element-web-Github/docs/usercontent.md

Usercontent

While decryption itself is safe to be done without a sandbox, letting the browser and user interact with the resulting data may be dangerous, previously usercontent.riot.im was used to act as a sandbox on a different origin to close the attack surface, it is now possible to do by using a combination of a sandboxed iframe and some code written into the app which consumes this SDK.

Usercontent is an iframe sandbox target for allowing a user to safely download a decrypted attachment from a sandboxed origin where it cannot be used to XSS your Element session out from under you.

Its function is to create an Object URL for the user/browser to use but bound to an origin different to that of the Element instance to protect against XSS.

It exposes a function over a postMessage API, when sent an object with the matching fields to render a download link with the Object URL:

{
    imgSrc: "", // the src of the image to display in the download link
    imgStyle: "", // the style to apply to the image
    style: "", // the style to apply to the download link
    download: "", // download attribute to pass to the <a/> tag
    textContent: "", // the text to put inside the download link
    blob: "", // the data blob to wrap in an object url and allow the user to download
}

If only imgSrc, imgStyle and style are passed then just update the existing link without overwriting other things about it.

It is expected that this target be available at usercontent/ relative to the root of the app, this can be seen in element-web's webpack config.