CVE-2026-42599: Svelte SSR vulnerable to cross-site scripting via spread attributes

When using spread syntax to render attributes from untrusted data, event handler properties are included in the rendered HTML output. If an application spreads user-controlled or external data as element attributes, an attacker can inject malicious event handlers that execute in victims’ browsers. Note that this vulnerability only triggers if the user’s browser has JavaScript enabled but Svelte’s hydration mechanism does not reach the vulnerable element before the event fires.

This is similar to but different from CVE-2026-27121.