Vercel preview deployments are public — and carry your env mistakes

Preview URLs ship real env values to anyone with the link. Audit previews like production — free scan, no account.

2026-08-234 min read

Previews exist to be shared — that is their feature and their risk. Each PR builds with project env attached, producing a live deployment whose discovery cost is low and whose audit rate is usually zero.

What previews inherit, three realistic leaks, and making previews part of monitored surface.

What actually happened

A feature branch added NEXT_PUBLIC_ support for a third-party widget; the preview carried it for weeks post-merge.

Branch pushes create preview deployments inheriting project env.
Staging DB connection string referenced client-side for “testing”.
Test-mode third-party key treated as harmless, promoted accidentally.
Merge happened; preview URLs remained alive and indexed-by-crawlers-that-guess.

Every step above is individually reasonable and none of them prints a warning. The value crosses into the bundle during substitution, not execution, so nothing in your runtime ever sees the moment it happened.

What someone can do with it

Preview exposure equals production exposure minus attention:

Whatever env the branch wired client-side.
Side-channel trust: reviewers share URLs widely.

Rotate first

Do the rotation first. From the moment this value reached a public URL, treat it as public knowledge: browser caches, shared proxies and automated scrapers all hold copies you cannot recall. Rotate anything client-referenced from branches; sweep open previews with scans. Code changes come after, because a clean repository with a compromised key is still compromised.

A rotated key left in old deploys is still discoverable in CDN caches and archived copies. Rotation plus redeploy closes both halves; either alone leaves the door ajar.

Move the call somewhere the browser cannot read

The structural fix is always the same shape: the call moves to a context that holds the key without serving it, and the browser asks your server instead.

// branch commit:
const db = postgres(process.env.POSTGRES_URL_NON_PROD!); // client file

// moved behind /api/branch-data route before merge.

Check whether yours is exposed

You can check manually right now: open the site, view source or open DevTools, and search the built JavaScript for vercel.app. A hit means the string shipped; decode or prefix-check it before deciding how bad the news is.

The faster path is to let a machine do the fetching. KeyDrift downloads the same JavaScript a visitor gets — HTML, every referenced chunk including ones named only in the route manifest, and the server-streamed data frameworks inline into the document — and reports credentials with a masked prefix, a fingerprint, and the exact file they live in. Paste your deployed URL into the scanner; no account needed.

Keep it from coming back

It bears saying because it happens constantly: the fix holds until the next prompt that needs the query to return rows. Drift monitoring exists for precisely this — it diffs consecutive scans and pages you when a previously resolved finding reappears, naming the regression as a regression rather than repeating the first alert.

What protection settings do and do not cover

Deployment Protection (auth-required previews) meaningfully reduces drive-by access on supported plans — credit where due. Residual risks remain: shared links among logged-in teams, older projects without protection, custom domains on previews, and integrations fetching unprotected aliases. Protection narrows the door; scanning verifies the room.

Why this keeps happening industry-wide

It helps to name the economics honestly. Fixing this class of leak costs minutes when caught at deploy time and days when caught at invoice time, because by then the credential has been harvested, validated, resold or drained — often all four. Detection latency is the entire game, which is why the monitoring half of KeyDrift exists alongside the scanning half.

How KeyDrift reports this exact finding

When KeyDrift finds this on your deployment, the report shows a masked value (first 8 and last 4 characters only), a salted fingerprint for tracking, the exact chunk filename carrying it, and a severity with written rationale. Public-by-design neighbours — anon keys, publishable keys, Firebase web constants — appear as informational context rather than noise, because knowing what should be there is what makes the real findings credible.

Manual check, step by step

A five-minute version you can run anywhere: view-source on the landing page, copy every src= script URL, fetch each and search the results for vercel.app. It misses manifest-only chunks and streamed payloads — which is precisely the gap between "I checked" and "it is clean" — but it catches the loud majority and builds the pattern-recognition that makes scanner output legible.

Close the loop with monitoring

Monitoring closes the loop that one-time verification leaves open. A scheduled scan refetches everything, diffs against history, and fires only on transitions: created, regressed, resolved. Regression alerts matter most here — they fire when a previously fixed finding returns, which in agent-era codebases is less a possibility than a schedule.

Common questions

Auto-cancel previews helps?

Yes for lifetime; historical URLs persist until purged — scan before assuming.

Monorepos multiply this?

Linearly: each project/env combination is its own surface; portfolio monitoring maps naturally.

Run a free scan at keydrift.dev/scan — paste a URL or the bundle source itself, no account. Findings arrive masked, with the exact chunk they live in.

KeyDrift is an independent product and is not affiliated with, endorsed by, or sponsored by Vercel. The name is referenced descriptively.