Qwik has a potential mXSS vulnerability due to improper HTML escaping

Summary

A potential mXSS vulnerability exists in Qwik for versions up to 1.6.0.

Details

Qwik improperly escapes HTML on server-side rendering. It converts strings according to the following rules:

https://github.com/QwikDev/qwik/blob/v1.5.5/packages/qwik/src/core/render/ssr/render-ssr.ts#L1182-L1208

• If the string is an attribute value:
  • " -> "
  • & -> &
  • Other characters -> No conversion
• Otherwise:
  • < -> <
  • > -> >
  • & -> &
  • Other characters -> No conversion

It sometimes causes the situation that the final DOM tree rendered on browsers is different from what Qwik expects on server-side rendering. This may be leveraged to perform XSS attacks, and a type of the XSS is known as mXSS (mutation XSS).

PoC

A vulnerable component:

import { component$ } from "@builder.io/qwik";
import { useLocation } from "@builder.io/qwik-city";

export default component$(() => {
  
  // user input
  const { url } = useLocation();
  const href = url.searchParams.get("href") ?? "https://example.com";

  return (
    <div>
      <noscript>
        <a href={href}>test</a>
      </noscript>
    </div>
  );
});

If a user accesses the following URL,

http://localhost:4173/?href=</noscript><script>alert(123)</script>

then, alert(123) will be executed.

Impact

XSS