SvelteKit : XSS via untrusted cache-control header in prerendered page

Impact

Applications that prerender pages and use setHeaders to set a cache-control header from an untrusted source are vulnerable to cross-site scripting (XSS).

SvelteKit inserts the header value into a <meta http-equiv="cache-control"> tag without escaping it. An attacker who can influence this value at prerender time can break out of the content attribute and inject HTML or JavaScript into the generated page. The injected code can then execute when users visit the page.

Patches

The fix escapes the header value before inserting it into the generated HTML. Upgrade to @sveltejs/kit version 3.0.0 or later.

Workarounds

Avoid forwarding untrusted cache-control header values with setHeaders on prerendered pages. Use a fixed, trusted cache policy instead.