Svelte has a potential mXSS vulnerability due to improper HTML escaping

Summary

A potential XSS vulnerability exists in Svelte for versions prior to 4.2.19.

Details

Svelte improperly escapes HTML on server-side rendering. It converts strings according to the following rules:

• If the string is an attribute value:
  • " -> "
  • & -> &
  • Other characters -> No conversion
• Otherwise:
  • < -> <
  • & -> &
  • Other characters -> No conversion

The assumption is that attributes will always stay as such, but in some situation the final DOM tree rendered on browsers is different from what Svelte expects on server-side rendering. This may be leveraged to perform XSS attacks. More specifically, this can occur when injecting malicious content into an attribute within a <noscript> tag.

PoC

A vulnerable page (+page.svelte):

<script>
import { page } from "$app/stores"

// user input
let href = $page.url.searchParams.get("href") ?? "https://example.com";
</script>

<noscript>
  <a href={href}>test</a>
</noscript>

If a user accesses the following URL,

http://localhost:4173/?href=</noscript><script>alert(123)</script>

then, alert(123) will be executed.

Impact

XSS, when using an attribute within a noscript tag