@sveltejs/kit vulnerable to Cross-site Scripting via tracked search_params

### Summary Unsanitized search param names cause XSS vulnerability. You are affected if you iterate over all entries of `event.url.searchParams` inside a server `load` function. Attackers can exploit it by crafting a malicious URL and getting a user to click a link with said URL. ### Details SvelteKit tracks which parameters in `event.url.searchParams` are read inside server `load` functions. If the application iterates over the these parameters, the `uses.search_params` array included in the boot script (embedded in the server-rendered HTML) will have any search param name included in unsanitized form. `packages/kit/src/runtime/server/utils.js:150` has the `stringify_uses(node)` function which prints these out. ### Reproduction In a `+page.server.js` or `+layout.server.js`: ```js /** @type {import('@sveltejs/kit').Load} */ export function load(event) { const values = {}; for (const key of event.url.searchParams.keys()) { values[key] = event.url.searchParams.get(key); } } ``` If a user visits the page in question via a link containing `?</script/><script>window.pwned%3D1</script/>`, the `</script>` will be included verbatim in the payload, causing the embedded script to be executed. It is not necessary to return the parameter value from `load` or render it in the page, only to read it (which causes it to be tracked as a dependency) while `load` is running. ### Impact Any application that iterates over all values in `event.url.searchParams` in a `load` function in `+page.server.js` or `+layout.server.js` (directly or indirectly) is vulnerable to XSS.