iFrame sandbox permissions tutorial

Jim rottinger.

Software Engineer at Looker

Understanding iFrame sandboxes and iFrame security

Embedding third-party JavaScript in web applications is a tale as old as time. Whether it’s dropping a widget onto your web page or including custom content from a client in your cloud application, it’s something that many developers have encountered in their career. We all know about the iframe element in HTML, but how much do we really know about how it works? What are the security concerns associated with running code inside of an iframe and, furthermore, how can the HTML5 sandbox attribute on the frame alleviate these concerns?

The goal of this tutorial is to walk through the various security risks associated with running third-party JavaScript on your page and explain how sandboxed iframes can alleviate those issues by restricting the permissions it is allowed to run with.

In this post, we’ll demonstrate setting up a demo application from the ground up that will simulate running JavaScript coming from a different origin. What we should end up with is a sandboxed environment in which we can execute any arbitrary JavaScript and still sleep well at night, knowing our host application will be safe from harm.

With all of that in mind, the guided walkthrough will consist of the following parts:

  • Setting up two node servers to simulate two different origins
  • Embedding the content of our client page in an iframe on the host page and investigating what the client iframe is and is not allowed to do
  • Applying the sandbox attribute to the iframe and exploring the various options for the sandbox.

Let’s get started!

Step 1: Setting up the servers for our demo application

To simulate executing code from a different origin, we are going to set up two node servers — one which we’ll call the host and second which we will call the client. We can do this using node’s http library to listen to and serve from two different ports.

Save this JS file to whatever name you’d like - I called it server.js. Then, to start our server, we can simply run:

This should start two different http servers, one on port 8000 and the second on port 8001. To test that it is working, you can individually visit your localhost on port 8000 and 80001, which should look like this:


Even though they are both running on localhost, the  same-origin policy  that browsers implement operate on a protocol-host-port tuple that will only be true if the protocol, the hostname, and port number all match up. In this case, the protocol and host are the same, but since the ports are different, these will be considered to be different origins.

Of course, just having a hard-coded response won’t get us very far. We’ll need to be able to serve both HTML and JS for this demo. To do this, I whipped up a function to serve assets from a given folder.

This function will take in a root folder path, a url, and the response object. If the file is not found, it will return 404. If it is found, it will set the header to be text/javascript or text/html, depending on the file suffix. To get this to work, we need to include two more dependencies at the top of the file:

Fun fact - Since we’re just using built-in node libraries, we do not have to install anything via npm! Once you instantiate fileSystem, path, and our asset function, go ahead and update your servers as well to call serveAsset.

These now look very similar. The only difference is that the host_server will look for its assets in the host folder and the client_server will look in the client folder. If we were to restart our server now, we would see the following error message:


This is because our serveAsset function is looking in either the host or client folder for an asset to serve, and we haven’t created them yet! Let’s create both of them, each with an index.html and a JS file.

Our file structure should look like this:

Now, if we start our server and visit our localhost location, we no longer get the 404, which means our server found the file! — but it has no content yet. To get some content in, let’s start with something very simple. For the host, we simply have the HTML as:

And the JavaScript as:

The client content is exactly the same, just with the word host changed to client. If we restart our server now, we should be able to go to both http://localhost:8000/ and http://localhost:8001/ and see our content in action! Each page should send an alert from the JS file and then render our html content to the page.


Each node server is serving an index.html file which includes a JavaScript file

Step 2: Embedding the client in the host without sandboxing and investigating its permissions

With our two servers running, we are now ready to begin testing some iframe scenarios. Before we do that, let’s add a second file to our host so that we can compare the permissions of an iframe from the same origin and an iframe from a different origin.

Fill in the content of these files with the same content as we used for our other html/JS pairs. We’ll call this one the “Hosted Client,” meaning an iframe client coming from the same origin as our host.

Once we do that, back in our host/index.html we can iframe both our same-origin client and our different-origin client.

Please note that we use localhost for the hosted-client and for the other. This will become important in the cookies section below. Refreshing the host page, you should now see two iframes, each with the content of our individual HTML files.

If you have alerts in your JS files like I do, you should have seen that each individual file triggers an alert at the top level of the browser. Should it be able to do this? That question brings us to our first area of concern — pop-ups and dialog boxes.

Pop-ups and dialog boxes

JavaScript has three different functions that trigger a popup — alert, prompt, and confirm. Each of these open up a dialog box at the top of the browsing context, regardless of whether it comes from the top-level window or not. The scary thing about these dialogs, as can be found in the  alert documentation , is:

“Dialog boxes are modal windows — they prevent the user from accessing the rest of the program’s interface until the dialog box is closed. For this reason, you should not overuse any function that creates a dialog box (or modal window).”

I am sure that most of us have, at some point in our lives, been forcefully redirected to a spammy site against your will that then bombarded you with these types of dialogs. Even when you try to close out of it, it just pops open another one. This annoying behavior completely blocks you out of using a site and sadly, it’s incredibly simple to reproduce. Try adding this to your client.js file:

Now, when you visit your host site, you get this:

These prompts make it impossible to ever interact with the page and you have to close the entire tab or kill the JavaScript execution to get rid of it. The worst part is that regardless of whether your embedded content is coming from a different host or the same origin, this behavior is exploitable.

You might think that you could just do something like this to get rid of it:

The problem with this is that each iframe operates within its own nested browsing context, so overriding the functions at the host level will not affect the functions at the frame level.

Fortunately, sandboxing can come to our rescue here, which we will see later in this post. For now, let’s move on to whether or not the iframe can navigate the page away from the current one.

Step 3: Top-level window navigation and opening new tabs

Let's now examine how and if the iframes are able to change the url of the top-level window and if they’re able to open a new window.

There are two different methods that we want to test here.

  • window.open for opening new windows and tabs, and
  • window.location for navigating the page away from the current url.

Helpful note: iframes can reference its top-level window using window.top. Similarly, it can reference its parent’s window with window.parent. In our case, they do the same thing.

Let’s remove all the code in client.js and replace it with:

This will attempt to redirect the top-level window to the client host. If we run this, we get the following error:


That’s great! It blocks the automatic redirect. But wait — what’s that last part? “… nor has it received a user gesture.”

What does that mean? Does it mean that a user-initiated gesture can navigate the window? Let’s try it.

If we add this code to our JS and HTML respectively, it will add a link to the client page. When we click it, the page navigates!

As it turns out, a user-initiated action can navigate the top-level window, which is the basic idea behind clickjacking. A typical  clickjacking  attack will put transparent click boxes over a page and then “hijack” the click to redirect the page to a different url. window.open works the same way.

If we have this, the browser will block the pop-up request of the outer function call, however, when we have it in a click handler, it will open the window regardless of whether we blocked pop-ups or not.

The other part of the above error message saying the iframe “does not have the same origin as the window it is targeting” means our iframe is the same origin as the host and it has full access to redirect the page, click-action or not.

If you run this, the browser will allow the redirect to happen since it is the same origin. This is not the case for window.open. Even if it is from the same origin, the browser will block the window.open unless you explicitly tell the browser to allow the pop-up.

Cookies and browser requests

The final thing we are going to look at is browser cookies. Before getting started, make sure your hosted-client iframe is pointed at localhost and your client.js iframe is pointed at

We need to do this because cookies care about domain and ignore the port. Once you have checked this, let’s set a cookie on the host.

This is representing a session ID, something that is often included in requests. Let’s see if our iframes can access the cookies.

As you can see, the client cannot access the cookies since it is a different origin, but the hosted-client can. Let’s try to make a request using the fetch API.

When we do this, we get a 200 response.

That means the server accepted the request and gave us a response, but did it send through the cookies? We can check this on the server side. Let’s add a console log to our host server request handler.

This will output the headers when we make a request to our host server. Make sure you restart the server after you add this line, and then reload your page and look for the request coming from the hosted-client. It looks like this:

As you can see, it sends the cookies through. If the server were to use just the session ID to authenticate the request, then it would think this is a legitimate request.

Step 4: Applying the sandbox attribute to the iFrame

So far, we’ve identified four areas of concern when working with iframes.

  • They can exploit pop-up dialog boxes to prevent interaction with the website
  • Navigating the top-level window through clickjacking even on different-origin iframes
  • Navigating the top-level window when the origin is the same even without user interaction
  • Same-origin iframes can make requests with cookies.

Now we’re going to begin making use of the sandbox attribute for iframes, introduced in HTML5. When added to an iframe, the sandboxed iframe restricts pretty much all scripts and browser behavior of any kind. It is not until we add the permissions in a space-separated list that we enable the exact permissions we want to set. To see its initial state, add the attribute as an empty string to both of our iframes.

When we sandbox the iframe, it blocks all scripts from executing


Sandboxed iframes with no permissions block all scripts from running

Getting this to work starts by allowing various permissions one at a time . The full list of string values can be found in the iframe documentation under the sandbox section. We will be starting with allow-scripts.

Allowing scripts

To begin here, let’s clear out our client.js and hosted-client.js and start with a simple console log.

Without defining any permissions, our sandbox won’t allow a console log to run. We can get our script running by adding the allow-scripts permission to our iframe attribute.

Once you do this, and refresh the page, you should see a console log from each of our pages.

Pop-ups and modals

One of the concerns we learned about is that an iframe can pop up dialog boxes at the top of the browsing context and prevent the user from interacting with the page. To see if this is exploitable in a sandbox, let’s add an alert to our client script.

When we run that we get the following error:

Even though we are allowing scripts to run, the sandbox still limits a lot of the browser behaviors. In order for the alert to work from the iframe, we would have to add the allow-modals property to the iframe.

Keep in mind that this is an all-or-nothing thing. We cannot allow some popups and block others. This is an okay restriction, in my opinion, and it crosses off our first iframe security concern.

Top-level window navigation and opening new tabs

Our second and third security concerns are related to navigating the page away from the original URL. We saw that a same-origin iframe could navigate the page without a user interaction, and that a different-origin iframe could do so with user interaction. Let’s try this in our sandbox.

This results in the following errors:


With this, we covered two cases at once. The initial command to change the location failed, as did the one on-click. By applying separate permissions to our iframe for each of these cases, we can allow any navigation with allow-top-navigation and user-activated navigation with allow-top-navigation-by-user-activation.

When we turn this on, the different-origin iframe can redirect the page upon user action. The case is the same for same-origin iframes, where you can explicitly set the navigation permissions, regardless of the origin.

The final concern to address is the ability to access cookies and make requests with same-origin iframes. Let’s try accessing the cookies with a sandboxed iframe.

Unlike last time, this results in the following error:

Similarly, when we try to make a request with the same request code as the previous section, we get a different error.

The above error is the browser blocking our request because we no longer have the same origin. This is because the sandbox property sets the origin of the frame to null, meaning it will now be a cross-origin request, even though the iframe is hosted on the same domain.

Adding the allow-same-origin sandbox attribute will prevent both of these errors from occurring. However, you should be careful and make sure you have complete control over the content of the frame before using it. As noted in the  Mozilla iframe documentation :

“When the embedded document has the same origin as the main page, it is strongly discouraged to use both allow-scripts and allow-same-origin at the same time, as that allows the embedded document to programmatically remove the sandbox attribute. Although it is accepted, this case is no more secure than not using the sandbox attribute.”

Generally speaking, if you find yourself needing both allow-scripts and allow-same-origin for your sandbox, you should ask yourself why you are iframing in the first place, and whether or not having the sandbox property is appropriate.

Putting it all together: how we use iFrames at Looker

As an example of a practical use of this, here at Looker, we use iframes to allow customers to create and run their own custom visualizations within our application.

Without any one way to vet every single line of code our customers could write for their custom visualizations, we needed to create a secure execution environment to run the code in. A diagram of this environment can be seen below.


Looker explore page using an iframe to render custom data visualization

We leverage the postMessage API to pass the data in and to receive back any events or errors that the visualization produces. Given the restrictions of the sandboxed iframe, it is not able to make calls outside of its own frame, nor is it able to read or modify anything about the parent page. This let’s us rest assured that both our application and our customers’ data is safe and secure.

Wrapping up

I hope you found this post helpful as you address security concerns related to sandboxed iframes. By walking through this tutorial, you should now have a better understanding of:

  • How sandboxed iframes without allow-modals permission prevent user interaction on the page
  • How sandboxed iframes without the allow-top-navigation or the allow-top-navigation-by-user-activation properties can alleviate same-origin iframes that can redirect the top-level page, as well as different-origin iframes with some user interaction.
  • Why sandboxed iframes without the allow-same-origin property prevent same-origin iframes from having access to the domain’s cookies and making requests as if they were the host.

If you have any questions about this tutorial, feel free to reach out on the  Looker Community . And if you’re curious to learn more about our Looker team, check out our  open positions .

Important notes

  • Although sandboxing is very powerful for security, don’t put all of your security eggs in the sandbox basket, as it is  not supported by IE9 and below .
  • If you struggled with any part of this tutorial, all of the code can be found in the  sandbox-sandbox repository .
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Play safely in sandboxed IFrames

Mike West X Homepage

Constructing a rich experience on today's web almost unavoidably involves embedding components and content over which you have no real control. Third-party widgets can drive engagement and play a critical role in the overall user experience, and user-generated content is sometimes even more important than a site's native content. Abstaining from either isn't really an option, but both increase the risk that Something Bad™ could happen on your site. Each widget that you embed -- every ad, every social media widget -- is a potential attack vector for those with malicious intent:

Content Security Policy (CSP) can mitigate the risks associated with both of these types of content by giving you the ability to whitelist specifically trusted sources of script and other content. This is a major step in the right direction, but it's worth noting that the protection that most CSP directives offer is binary: the resource is allowed, or it isn't. There are times when it would be useful to say "I'm not sure I actually trust this source of content, but it's soooo pretty! Embed it please, Browser, but don't let it break my site."

Least Privilege

In essence, we're looking for a mechanism that will allow us to grant content we embed only the minimum level of capability necessary to do its job. If a widget doesn't need to pop up a new window, taking away access to window.open can't hurt. If it doesn't require Flash, turning off plugin support shouldn't be a problem. We're as secure as we can be if we follow the principle of least privilege , and block each and every feature that isn't directly relevant to functionality we'd like to use. The result is that we no longer have to blindly trust that some piece of embedded content won't take advantage of privileges it shouldn't be using. It simply won't have access to the functionality in the first place.

iframe elements are the first step toward a good framework for such a solution. Loading some untrusted component in an iframe provides a measure of separation between your application and the content you'd like to load. The framed content won't have access to your page's DOM, or data you've stored locally, nor will it be able to draw to arbitrary positions on the page; it's limited in scope to the frame's outline. The separation isn't truly robust, however. The contained page still has a number of options for annoying or malicious behavior: autoplaying video, plugins, and popups are the tip of the iceberg.

The sandbox attribute of the iframe element gives us just what we need to tighten the restrictions on framed content. We can instruct the browser to load a specific frame's content in a low-privilege environment, allowing only the subset of capabilities necessary to do whatever work needs doing.

Twust, but verify

Twitter's "Tweet" button is a great example of functionality that can be more safely embedded on your site via a sandbox. Twitter allows you to embed the button via an iframe with the following code:

To figure out what we can lock down, let's carefully examine what capabilities the button requires. The HTML that's loaded into the frame executes a bit of JavaScript from Twitter's servers, and generates a popup populated with a tweeting interface when clicked. That interface needs access to Twitter's cookies in order to tie the tweet to the correct account, and needs the ability to submit the tweeting form. That's pretty much it; the frame doesn't need to load any plugins, it doesn't need to navigate the top-level window, or any of a number of other bits of functionality. Since it doesn't need those privileges, let's remove them by sandboxing the frame's content.

Sandboxing works on the basis of a whitelist. We begin by removing all permissions possible, and then turn individual capabilities back on by adding specific flags to the sandbox's configuration. For the Twitter widget, we've decided to enable JavaScript, popups, form submission, and twitter.com's cookies. We can do so by adding a sandbox attribute to the iframe with the following value:

That's it. We've given the frame all the capabilities it requires, and the browser will helpfully deny it access to any of the privileges that we didn't explicitly grant it via the sandbox attribute's value.

Granular Control over Capabilities

We saw a few of the possible sandboxing flags in the example above, let's now dig through the inner workings of the attribute in a little more detail.

Given an iframe with an empty sandbox attribute, the framed document will be fully sandboxed, subjecting it to the following restrictions:

  • JavaScript will not execute in the framed document. This not only includes JavaScript explicitly loaded via script tags, but also inline event handlers and javascript: URLs. This also means that content contained in noscript tags will be displayed, exactly as though the user had disabled script herself.
  • The framed document is loaded into a unique origin, which means that all same-origin checks will fail; unique origins match no other origins ever, not even themselves. Among other impacts, this means that the document has no access to data stored in any origin's cookies or any other storage mechanisms (DOM storage, Indexed DB, etc.).
  • The framed document cannot create new windows or dialogs (via window.open or target="_blank" , for instance).
  • Forms cannot be submitted.
  • Plugins will not load.
  • The framed document can only navigate itself, not its top-level parent. Setting window.top.location will throw an exception, and clicking on link with target="_top" will have no effect.
  • Features that trigger automatically (autofocused form elements, autoplaying videos, etc.) are blocked.
  • Pointer lock cannot be obtained.
  • The seamless attribute is ignored on iframes the framed document contains.

This is nicely draconian, and a document loaded into a fully sandboxed iframe poses very little risk indeed. Of course, it also can't do much of value: you might be able to get away with a full sandbox for some static content, but most of the time you'll want to loosen things up a bit.

With the exception of plugins, each of these restrictions can be lifted by adding a flag to the sandbox attribute's value. Sandboxed documents can never run plugins, as plugins are unsandboxed native code, but everything else is fair game:

  • allow-forms allows form submission.
  • allow-popups allows (shock!) popups.
  • allow-pointer-lock allows (surprise!) pointer lock.
  • allow-same-origin allows the document to maintain its origin; pages loaded from https://example.com/ will retain access to that origin's data.
  • allow-scripts allows JavaScript execution, and also allows features to trigger automatically (as they'd be trivial to implement via JavaScript).
  • allow-top-navigation allows the document to break out of the frame by navigating the top-level window.

With these in mind, we can evaluate exactly why we ended up with the specific set of sandboxing flags in the Twitter example above:

  • allow-scripts is required, as the page loaded into the frame runs some JavaScript to deal with user interaction.
  • allow-popups is required, as the button pops up a tweeting form in a new window.
  • allow-forms is required, as the tweeting form should be submittable.
  • allow-same-origin is necessary, as twitter.com's cookies would otherwise be inaccessible, and the user couldn't log in to post the form.

One important thing to note is that the sandboxing flags applied to a frame also apply to any windows or frames created in the sandbox. This means that we have to add allow-forms to the frame's sandbox, even though the form only exists in the window that the frame pops up.

With the sandbox attribute in place, the widget gets only the permissions it requires, and capabilities like plugins, top navigation, and pointer lock remain blocked. We've reduced the risk of embedding the widget, with no ill-effects. It's a win for everyone concerned.

Privilege Separation

Sandboxing third-party content in order to run their untrusted code in a low-privilege environment is fairly obviously beneficial. But what about your own code? You trust yourself, right? So why worry about sandboxing?

I'd turn that question around: if your code doesn't need plugins, why give it access to plugins? At best, it's a privilege you never use, at worst it's a potential vector for attackers to get a foot in the door. Everyone's code has bugs, and practically every application is vulnerable to exploitation in one way or another. Sandboxing your own code means that even if an attacker successfully subverts your application, they won't be given full access to the application's origin; they'll only be able to do things the application could do. Still bad, but not as bad as it could be.

You can reduce the risk even further by breaking your application up into logical pieces and sandboxing each piece with the minimal privilege possible. This technique is very common in native code: Chrome, for example, breaks itself into a high-privilege browser process that has access to the local hard-drive and can make network connections, and many low-privilege renderer processes that do the heavy lifting of parsing untrusted content. Renderers don't need to touch the disk, the browser takes care of giving them all the information they need to render a page. Even if a clever hacker finds a way to corrupt a renderer, she hasn't gotten very far, as the renderer can't do much of interest on its own: all high-privilege access must be routed through the browser's process. Attackers will need to find several holes in different pieces of the system order to do any damage, which hugely reduces the risk of successful pwnage.

Safely sandboxing eval()

With sandboxing and the postMessage API , the success of this model is fairly straightforward to apply to the web. Pieces of your application can live in sandboxed iframe s, and the parent document can broker communication between them by posting messages and listening for responses. This sort of structure ensures that exploits in any one piece of the app do the minimum damage possible. It also has the advantage of forcing you to create clear integration points, so you know exactly where you need to be careful about validating input and output. Let's walk through a toy example, just to see how that might work.

Evalbox is an exciting application that takes a string, and evaluates it as JavaScript. Wow, right? Just what you've been waiting for all these long years. It's a fairly dangerous application, of course, as allowing arbitrary JavaScript to execute means that any and all data an origin has to offer is up for grabs. We'll mitigate the risk of Bad Things™ happening by ensuring that the code is executed inside of a sandbox, which makes it quite a bit safer. We'll work our way through the code from the inside out, starting with the frame's contents:

Inside the frame, we have a minimal document that simply listens for messages from its parent by hooking into the message event of the window object. Whenever the parent executes postMessage on the iframe's contents, this event will trigger, giving us access to the string our parent would like us to execute.

In the handler, we grab the source attribute of the event, which is the parent window. We'll use this to send the result of our hard work back up once we're done. Then we'll do the heavy lifting, by passing the data we've been given into eval() . This call has been wrapped up in a try block, as banned operations inside a sandboxed iframe will frequently generate DOM exceptions; we'll catch those and report a friendly error message instead. Finally, we post the result back to the parent window. This is pretty straightforward stuff.

The parent is similarly uncomplicated. We'll create a tiny UI with a textarea for code, and a button for execution, and we'll pull in frame.html via a sandboxed iframe , allowing only script execution:

Now we'll wire things up for execution. First, we'll listen for responses from the iframe and alert() them to our users. Presumably a real application would do something less annoying:

Next, we'll hook up an event handler to clicks on the button . When the user clicks, we'll grab the current contents of the textarea , and pass them into the frame for execution:

Easy, right? We've created a very simple evaluation API, and we can be sure that code that's evaluated doesn't have access to sensitive information like cookies or DOM storage. Likewise, evaluated code can't load plugins, pop up new windows, or any of a number of other annoying or malicious activities.

You can do the same for your own code by breaking monolithic applications into single-purpose components. Each can be wrapped in a simple messaging API, just like what we've written above. The high-privilege parent window can act as a controller and dispatcher, sending messages into specific modules that each have the fewest privileges possible to do their jobs, listening for results, and ensuring that each module is well-fed with only the information it requires.

Note, however, that you need to be very careful when dealing with framed content that comes from the same origin as the parent. If a page on https://example.com/ frames another page on the same origin with a sandbox that includes both the allow-same-origin and allow-scripts flags, then the framed page can reach up into the parent, and remove the sandbox attribute entirely.

Play in your sandbox

Sandboxing is available for you now in a variety of browsers: Firefox 17+, IE10+, and Chrome at the time of writing ( caniuse, of course, has an up-to-date support table ). Applying the sandbox attribute to iframes you include allows you to grant certain privileges to the content they display, only those privileges which are necessary for the content to function correctly. This gives you the opportunity to reduce the risk associated with the inclusion of third-party content, above and beyond what is already possible with Content Security Policy .

Moreover, sandboxing is a powerful technique for reducing the risk that a clever attacker will be able to exploit holes in your own code. By separating a monolithic application into a set of sandboxed services, each responsible for a small chunk of self-contained functionality, attackers will be forced to not only compromise specific frames' content, but also their controller. That's a much more difficult task, especially since the controller can be greatly reduced in scope. You can spend your security-related effort auditing that code if you ask the browser for help with the rest.

That's not to say that sandboxing is a complete solution to the problem of security on the internet. It offers defense in depth, and unless you have control over your users' clients, you can't yet rely on browser support for all your users (if you do control your users clients -- an enterprise environment, for example -- hooray!). Someday… but for now sandboxing is another layer of protection to strengthen your defenses, it's not a complete defense upon which you can soley rely. Still, layers are excellent. I suggest making use of this one.

Further Reading

" Privilege Separation in HTML5 Applications " is an interesting paper that works through the design of a small framework, and its application to three existing HTML5 apps.

Sandboxing can be even more flexible when combined with two other new iframe attributes: srcdoc , and seamless . The former allows you to populate a frame with content without the overhead of an HTTP request, and the latter allows style to flow into the framed content. Both have fairly miserable browser support at the moment (Chrome and WebKit nightlies). but will be an interesting combination in the future. You could, for example, sandbox comments on an article via the following code:

Except as otherwise noted, the content of this page is licensed under the Creative Commons Attribution 4.0 License , and code samples are licensed under the Apache 2.0 License . For details, see the Google Developers Site Policies . Java is a registered trademark of Oracle and/or its affiliates.

Last updated 2013-01-04 UTC.

You are totally ignoring my context…

As mentioned above, iframes in an extension page are protected by the CSP and no arbitrary 3rd party scripts may run in it . Scripts run in the iframe can only come from the extension itself, and, as the extension itself doesn’t contain any code that removes the sandboxing (which can be verified by reviewing), it is impossible that the sandbox will break.

As such, your statement that combining allow-scripts allow-same-origin means no sandboxing at all is definitely false in this context—the sandbox WON’T break, and it WILL add security by forbidding form submission, top navigation, etc., than no sandbox attribute.

Read full topic

Actually, this should be no longer an issue in the review process.

The whole point of that requirement is to avoid giving false sense of security . If you have the sandbox attribute, it’s expected to mean that you want sandboxing. If you use combination of flags that allow removing the sandboxing at runtime, it’s better to remove the whole sandbox attribute to make it obvious that there’s no sandboxing at all.

Note that if you use combination allow-scripts allow-same-origin that allows the supposedly sandboxed code to remove the sandbox attribute at runtime so there’s zero additional security if you add the sandbox attribute.

You can scan for scripts related with browser.tabs.executeScript for that. I don’t think this is relevant with the iframe issue.

I don’t think this is true. The default content security policy will block all 3rd party scripts that are loaded into an extension iframe.

This is the basic logic: iframe[sandbox="allow-scripts allow-same-origin"] is no more open than a normal iframe . If iframe is considered safe enough and is generally allowed, there is no point to (generally) disallow iframe[sandbox="allow-scripts allow-same-origin"] .

I do can workaround by replacing iframe[sandbox="allow-scripts allow-same-origin"] with a normal iframe , it’s just stupid and imperformant to block forms using more complicated scripts than a simple sandboxed iframe without the “allow-forms” value. And actually I have done this revision but the re-submission is still halted for more than 2 weeks—you don’t have a mechanism to review a re-submission together with the previous submission with the same reviewer?

I’m not sure you understand the risks enough.

3rd party remote script can be executed from simple string without any “content_security_policy” issues - simply by using browser.tabs.executeScript API. This is how most malicious addons works and the future Manifest V3 will remove this feature for this reason.

If you are loading HTTP page into iframe, than the remote page will load and run 3rd party script!

The reviewers are skilled people so if you give them bad arguments than you sound suspicious. Note that all those malicious addons are made by developers like us so they may try to persuade them the same way you do! So reviewer needs to be able distinguish who is good and who is bad!

But I agree with you regarding sandbox="allow-scripts allow-same-origin" , I don’t see a problem as long as the iframe cannot communicate with your addon somehow.

My project ID is [email protected]

3rd party remote scripts are NOT possible in extensions (unless an arbitrary “content_security_policy” manifest is set, which is not the case for my extension), as previously mentioned: "As extensions are protected by the content security policy, only explicit extension scripts may be run and no external or arbitrary scripts are possible. "

As NO 3rd party script is involved, there can’t be any more security issue for a sandboxed iframe that allows scripts than a normal iframe. I request that the silly restriction for addon reviewing “Sandboxed iframes with attributes ‘allow-scripts’ and ‘allow-same-origin’ are not allowed for security reasons.” should be revoked, and addons that are rejected only due to such reason should be unrejected.

You mean script running in the page inside the iframe can modify its own attributes in the parent HTML document? For sure not.

Anyway, it sounds very strange. The only thing that can be an issue is if the 3rd party remote script could actually control your extensions somehow.

Could you share more info about your project here?

This topic is here for more than 2 weeks. The reviewer hasn’t provide any more explanation. I submitted another temp workaround and the review is also pending without any reviewer feedback for over 2 weeks. Can anyone help with this question?

A reviewer has rejected my addon with the reason: Sandboxed iframes with attributes ‘allow-scripts’ and ‘allow-same-origin’ are not allowed for security reasons.

He also said that I may simply remove the sandbox attribute of the iframe, i.e. make it a normal iframe.

I would argue that this restriction is pointless, and even worse, it makes extensions less secure.

As extensions are protected by the content security policy, only explicit extension scripts may be run and no external or arbitrary scripts are possible. This applies to normal extension pages and normal frames, and sandboxed iframes (even though scripts are allowed) are no different from them.

I am fully aware that a sandboxed iframe with attributes ‘allow-scripts’ and ‘allow-same-origin’ generally means that any script can be run in it, and the script can possibly modify the sandbox attribute, making the whole sandbox breakable. However, as my extension scripts won’t do something as silly as breaking the sandbox, a sandboxed iframe is still more restrictive than a normal iframe, which is not considered unacceptable. As a result, this restriction won’t bring any help on security.

My extension runs scripts in the iframe, and it won’t work without “allow-scripts” and “allow-same-origin”. The point of using a sandboxed iframe instead of a normal iframe is for restricting form submission and top frame navigation (via link etc.). The request of replacing it with a normal iframe will break this protection and introduce security hazards.

As the policy never restrict normal frames, please re-evaluate the rationale and necessity of forbidding sandboxed iframes with attributes ‘allow-scripts’ and ‘allow-same-origin’. Thank you.

HTML References

Html <iframe> sandbox attribute.

❮ HTML <iframe> tag

An <iframe> with extra restrictions:

More "Try it Yourself" examples below.

Definition and Usage

The sandbox attribute enables an extra set of restrictions for the content in the iframe.

When the sandbox attribute is present, and it will:

  • treat the content as being from a unique origin
  • block form submission
  • block script execution
  • disable APIs
  • prevent links from targeting other browsing contexts
  • prevent content from using plugins (through <embed> , <object> , <applet> , or other)
  • prevent the content to navigate its top-level browsing context
  • block automatically triggered features (such as automatically playing a video or automatically focusing a form control)

The value of the sandbox attribute can either be empty (then all restrictions are applied), or a space-separated list of pre-defined values that will REMOVE the particular restrictions.

Browser Support

The numbers in the table specify the first browser version that fully supports the attribute.


Attribute Values

More examples.

An <iframe> sandbox allowing form submission:

An <iframe> sandbox allowing scripts:

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Facebook One Box Error

Facebook One Box Error on Latest 2.7.0.beta1 version.

ErrorUtils caught an error: Failed to set the ‘domain’ property on ‘Document’: Assignment is forbidden for sandboxed iframes. [Caught in: Module “lowerDomain”] Subsequent non-fatal errors won’t be logged; see https://fburl.com/debugjs .

Same error show when I do F12 on following post.

Can you provide an example?


This error is coming from Facebook API because we are loading the video in a sandbox iframe for security reasons.

As I can see here the video works fine even with the error from Facebook so it’s not blocking the feature? The alternate here would be to not onebox the video at all which seems even worse.

This topic was automatically closed after 39 hours. New replies are no longer allowed.

Related Topics

Can the FB API be used in Codepen?

Evening all, My random quote generator is still a work in progress, but I wanted to see if this was possible at all. I added a FB post button here , which will first prompt you to log in to facebook, and then post the quote to your wall. However, when running this via Codepen, I receive an error such as this:

After doing some googling, it doesn’t look like it’d be possible and I have been able to post on my own local host so it shouldn’t be the code itself. Is there a way from Codepen (or other sandboxed iframes) that this would be possible?

From what I understand from a quick couple of minutes (very quick so I might be explaining it wrong) of reading is codepen doesn’t allow iframes and this Facebook API creates an iframe for the share window. This will work in a site not hosted/created in codepen. So what you can do is fake this action by creating a new pop up window to mimic the iframe with the facebook share page. I created a small codpen to show you how.


Features/Platform/Iframe Sandbox

Open issues/risks, stage 1: definition, 1. feature overview.

The HTML5 spec specifies a new attribute for the IFRAME element, "sandbox", see http://dev.w3.org/html5/spec/the-iframe-element.html#attr-iframe-sandbox . See also bug 341604 "Implement HTML5 sandbox attribute for IFRAMEs" and bug 671389 "Implement CSP sandbox directive"

2. Users & use cases

Users are web developers looking for a way to isolate content on their site and prevent it from having its default same origin privileges, ability to execute scripts and other normally granted abilities. The HTML5 spec specifies some modifying attributes that can grant permissions such as same origin , executing scripts, navigating the top window and submitting forms, etc. There are other abilities which are always removed in a sandboxed document that cannot be granted.

3. Dependencies

4. requirements.

This feature should be designed and implemented in a way that makes it usable for also implementing the sandboxing required to support the CSP (Content Security Policy) sandbox value also.

This feature requires a comprehensive test suite, as automated as possible for inclusion in the Firefox unit tests - Boris Zbarsky has suggested we also submit this test suite to the W3C for inclusion in their HTML5 test suite.

  • Providing sandboxing above and beyond what's described in the HTML5 spec
  • implementing the IFRAME seamless attribute and interactions between it the sandbox attribute.
  • implementing @sandbox on <object> - this was discussed on the whatwg list, currently we don't want to do this, since the meaning of sandbox on <object> is confusing - it would apply in some contexts and not others
  • implementing @sandbox on <frame> - this was discussed on the whatwg list and @sandbox for <frame> will not be implemented at the current time (this matches at least one other implementation)
  • implementing @sandbox on <xul:iframe/browser/editor> - the current implementation shouldn't prevent this enhancement from being implemented but there are no plans to implement this at the current time

Stage 2: Design

5. functional specification.

An IFRAME with the sandbox attribute (and its various modifying attributes) should behave as outlined in the HTML5 spec. See W3C Working Draft at http://www.w3.org/TR/html5/the-iframe-element.html#the-iframe-element and W3C Editor's Draft at http://dev.w3.org/html5/spec/Overview.html#the-iframe-element . This feature attempts to be compatible with the CSP sandbox directive (see https://wiki.mozilla.org/Security/CSP/Sandbox ) by establishing infrastructure where CSP only needs to set the sandbox flags on content documents to have them sandboxed as intended.

6. User experience design

Stage 3: planning, 7. implementation plan.

  • The general approach for removing same-origin privileges is to give a sandboxed IFRAME a null principal to remove its normal domain/principal. This removes its same origin privileges unless allow-same-origin is specified as part of the sandbox attribute. In some case, there are additional checks that need to be added to the code to see if a document is sandboxed in a way that would prevent an operation from occurring.
  • We will use nsDocShell::SetAllowPlugins(false) to prevent plugins being loaded by a sandboxed IFRAME
  • We will create the flags as described in the HTML5 spec's description of the IFRAME sandbox attribute on both the docshell and the document when it is loaded

content/base/src/nsSandboxFlags.h contains :

  • we will implement this using nsHTMLIFrameElement::AfterSetAttr,
  • the document needs to keep the exact set of flags assigned at load time (these are immutable - changing sandbox attributes does NOT dynamically affect already loaded content)
  • sandbox flags for a document are set based on the sandbox flags of its parent document and the sandbox flags of the embedding frame (stored in the docshell)
  • sandboxed IFRAME's need to block access (read and write) to document.cookie

and local and session storage unless the allow-same-origin keyword is specified

  • sandboxed IFRAME's are also supposed to block access to content automatically doing something, the examples given automatically playing a video or automatically focusing a form control (unless allow-scripts is specified since scripts can do this anyways) - while disabling JS will take care of many of these cases, HTML5 video autoplay may need to be handled separately and there could be many other edge cases here as well ! - i would prefer to address this in a follow-up patch/bug as playing video or auto focusing a text field doesn't IMO diminish the security benefits provided by landing the rest of the functionality as defined in the HTML5 spec
  • this also requires modifying surrounding code to be able to force an owner to be set on the loading channel.
  • we may be able to implement both cases in one place if we do the null principal assignment at a low enough level in the docshell loading code
  • The nsFrameLoader will check if its owner content has the sandbox attribute set and is an nsHTMLIFrameElement to determine whether to sandbox the frame being loaded (again, if the allow-same-domain modifier isn't present in the sandbox attribute)
  • when we support the CSP sandbox directive, similar logic to that in nsFrameLoader will need to be implemented for the loaders for other CSP-protectable resources
  • for CSP sandbox, the flags will only be stored on the document itself - when content is navigated to, the CSP sandbox flags won't be persisted (unless the new content also specifies a CSP sandbox directive)
  • the HTML5 spec provides examples of how to apply flags with nested IFRAMEs, abarth has proposed that if both CSP and IFRAME sandbox can apply to content, the algorithm used in these example should be used to merge the policies which sounds reasonable
  • Disabling Javascript uses the same script choke points as CSP to maximize the functionality available in a sandboxed document while still not allowing the document to load a remote script or execute inline script.
  • After discussion, we allow workers to be loaded inside a sandboxed document with 'allow-scripts' (but without 'allow-same-origin') from a data: URL or a blob URL created by the same sandboxed document creating the worker. This requires modifying either worker code or CheckMayLoad() code most likely.
  • After discussion, although the sandbox attribute is specified as DOMSettableTokenList in the HTML5 spec, this implementation implements it as a DOMString.

Security review

This feature will definitely need a full security review from the Security Assurance team. I've posted this feature page on dev.security and updated the bug with decisions and implementation plans as implementation has proceeded. Before the security review I will post again to dev.security and encourage review of this feature page and the HTML5 iframe sandbox spec.

Privacy review

Localization review, accessibility, quality assurance review.

We will need a test suite for this feature. Microsoft has released test cases for sandboxing publically that have been submitted to the W3C for inclusion in the HTML5 test suite. We will definitely want to compare our implementation to other browsers' implementation for consistency etc. and address inconsistencies via suggested modifications to the HTML5 spec and discussion on the whatwg list. Boris Zbarsky has suggested submitting our sandbox test suite to the W3C also.

A mochitest test suite has been written to provide consistent automated testing for this feature.

Operations review

Stage 4: development, 9. implementation.

See https://bugzilla.mozilla.org/show_bug.cgi?id=341604 for a patch that attempts to implement HTML5 iframe sandbox.

There's an set of tests for various pieces of functionality in that bug as well.

Please see the bug for updates to current patches and iteration based on feedback/review.

Stage 5: Release

10. landing criteria.

  • Needs a test suite - DONE
  • Needs to be compared against other implementations and public test suites for consistency - DONE
  • Needs a full security review - DONE
  • After discussion, although the sandbox attribute is specified as DOMSettableTokenList in the HTML5 spec, this implementation implements it as a DOMString." contains strip markers and therefore it cannot be parsed sufficiently.

Feature details

Team status notes.

  • Feature Page

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Uncaught DOMException: Failed to set the 'domain' property on 'Document': Assignment is forbidden for Sandboxed iFrames #16262


nileshpandey3 commented Apr 29, 2021

Nileshpandey3 commented apr 29, 2021 • edited.

Sorry, something went wrong.


jennifer-shehane commented Apr 29, 2021


nileshpandey3 commented Apr 29, 2021 • edited by jennifer-shehane

Nileshpandey3 commented apr 30, 2021.

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HTML | <iframe> sandbox Attribute

The sandbox attribute permits an additional set of restrictions for the content within the iframe. When the sandbox attribute exists, and it will:   

  • treat the content as being from a singular origin
  • It blocks form submission
  • It blocks script execution
  • It disables APIs
  • It also preventing links from targeting other browsing contexts
  • It stop the content to navigate its top-level browsing context
  • block automatically triggered features (such as automatically playing a video or automatically focusing a form control)

The value of the sandbox attribute will either be simply sandboxed (then all restrictions are applied) or a space-separated list of pre-defined values which will take away the actual restrictions.  


Attribute Values  

  • no-values: applies all restriction
  • allow-forms: Re-enables form submission
  • allow-pointer-lock: Re-enables APIs
  • allow-popups: Re-enables popups
  • allow-same-origin: It allows the content of iframe to be treated as being from same origin
  • allow-scripts: Re-enables scripts
  • allow-top-navigation: It Allows the content of iframe to navigate its top-level browsing context



assignment is forbidden for sandboxed iframes

Supported Browsers: The browsers supported by HTML IFrame sandbox Attribute are listed below 

  • Google Chrome 4.0 and above
  • Edge 12.0 and above
  • Firefox 17.0 and above
  • Internet Explorer 10.0 and above
  • Opera 15.0 and above
  • Safari 5.0 and above

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Document: domain property

Deprecated: This feature is no longer recommended. Though some browsers might still support it, it may have already been removed from the relevant web standards, may be in the process of being dropped, or may only be kept for compatibility purposes. Avoid using it, and update existing code if possible; see the compatibility table at the bottom of this page to guide your decision. Be aware that this feature may cease to work at any time.

The domain property of the Document interface gets/sets the domain portion of the origin of the current document, as used by the same-origin policy .

Use of this feature was blocked by a Permissions Policy .

Getting the domain

For code running at the URL https://developer.mozilla.org/en-US/docs/Web , this example would set currentDomain to the string " developer.mozilla.org ".

The getter for this property returns the domain portion of the current document's origin. In most cases, this will be the hostname portion of the document's URL. However, there are some exceptions:

  • If the page has an opaque origin , e.g. for a page with a data URL , then it will return the empty string.
  • If the document.domain setter has been used, then it will return the value that was set.

Although the getter is not dangerous in the same way that the setter is, it is likely simpler and more useful to use the Location.hostname property instead. Then you can avoid document.domain entirely:

For the URL https://developer.mozilla.org/en-US/docs/Web , currentHostname is also the string " developer.mozilla.org ". Other alternatives that provide slightly different information are Location.host , which includes the port, and origin , which provides the full origin.

Setting the domain

The setter for this property can be used to change a page's origin , and thus modify how certain security checks are performed. It can only be set to the same or a parent domain. For example, if https://a.example.com and https://b.example.com both use

then they have both modified their origin to have the same domain, and they can now access each other's DOM directly—despite being cross-origin, which would normally prevent such access.

Note that setting document.domain to its current value is not a no-op. It still changes the origin. For example, if one page sets

then it will be counted as cross-origin from any other normally-same-origin pages that have not done the same thing.


The document.domain setter is deprecated. It undermines the security protections provided by the same origin policy , and complicates the origin model in browsers, leading to interoperability problems and security bugs.

Attempting to set document.domain is dangerous. It opens up full access to a page's DOM from all subdomains, which is likely not what is intended. It also removes the port component from the origin, so now your page can be accessed by other pages with the same IP address or same host component, even on a different port.

This is especially insecure on shared hosting. For example, another shared hosting customer is able to host a site at the same IP address but on a different port, then setting document.domain will remove the same-origin protection that normally protects you from that other customer's site accessing your site's data.

Similar problems occur with shared hosting sites that give each customer a different subdomain. If a site sets document.domain , any other customer on a different subdomain can now do the same thing, and start accessing the data of the original site.

Instead of using document.domain to facilitate cross-origin communication, you should use Window.postMessage to send an asynchronous message to the other origin. This controlled access via message-passing is much more secure than the blanket exposure of all data caused by document.domain .

The setter will throw a " SecurityError " DOMException in several cases:

  • The document-domain Permissions-Policy is disabled.
  • The document is inside a sandboxed <iframe> .
  • The document has no browsing context .
  • The document's effective domain is null .
  • The given value is neither the same as the page's current hostname, nor a parent domain of it.

As an example of this last failure case, trying to set document.domain to "example.org" when on https://example.com/ will throw.

Additionally, as part of its deprecation, it will do nothing when combined with certain modern isolation features:

  • If used on a cross-origin isolated page, i.e. one that uses the appropriate values for the Cross-Origin-Opener-Policy and Cross-Origin-Embedder-Policy HTTP headers
  • If used on an origin-isolated page, i.e. one that uses the Origin-Isolation HTTP header

Finally, setting document.domain does not change the origin used for origin-checks by some Web APIs, preventing sub-domain access via this mechanism. Affected APIs include (but are not limited to): Window.localStorage , IndexedDB_API , BroadcastChannel , SharedWorker .


Browser compatibility.

BCD tables only load in the browser with JavaScript enabled. Enable JavaScript to view data.

  • Same-origin policy
  • Location.hostname
  • Location.host


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    1. Note that since you are willing to set this document.domain to allow access between the two contexts, the sandbox is completely useless: The iframe could remove its own sandbox attributes. So as has been said in the previous comment, don't set the sandbox at all if it's really the way you want to go. But in your position, I would reconsider ...

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  12. Can the FB API be used in Codepen?

    Uncaught DOMException: Failed to set the 'domain' property on 'Document': Assignment is forbidden for sandboxed iframes. After doing some googling, it doesn't look like it'd be possible and I have been able to post on my own local host so it shouldn't be the code itself.

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  14. Uncaught DOMException: Failed to set the 'domain' property on ...

    Uncaught DOMException: Failed to set the 'domain' property on 'Document': Assignment is forbidden for Sandboxed iFrames #16262. nileshpandey3 opened this issue Apr 29, 2021 · 9 comments Labels. stage: needs information Not enough info to reproduce the issue. Comments. Copy link

  15. iFrame Sandbox Permissions Tutorial

    How sandboxed iframes without the allow-top-navigation or the allow-top-navigation-by-user-activation properties can alleviate same-origin iframes that can redirect the top-level page, as well as ...

  16. 175281

    The SandboxingFlag enumeration is missing an option for "Document.Domain" access. We should add this so that websites can block frames from manipulating the frame's domain.

  17. HTML

    HTML | <iframe> sandbox Attribute. Read. Courses. The sandbox attribute permits an additional set of restrictions for the content within the iframe. When the sandbox attribute exists, and it will: treat the content as being from a singular origin. It blocks form submission. It blocks script execution. It disables APIs.

  18. Document: domain property

    const currentDomain = document.domain; The getter for this property returns the domain portion of the current document's origin. In most cases, this will be the hostname portion of the document's URL. However, there are some exceptions: If the page has an opaque origin, e.g. for a page with a data URL, then it will return the empty string.

  19. javascript

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