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
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