Stored Cross-site Scripting (XSS) in excalidraw's web embed component

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.