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How does session replay work Part4: Sandbox

In this series of posts, I would like to share how does session replay products(hotjar, logrocket, etc) work.

session replay

I've also maintained an open-source library, rrweb, which contains all the functions being described in these posts.


In the serialization design we mentioned the "de-scripting" process, that is, we will not execute any JavaScript in the recorded page during replay, but instead reproduce its effects on the snapshots. The script tag is rewritten as a noscript tag to solve some of the problems. However, there are still some scripted behaviors that are not included in the script tag, such as inline scripts in HTML, form submissions, and so on.

There are many kinds of scripting behaviors. A filtering approach to getting rid of these scripts will never be a complete solution, and once a script slips through and is executed, it may cause irreversible unintended consequences. So we use the iframe sandbox feature provided by HTML for browser-level restrictions.

iframe sandbox

We reconstruct the recorded DOM in an iframe element when we rebuild the snapshot. By setting its sandbox attribute, we can disable the following behavior:

  • Form submission
  • pop-up window such as
  • JS script (including inline event handlers and javascript: URLs)

This is in line with our expectations, especially when dealing with JS scripts is safer and more reliable than implementing this security ourselves.

Avoid link jumps

When you click the element link, the default event is to jump to the URL corresponding to its href attribute. During replay, we will ensure visually correct replay by rebuilding the page DOM after the jump, and the original jump should be prohibited.

Usually, we will capture all elements click events through the event handler proxy and disable the default event via event.preventDefault(). But when we put the replay page in the sandbox, all the event handlers will not be executed, and we will not be able to implement the event delegation.

When replaying interactive events, note that replaying the JS click event is not necessary because click events do not have any impact when JS is disabled. However, in order to optimize the replay effect, we can add special animation effects to visualize elements being clicked with the mouse, to clearly show the viewer that a click has occurred.

iframe style settings

Since we're rebuilding the DOM in an iframe, we can't affect the elements in the iframe through the CSS stylesheet of the parent page. But if JS scripts are not allowed to execute, the noscript tag will be displayed, and we want to hide it. So we need to dynamically add styles to the iframe. The sample code is as follows:

const injectStyleRules: string[] = [
  'iframe { background: #f1f3f5 }',
  'noscript { display: none !important; }',

const styleEl = document.createElement('style');
const { documentElement, head } = this.iframe.contentDocument!;
documentElement!.insertBefore(styleEl, head);
for (let idx = 0; idx < injectStyleRules.length; idx++) {
  (styleEl.sheet! as CSSStyleSheet).insertRule(injectStyleRules[idx], idx);
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Note that this inserted style element does not exist in the originally recorded page, so we can't serialize it, otherwise, the id -> Node mapping will be wrong.

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