Launch Ready API security Checklist for AI-built SaaS app: Ready for first 100 users in marketplace products?.

Launch Ready API Security Checklist for AI-built SaaS app: Ready for first 100 users in marketplace products?

For an AI-built SaaS app, "ready" does not mean "it works on my machine" or "users can sign up." It means a stranger can hit your marketplace listing, create an account, connect data, make a payment or start a trial, and not break your app, leak data, or create a support fire.

For the first 100 users, I want to see five things before I call it launch ready:

• No exposed secrets in code, logs, or client-side bundles.
• No critical auth bypasses or broken access control.
• p95 API response time under 500ms for core actions like login, create workspace, and fetch dashboard.
• Email deliverability working with SPF, DKIM, and DMARC passing.
• Monitoring in place so you know within 5 minutes if the app goes down or starts failing.

If any of those are missing, you do not have a launch problem. You have a production risk problem that will show up as failed onboarding, refund requests, marketplace rejection, and support load.

For marketplace products aiming at the first 100 users, I focus on the boring stuff that keeps the app alive long enough to convert.

Quick Scorecard

| Check | Pass criteria | Why it matters | What breaks if it fails | |---|---|---|---| | Authentication | No shared admin accounts; login protected by secure session or token flow | Prevents account takeover | Users can access other workspaces | | Authorization | Users only see their own org/data; object-level checks exist | Stops cross-tenant leaks | Marketplace trust is lost fast | | Secrets handling | Zero secrets in frontend; env vars only server-side | Protects APIs and databases | Keys get scraped and abused | | Input validation | All API inputs validated server-side | Blocks malformed payloads and injection | Broken records and exploit paths | | Rate limiting | Login, OTP, webhook, and public endpoints limited | Reduces abuse and brute force | Spam signups and account attacks | | CORS policy | Only approved origins allowed; no wildcard with credentials | Prevents browser-based data exposure | Frontend can leak authenticated data | | Logging hygiene | No tokens, passwords, PII in logs | Limits damage during incidents | Sensitive data ends up in log tools | | Email deliverability | SPF/DKIM/DMARC pass; inbox placement tested | Needed for onboarding emails and alerts | Users miss verification and password reset mail | | Deployment safety | Production deploy has rollback path and env separation | Reduces release risk | One bad deploy takes down signup flow | | Monitoring/alerts | Uptime alerting and error tracking active within 5 min SLA | Lets you catch failures early | You find out from angry users |

The Checks I Would Run First

1. Auth bypass and broken access control

The signal I look for is simple: can one user fetch another user's workspace by changing an ID in the URL or API request? For marketplace products with multi-tenant data, this is the most expensive bug because it turns into a trust failure instead of a small defect.

I test this with browser devtools plus direct API calls using Postman or curl. Then I compare responses across two test accounts to see whether object-level authorization is enforced on every route.

Fix path:

• Add server-side authorization checks on every tenant-scoped endpoint.
• Never trust client-supplied org IDs without ownership validation.
• Add tests for horizontal privilege escalation before shipping.

2. Secret exposure in frontend bundles and logs

The signal is any API key, webhook secret, private token, or service credential visible in source maps, network requests, build artifacts, or logs. If a key is in the browser bundle even once, assume it is public.

I check the built app output plus environment variables used by the frontend framework. I also inspect server logs for tokens accidentally printed during auth failures or webhook handling.

Fix path:

• Move all privileged keys to server-only environment variables.
• Rotate anything already exposed.
• Strip sensitive fields from logs at the edge and application layer.

3. CORS and cookie/session security

The signal here is overly permissive cross-origin settings like `Access-Control-Allow-Origin: *` combined with credentials or weak cookie flags. This creates unnecessary exposure when your app is embedded in marketplace flows or uses third-party integrations.

I review CORS headers from production endpoints and confirm cookie flags are set correctly. For session-based auth, cookies should be `HttpOnly`, `Secure`, and `SameSite` appropriate for your flow.

A safe baseline looks like this:

Access-Control-Allow-Origin: https://app.yourdomain.com
Access-Control-Allow-Credentials: true
Set-Cookie: session=...; HttpOnly; Secure; SameSite=Lax

Fix path:

• Allowlist only known origins.
• Avoid wildcard CORS on authenticated routes.
• Use secure cookie settings or signed tokens with short expiry.

4. Rate limiting on high-risk endpoints

The signal is repeated login attempts, OTP abuse, webhook floods, or AI endpoint spam without throttling. First 100 users sounds small until one bot hits your signup form 5,000 times overnight.

I test login forms, password reset flows, invite endpoints, file uploads, and any public API route that can be abused cheaply. If there is no rate limit response after repeated calls from one IP or identity bucket, it is not launch ready.

Fix path:

• Add per-IP and per-account limits.
• Put stricter rules on auth endpoints than read-only routes.
• Return clear but non-revealing errors.

5. Webhook verification and replay protection

The signal is whether inbound webhooks are verified with a signature check before processing. Marketplace products often depend on Stripe-like billing events or external platform callbacks that attackers can spoof if verification is weak.

I replay known webhook payloads with altered signatures to confirm rejection. I also check whether duplicate events are idempotent so retries do not create duplicate subscriptions or double writes.

Fix path:

• Verify signatures before any business logic runs.
• Store event IDs to block replays.
• Make handlers idempotent by design.

6. Production readiness of email delivery

The signal is whether onboarding emails actually arrive in inboxes instead of spam folders or bouncing entirely. For first-time users from a marketplace listing, missed verification mail equals lost conversion.

I validate DNS records for SPF/DKIM/DMARC and then send test messages to Gmail and Outlook accounts. I also confirm password reset emails are sent from a real domain with consistent branding.

Fix path:

• Publish SPF/DKIM/DMARC correctly.
• Use a reputable sending provider.
• Separate transactional email from marketing email streams.

Red Flags That Need a Senior Engineer

These are the signs I would not DIY if I wanted to protect launch speed.

1. You have multiple AI-generated auth flows stitched together with no clear session model.

• This usually creates edge cases around refresh tokens, role checks, and logout behavior.

2. Your app stores customer data across several tools with unclear ownership rules.

• That leads to accidental cross-account access when one integration fails open.

3. You cannot tell me where secrets live right now.

• If you need to search Slack threads or old prompts to find keys, you already have operational risk.

4. The product depends on external APIs that fail unpredictably.

• Without retries, queues, timeouts around 10 to 30 seconds max depending on task type,

and fallback states, your onboarding will stall under normal load.

5. You have no deployment rollback plan.

• One bad release can take down signup pages,

break webhooks, or expose unfinished features to paying users.

DIY Fixes You Can Do Today

If you want to reduce risk before bringing me in, do these five things today:

1. Remove all hardcoded secrets from code files.

• Search for `.env`, API keys,

private tokens, Stripe secrets, Supabase keys, Firebase admin keys, and webhook secrets.

2. Turn on MFA for every admin account.

• Start with hosting,

DNS, GitHub, Cloudflare, email provider, database console, analytics, and payment tools.

3. Check your public pages for exposed debug data.

• Open devtools,

inspect network requests, confirm no internal IDs, tokens, or PII are visible where they should not be.

4. Test password reset and signup emails manually.

• Send them to Gmail plus Outlook so you catch deliverability issues early.

5. Add basic uptime monitoring now.

• Even free monitoring beats finding out about downtime from a user message at midnight.

Where Cyprian Takes Over

When these checks fail, my Launch Ready sprint maps directly to the fix list instead of guessing at symptoms.

| Failure area | What I fix in Launch Ready | Timeline | |---|---|---| | Domain not configured correctly | DNS setup plus redirects plus subdomains cleanup | Hours 1 to 8 | | Weak edge protection | Cloudflare setup with SSL caching DDoS protection basics enabled properly | Hours 1 to 12 | | Broken email flow | SPF/DKIM/DMARC configuration plus sender alignment checks | Hours 6 to 18 | | Unsafe deployment state | Production deployment review plus environment variable cleanup plus secret handling pass | Hours 8 to 24 | | Missing monitoring | Uptime monitoring plus alert routing plus basic incident visibility | Hours 18 to 30 | | Missing handover clarity | Final checklist covering access ownership rollback notes next steps |\nHours 30 to 48 |

My recommendation is straightforward: do not spend days polishing UI while auth logging secrets email delivery or deployment are still shaky. For marketplace products chasing the first 100 users the fastest win is making sure every signup payment reset email webhook call and dashboard request survives real traffic without leaking data or collapsing under avoidable mistakes.

References

• roadmap.sh - API Security Best Practices: https://roadmap.sh/api-security-best-practices
• roadmap.sh - Cyber Security: https://roadmap.sh/cyber-security
• roadmap.sh - Code Review Best Practices: https://roadmap.sh/code-review-best-practices
• OWASP API Security Top 10: https://owasp.org/www-project-api-security/
• Cloudflare SSL/TLS documentation: https://developers.cloudflare.com/ssl/

---

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.*