Launch Ready API security Checklist for paid acquisition funnel: Ready for first 100 users in marketplace products?.

What "ready" means for a paid acquisition funnel in a marketplace product

For this kind of product, "ready" does not mean the app works on your laptop. It means paid traffic can land, sign up, verify email, create an account, and complete the first valuable action without security gaps, broken redirects, or support chaos.

If I were self-assessing, I would want these outcomes before spending on ads:

• Zero exposed secrets in code, logs, or environment files.
• Auth flows protected against bypass, replay, and weak session handling.
• API p95 under 500ms for the signup and onboarding path.
• SPF, DKIM, and DMARC all passing for transactional email.
• Cloudflare, SSL, redirects, and subdomains working with no mixed-content errors.
• Uptime monitoring live before launch.
• No critical auth or access-control issues in the first 100-user path.

For marketplace products, the failure mode is expensive. A small auth bug can create fake sellers, duplicate listings, payment abuse, or support tickets that burn your first ad budget.

Quick Scorecard

| Check | Pass criteria | Why it matters | What breaks if it fails | |---|---|---|---| | Domain and SSL | Domain resolves correctly and HTTPS is forced on all pages | Trust and conversion | Browser warnings, lost signups | | Redirects | One canonical URL per page with no loops | Paid traffic landing quality | Ad clicks waste money | | Email auth | SPF, DKIM, DMARC pass | Inbox delivery for verification and receipts | Emails go to spam | | Secrets handling | No secrets in repo or client bundle | Prevents account takeover and abuse | API keys get stolen | | Auth controls | Login and signup cannot be bypassed | Core access security | Fake accounts and unauthorized access | | API validation | All inputs validated server-side | Stops bad data and injection paths | Broken records and exploit risk | | Rate limiting | Signup, login, reset password are rate limited | Blocks abuse and credential stuffing | Bot signups and lockouts | | CORS policy | Only approved origins allowed | Protects browser-based APIs | Data exposure to other sites | | Monitoring | Uptime alerts plus error tracking enabled | Fast detection of launch issues | Silent outages during ad spend | | Performance | Key pages load fast; LCP under 2.5s on mobile | Paid acquisition conversion depends on speed | Higher bounce rate and lower ROAS |

The Checks I Would Run First

1) Authentication cannot be bypassed

Signal: I test whether protected routes or APIs can be reached without a valid session. If a user can hit marketplace actions like listing creation, checkout initiation, or profile changes unauthenticated, that is a launch blocker.

Tool or method: I use browser dev tools plus direct API calls with curl or Postman. I also test expired tokens, tampered JWTs, missing cookies, and role changes.

Fix path: Enforce server-side authorization on every protected endpoint. Do not rely on frontend route guards. If roles exist for buyer versus seller versus admin, check them at the API layer every time.

2) Secrets are not exposed anywhere public

Signal: I look for API keys in frontend bundles, `.env` leaks, Git history, logs, build output, and error pages. For a paid funnel product, one leaked Stripe key or email key can become a support incident within hours.

Tool or method: I scan the repo for common secret patterns and inspect built assets. I also check CI logs and deployment variables.

Fix path: Move all secrets to server-side environment variables only. Rotate anything exposed immediately. If the secret has already shipped to users' browsers once, assume it is compromised.

3) Input validation exists on the server

Signal: I submit empty fields, long strings, script tags, invalid emails, duplicate records, malformed JSON, and unexpected file types. If the backend accepts garbage because the UI blocks it locally only once in production.

Tool or method: Use Postman collections or automated tests against every signup and onboarding endpoint. Test both happy path and hostile input.

Fix path: Add schema validation at the API boundary. Reject invalid payloads early with clear errors. Sanitize where needed but do not depend on sanitization alone for security.

4) Rate limits protect signup and login

Signal: I try repeated signups with the same IP address or email pattern. Then I hammer password reset and login endpoints to see whether they throttle abuse.

Tool or method: Use a simple script plus manual testing from one IP. Watch for captcha bypasses if you use them.

Fix path: Add per-IP and per-account rate limits on authentication endpoints. For marketplace products with paid acquisition traffic from multiple geographies you want sensible thresholds that stop bots without blocking real users.

5) Email delivery is production-safe

Signal: Verification emails arrive reliably in Gmail and Outlook. If SPF/DKIM/DMARC fail then your onboarding funnel loses users before they even log in again.

Tool or method: Send test emails to multiple inboxes and inspect authentication results in headers. Check DNS records directly.

Fix path: Publish correct SPF/DKIM/DMARC records before launch. Use a transactional email provider with bounce handling and suppression lists turned on.

6) CORS and browser access are locked down

Signal: I inspect whether APIs accept requests from any origin by default. For browser-based marketplace apps this can expose user data through overly permissive cross-origin settings.

Tool or method: Check response headers from authenticated endpoints using dev tools or curl with custom Origin headers.

Fix path: Allow only known production domains plus any required preview environments. Avoid wildcard origins when credentials are involved.

Red Flags That Need a Senior Engineer

1. You have an AI-built app but do not know where secrets live.

• This usually means deployment was stitched together fast enough that one leak could already exist in build logs or client code.

2. Your auth logic exists in multiple places.

• If frontend checks say one thing and backend checks say another then you are one bug away from unauthorized access.

3. You are sending paid traffic to an app with no monitoring.

• Without uptime alerts you can burn through ad spend for hours before noticing that checkout is broken.

4. Email verification sometimes works.

• "Sometimes" means inbox deliverability is already hurting activation rates and support load will rise after launch.

5. The product handles buyer-seller data but nobody has reviewed permissions.

• Marketplace apps often fail on object-level authorization where users can see or edit someone else's records.

DIY Fixes You Can Do Today

1. Turn on HTTPS everywhere.

• Force redirect HTTP to HTTPS at the edge.
• Make sure there are no mixed-content warnings on key pages.

2. Audit your environment variables.

• Remove anything sensitive from frontend code.
• Rotate keys if they have ever been committed to GitHub or pasted into chat tools.

3. Verify email DNS records now.

• Confirm SPF includes your provider only once.
• Check DKIM signing is active.
• Publish DMARC with at least `p=quarantine` once you confirm delivery is stable.

4. Test your signup flow like a stranger would.

• Use incognito mode.
• Try invalid emails.
• Try duplicate accounts.
• Try refreshing mid-flow to see whether state breaks.

5. Add basic uptime monitoring today.

• Monitor homepage availability plus one authenticated endpoint if possible.
• Set alerts to email plus Slack so outages do not sit unnoticed during ad spend hours.

A simple starting point for DMARC looks like this:

v=DMARC1; p=quarantine; rua=mailto:dmarc@yourdomain.com; pct=100

Where Cyprian Takes Over

When these checks fail across domain setup, deployment safety, secrets handling, monitoring, or email deliverability I take over with Launch Ready so you do not lose first-user momentum while debugging infrastructure yourself.

Here is how the failures map to the service deliverables:

• Domain resolution issues -> DNS setup plus redirects plus subdomain cleanup
• Mixed content or SSL errors -> Cloudflare plus SSL configuration
• Slow landing page responses -> caching setup plus edge optimization
• Exposed secrets -> environment variable hardening plus secret cleanup
• Broken email verification -> SPF/DKIM/DMARC setup
• Missing production controls -> deployment review plus handover checklist
• No visibility into outages -> uptime monitoring setup
• Risky launch process -> production deployment verification

My goal is to make the product safe enough for first traffic fast so you can start learning from real users instead of guessing in staging forever.

My handover includes:

• DNS verified
• Redirects tested
• Subdomains checked
• Cloudflare configured
• SSL active
• Caching enabled where safe
• DDoS protection enabled
• SPF/DKIM/DMARC passing
• Production deployment confirmed
• Environment variables reviewed
• Secrets removed from unsafe locations
• Uptime monitoring live
• Handover checklist completed

References

1. roadmap.sh code review best practices: https://roadmap.sh/code-review-best-practices 2. roadmap.sh API security best practices: https://roadmap.sh/api-security-best-practices 3. roadmap.sh cyber security roadmap: https://roadmap.sh/cyber-security 4. OWASP ASVS: https://owasp.org/www-project-application-security-verification-standard/ 5. Cloudflare SSL/TLS docs: https://developers.cloudflare.com/ssl/

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Take the next step

If this is a problem in your product right now, here is what to do next:

• [Use the free Cyprian tools](/tools) - estimate cost, score app risk, check launch readiness, or pick the right service sprint.

• [Book a discovery call](/contact) - I will tell you honestly whether you need a sprint or if you can DIY the next step.

*Written by Cyprian Tinashe Aarons - senior full-stack and AI engineer helping founders rescue, launch, automate, and scale AI-built products.*