svelte is vulnerable to XSS with textarea bind:value

Summary

A server-side rendered <textarea> with two-way bound value does not have its value correctly escaped in the rendered HTML.

Details

In SSR, <textarea bind:value={...}> does not have its value escaped when it is rendered into the HTML as <textarea>...</textarea>.

PoC

Put this in a server-side-rendered Svelte component:

<script>
  let value = `test'"></textarea><script` + `>alert('BIM');</sc` + `ript>`;
</script>

<textarea bind:value />

Impact

• Only affects SSR
• Needs a <textarea bind:value> filled by user content via two-way binding