excalidraw is an open source virtual hand-drawn style whiteboard. A stored XSS vulnerability in Excalidraw's web embeddable component. This allows arbitrary JavaScript to be run in the context of the domain where the editor is hosted. There were two vectors. One rendering untrusted string as iframe's `srcdoc` without properly sanitizing against HTML injection. Second by improperly sanitizing against attribute HTML injection. This in conjunction with allowing `allow-same-origin` sandbox flag (necessary for several embeds) resulted in the XSS. This vulnerability is fixed in 0.17.6 and 0.16.4.

Affected: npm: @excalidraw/excalidraw

### Summary

A stored XSS vulnerability in Excalidraw's web embeddable component. This allows arbitrary JavaScript to be run in the context of the domain where the editor is hosted.

### Poc

Inserting an embed with the below url (can be copy/pasted onto canvas to insert as embed) will log `42` to the console:

``` https://gist.github.com/vv=v<script>console.log(42)</script> ```

### Details

There were two vectors. One rendering untrusted string as iframe's `srcdoc` without properly sanitizing against HTML injection. Second by improperly sanitizing against attribute HTML injection. This in conjunction with allowing `allow-same-origin` sandbox flag (necessary for several embeds) resulted in the XSS.

Former was fixed by no longer rendering unsafe `srcdoc` content verbatim, and instead strictly parsing the supplied content and constructing the `srcdoc` manually. The latter by sanitizing properly.

The `allow-same-origin` flag is now also set only in cases that require it, following the principle of least privilege.

### Impact

This is a cross site scripting vulnerability, for more information, please see: https://portswigger.net/web-security/cross-site-scripting

Two npm `@excalidraw/excalidraw` stable version releases were affected (`0.16.x`, `0.17.x`), and both are now patched.