How to do a security audit of a Fly.io app?

Why a Fly.io-specific audit (not a generic web audit)

A generic OWASP audit will tell you your Fly.io app "needs CSP headers." A Fly.io-aware audit tells you that your specific Fly.io app has an RPC function callable without auth or a service key in a client bundle — the issues that actually appear when Fly.io apps get compromised. The difference in output value is why the audit should be scoped to Fly.io's real failure modes.

Step 1 — Fingerprint the deployment

Confirm the Fly.io stack components: database (postgres, supabase), hosting, auth provider, third-party integrations. For Fly.io apps this is often visible in the Supabase endpoint URL in network requests. Document every component — each is an independent audit target.

Step 2 — Automated scan with Fly.io-aware rules

Run VAS against the deployed URL. The scan probes the specific issue classes found in Fly.io apps: secrets config, network security, app security, headers. This is the 80/20 — most critical and high findings surface here. Fix anything critical before continuing to manual steps.

Step 3 — Manual Row Level Security (RLS) policies review

Open the Supabase dashboard → Authentication → Policies. For each table: is RLS enabled? Do policies check `(select auth.uid()) = user_id` or equivalent? Are there policies scoped to the anon role that shouldn't exist? The automated scan catches missing RLS; this step catches overly permissive RLS — a subtler but equally dangerous failure mode.

Step 4 — Authentication & authorization probing

Test every endpoint with no session (expect 401), with a valid session for a different user (expect 403 on user-owned resources), and with session tokens that have been tampered with (expect 401 if signatures are enforced). Rate limiting on login/password-reset is a pass/fail check here, not a nuance.

Step 5 — Re-scan to verify

Fix findings in severity order (critical → high → medium → low), re-scan after each batch of fixes. "I applied the fix" is not evidence — the fix might not have been deployed, might have been partial, or might have been reverted. Only the scan output proves the gap is closed. Log each finding + fix + verification scan for compliance records.

Fly.io-specific checks often missed

• Secrets synchronization across regions (fix: Move all secrets server-side (environment variables, serverless functions))
• Edge deployment security (fix: Scan your deployed application with a security tool that understands this stack)
• Private networking configuration (fix: Scan your deployed application with a security tool that understands this stack)
• Volume and persistence security (fix: Scan your deployed application with a security tool that understands this stack)