A redirect route rule like:

ts routeRules: { "/legacy/": { redirect: "/" } }

is intended to rewrite paths within the same host. Before the patch, an attacker could turn the rewrite into a cross-host redirect by sliding an extra slash in after the rule prefix. Example exploit:

GET /legacy//evil.com

Nitro stripped /legacy from the matched pathname and joined the remainder against the rule's target. The remainder was //evil.com, which the join preserved verbatim, so Nitro responded with Location: //evil.com. Browsers resolve //evil.com as a protocol-relative URL against the current scheme, sending the user to https://evil.com.

Are you affected?

Users may be affected if all of the following are true:

1. Their project uses Nitro's routeRules with a redirect entry. 2. The target uses a / wildcard suffix to forward sub-paths (e.g. redirect: "/", redirect: "/new/", proxy: { to: "http://upstream/" }). 3. The redirect rule is not handled natively at the CDN layer. The vercel, netlify, cloudflare-pages, and edgeone presets translate routeRules.redirect into platform config (vercel.json, redirects, EdgeOne v3 config) and serve the redirect at the edge — those deployments bypass the Nitro runtime entirely and are not affected. Every other preset executes the redirect through the Nitro runtime and can be vulnerable.

Impact

Open redirect from any host serving Nitro with a wildcard redirect rule. The redirect target is fully attacker-controlled, the URL looks legitimate (it starts with the victim's domain), and the browser silently follows it.

Patched versions

Upgrade to one of:

- 2.13.4 or later (or upgrade lockfile with latest ufo 1.6.4+) - 3.0.260429-beta or later (https://github.com/nitrojs/nitro/pull/4236)

The fix has two parts:

1. ufo is bumped to ^1.6.4 (unjs/ufo@5cd9e67), which collapses any run of leading slashes to a single / inside withoutBase. This covers the typical "/scope/" rule. 2. The Nitro runtime additionally collapses leading // before joining when the rule path itself is / (in rare case which case withoutBase is never called and the raw pathname flows straight into joinURL("", …)).