Fix redirect target comes from the caller in Fastify

The destination of a redirect is taken from the request without being checked. An attacker can send a link that starts with your domain and ends on theirs, which is what makes a phishing page credible - and in an OAuth callback it hands the authorisation code to whoever asked. Resolve the target against your own origin and refuse anything else.

medium likely Fastify CWE-601 / OWASP A01:2021

The vulnerable pattern in Fastify

MEDIUM likely Redirect target comes from the caller A01:2021 src/server.ts:68:10 66 │ // open-redirect 67 │ const next = (request.query as { next?: string }).next as string 68 │ return reply.redirect(next) │ ~~~~~~~~~~~~~~~~~~~~ destination chosen by the caller 69 │ })

This finding comes from the Fastify fixture in the owlwarden test suite. The whole redirect target comes from the request, so a link to this endpoint can send a visitor anywhere. The URL they click genuinely belongs to you, which is what makes the page they land on convincing - and on an OAuth callback the authorisation code goes with them.

The corrected handler

Validate before reply.redirect().

const base = `${request.protocol}://${request.hostname}`
const next = (request.query as { next?: string }).next
return reply.redirect(safeRedirect(next, base))

If you are not using Fastify

Resolve the target against your own origin and refuse anything that lands elsewhere. Do not use a startsWith('/') check: '//evil.com' passes it and leaves the site.

Check your own repository

npx owlwarden scan
npx owlwarden explain open-redirect

Runs on your machine. No account, no telemetry, no network unless you ask. In CI, SARIF uploads to code scanning and the exit code is the gate.

Other Fastify checks

Rules with a tested Fastify example.

open-redirect for every framework / All rules / owlwarden