Vercel preset: `/_vercel/cron` handler is fully public when `CRON_SECRET` is unset — consider a fail-closed default or build-time warning

Description

While reviewing the Vercel preset I noticed the auto-registered cron handler only authenticates when CRON_SECRET happens to be set:

// src/presets/vercel/runtime/cron-handler.ts (main @ e36e7a6, unchanged as of 2026-08-21)
const cronSecret = process.env.CRON_SECRET;
if (cronSecret) {
  // ... timingSafeEqual check, 401 on mismatch
}
// no else — with no CRON_SECRET the request proceeds unauthenticated
const cron = event.req.headers.get("x-vercel-cron-schedule");
if (!cron) throw new HTTPError("Missing x-vercel-cron-schedule header", { status: 400 });
await runCronTasks(cron, { ... });
return { success: true };

Concretely, when experimental.tasks is enabled with non-empty scheduledTasks and CRON_SECRET is unset:

• /_vercel/cron answers to any HTTP verb from anyone (the handler registers without a method, so rou3 matches all verbs);
• an attacker chooses the schedule via x-vercel-cron-schedule, and runCronTasks exact-matches it against the configured keys — so any configured scheduled task can be executed on demand, repeatedly;
• the { success: true } response confirms execution (400 if the header is missing; 401 only when CRON_SECRET is set);
• minor extra leak: runTask is single-flight via __runningTasks__ (src/runtime/internal/task.ts:23-25), so a request timed while the real scheduled run is in-flight resolves with that run's task result value, which is returned to the caller if tasks return data.

There are currently no functional tests for this handler at all (test/presets/vercel.test.ts only snapshots the config output), so neither the 401 path nor the public-by-default path is pinned by tests.

I understand this mirrors Vercel's documented securing model (the code comment cites vercel.com/docs "securing cron jobs", and the nitro docs recommend setting CRON_SECRET), so I'm filing this as a hardening suggestion rather than a vulnerability claim. Still, the current default is fail-open for a code-execution endpoint, and nothing at build time or in the runtime output warns the deployer; a user who skips one env var silently publishes their scheduled tasks.

Suggestions (any one would close the gap)

1. Fail closed: return 401/403 when CRON_SECRET is unset. (Would be a behavior change; a major-version candidate.)
2. Build-time warning/error: when the preset registers /_vercel/cron without CRON_SECRET present, emit a prominent build warning (or refuse if strict).
3. Docs: state explicitly in docs/2.deploy/20.providers/vercel.md that the endpoint is public by default without the secret.
4. Tests: there are currently no tests covering the handler's auth behavior; a case for the unset-secret path would pin whatever default you choose.

Happy to send a PR for options 2-4. Context: verified against main @ e36e7a6; no live deployments were tested.

Reported by zz-protocol

Start with src/presets/vercel/runtime/cron-handler.ts and inspect how the handler behaves when CRON_SECRET is unset. Review test/presets/vercel.test.ts and the task path in src/runtime/internal/task.ts:23-25. Confirm the chosen hardening behavior with maintainers, then add coverage for the authentication behavior and update docs/2.deploy/20.providers/vercel.md if documentation is included.