CVE-2025-21620

When you send a request with the Authorization header to one domain, and the response asks to redirect to a different domain, Deno'sfetch() redirect handling creates a follow-up redirect request that keeps the original Authorization header, leaking its content to that second domain.

Details

The right behavior would be to drop the Authorization header instead, in this scenario. The same is generally applied to Cookie and Proxy-Authorization headers, and is done for not only host changes, but also protocol/port changes. Generally referred to as "origin".

The documentation states:

Deno does not follow the same-origin policy, because the Deno user agent currently does not have the concept of origins, and it does not have a cookie jar. This means Deno does not need to protect against leaking authenticated data cross origin

Reproduction

const ac = new AbortController()

const server1 = Deno.serve({ port: 3001, signal: ac.signal }, (req) => {
  return new Response(null, {
    status: 302,
    headers: {
      'location': 'http://localhost:3002/redirected'
    },
  })
})

const server2 = Deno.serve({ port: 3002, signal: ac.signal }, (req) => {
  const body = JSON.stringify({
    url: req.url,
    hasAuth: req.headers.has('authorization'),
  })
  return new Response(body, {
    status: 200,
    headers: {'content-type': 'application/json'},
  })
})

async function main() {
  const response = await fetch("http://localhost:3001/", {
    headers: {authorization: 'Bearer foo'}
  })
  const body = await response.json()
  
  ac.abort()
  
  if (body.hasAuth) {
    console.error('ERROR: Authorization header should not be present after cross-origin redirect')
  } else {
    console.log('SUCCESS: Authorization header is not present after cross-origin redirect')
  }
}

setTimeout(main, 500)

CVE-2025-21620 has a CVSS score of 7.5 (High). The vector is network-reachable, no privileges required, and no user interaction. A CVSS score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether this affects your application depends on whether the vulnerable code is present and reachable in your environment. A fixed version is available (0.204.0, 2.1.2); upgrading removes the vulnerable code path.

Kodem intelligence

1. What is CVE-2025-21620? CVE-2025-21620 is a high-severity security vulnerability in deno_fetch (rust), affecting versions >= 0.0.1, < 0.204.0. It is fixed in 0.204.0, 2.1.2.
2. How severe is CVE-2025-21620? CVE-2025-21620 has a CVSS score of 7.5 (High). This score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether it represents real risk in your environment depends on whether the vulnerable code is present and reachable.
3. Which packages are affected by CVE-2025-21620?
   • deno_fetch (rust) (versions >= 0.0.1, < 0.204.0)
   • deno (rust) (versions <= 1.46.3)
4. Is there a fix for CVE-2025-21620? Yes. CVE-2025-21620 is fixed in 0.204.0, 2.1.2. Upgrade to this version or later.
5. Is CVE-2025-21620 exploitable, and should I be worried? Whether CVE-2025-21620 is exploitable in your environment depends on whether the vulnerable code is present and reachable. A CVSS score is a worst-case rating; it does not account for your specific deployment, configuration, or usage patterns. Kodem, an Intelligent Application Security platform, uses runtime intelligence to show which vulnerabilities actually execute in production, so you can focus on the ones that represent real risk. Get a demo
6. What actually determines whether CVE-2025-21620 is exploitable, and how bad it is? Exploitability and impact are not fixed properties of a CVE. They depend on runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A high CVSS score on a dependency that never runs is not the same as real risk. Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter.
7. How do I fix CVE-2025-21620?
   • Upgrade deno_fetch to 0.204.0 or later
   • Upgrade deno to 2.1.2 or later