svelte vulnerable to Cross-site Scripting

Summary

An XSS vulnerability exists in Svelte 5.46.0-2 resulting from improper escaping of hydratable keys. If these keys incorporate untrusted user input, arbitrary JavaScript can be injected into server-rendered HTML.

Details

When using the hydratable function, the first argument is used as a key to uniquely identify the data, such that the value is not regenerated in the browser.

This key is embedded into a <script> block in the server-rendered <head> without escaping unsafe characters. A malicious key can break out of the script context and inject arbitrary JavaScript into the HTML response.

Impact

This is a cross-site scripting vulnerability affecting applications that have the experimental.async flag enabled and use hydratable with keys incorporating untrusted user input.

• Impact: Arbitrary JS execution in the client’s browser.

• Exploitability: Remote, single-request if key is attacker-controlled.

• Typical Outcomes:

  • Session/token theft

  • DOM defacement

  • CSRF bypass via injected JS

  • Account takeover depending on cookie/session strategy

Affected applications should upgrade to a patched version immediately.