credentials request mode. This determines how user credentials, such as cookies are handled. The options are:

  • omit: Never send or receive cookies.
  • same-origin: This is the default, that allows user credentials to be sent to the same origin.
  • include: Send user credentials even if cross-origin.

Let’s see what happens.

Modify the fetch call in the onPut() function in frontend/Control.js:

fetch(url, {
    method : "PUT",
    mode: 'cors',
    credentials: 'include',
    headers: headers,
    body: new URLSearchParams(new FormData(document.getElementById("form2"))),
})

Now, make sure that the client and server are running and reload the web page. Send a message as before. You will get another CORS error in the developer console:

Access to fetch at ‘http://localhost:8000/api/v2/messages/0’ from origin ‘http://localhost:8080’ has been blocked by CORS policy: Response to preflight request doesn’t pass access control check: The value of the ‘Access-Control-Allow-Credentials’ header in the response is ‘’ which must be ‘true’ when the request’s credentials mode is ‘include’.

The fix for this is to add another header to both the OptionMessage(), and PutMessage() functions in cors/server.go as the header needs to be present in both the preflight request and the actual request:

c.Header("Access-Control-Allow-Credentials", "true")

The request should now work correctly.

The credentials issue can also be resolved on the client by setting the credentials mode to omit and sending credentials as request parameters, instead of using cookies.

Control CORS Cache Configuration

The is one more CORS header that hasn’t yet been discussed. The browser can cache the preflight request results. The Access-Control-Max-Age header specifies the maximum time, in seconds, the results can be cached. It should be added to the preflight response headers as it controls how long the Access-Control-Allow-Methods and Access-Control-Allow-Headers results can be cached.

Access-Control-Max-Age: 600

Browsers typically have a cap on the maximum time. Setting the time to -1 prevents caching and forces a preflight check on all calls that require it.

How to Prevent CORS Issues with Okta

Authentication providers, such as Okta, need to handle cross-domain requests to their authentication servers and API servers. This is done by providing a list of trusted origins. See Okta Enable CORS for more information.

How CORS Prevents Security Issues

CORS is an important security feature that is designed to prevent JavaScript clients from accessing data from other domains without authorization.

Modern web browsers implement CORS to block cross-domain JavaScript requests by default. The server needs to authorize cross-domain requests by setting the correct response headers.

CORS has several layers. Simply allowing an origin domain only works for a subset of request methods and request headers. Some request methods and headers force a preflight request as a further means of protecting data. The actual request is only allowed if the preflight response headers permit it.

CORS is a major pain point for developers. If a request is blocked, it is not easy to understand why, and how to fix it. An understanding of the nature of the blocked request, and of the CORS mechanisms, can easily overcome such issues.

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