@sveltejs/kit has unescaped error message included on error page

Summary

The static error.html template for errors contains placeholders that are replaced without escaping the content first.

Details

From https://kit.svelte.dev/docs/errors:

error.html is the page that is rendered when everything else fails. It can contain the following placeholders: %sveltekit.status% — the HTTP status %sveltekit.error.message% — the error message

This leads to possible injection if an app explicitly creates an error with a message that contains user controlled content that ends up being something like this inside a server handle function:

error(500, '<script>alert("boom")</script>');

Uncaught errors cannot be exploited like this, as they always render the message "Internal error".

Escaping the message string in the function that creates the html output can be done to improve safety for applications that are using custom errors on the server.

PoC

None provided

Impact

Only applications where user provided input is used in the Error message will be vulnerable, so the vast majority of applications will not be vulnerable