Most security advice is written for teams with a security budget. If you are one person shipping a side project this weekend, you do not need a 200-item audit. You need to catch the handful of mistakes that actually get small apps breached.
Here is the short list, in the order things usually go wrong.
Secrets in the code
The single most common way indie apps leak is a hardcoded API key, database URL, or token that ends up in a public repo or a shipped bundle. Grep your project for anything that looks like a key before every release. Move it to an environment variable, and confirm the built output does not contain it.
Wide-open database rules
Firebase, Supabase, and similar backends ship with permissive defaults that are fine for a demo and dangerous in production. If your rules still say anyone can read and write, you are one curious visitor away from a data dump. Lock reads and writes to the authenticated owner before launch.
Missing security headers
No Content-Security-Policy, no HTTPS redirect, source maps served in production. None of these will stop your app from working, which is exactly why they get skipped. They are also the first things an attacker checks.
You do not have to be a security expert. You have to not ship the five obvious mistakes.
Auth you wired up in a hurry
JWTs decoded instead of verified, sessions without expiry, password resets that leak whether an account exists. Auth is the part everyone copy-pastes at midnight, and it is the part that matters most.
Run the check before you ship, not after
The reason these slip through is not ignorance. It is that the check happens after launch, if at all. Make "am I safe to ship?" a step in your release, the same way you run the build. A one-minute scan of your source catches the boring, dangerous stuff while it is still cheap to fix.