I've audited dozens of SaaS apps built by early-stage teams. The same vulnerabilities show up almost every time. Here's what to check before an attacker does:

1. Broken access control

Users can access or modify data belonging to other users by changing an ID in the URL or API request. This is the #1 most common issue I find. Always verify ownership server-side.

2. Exposed API keys and secrets

Hardcoded in frontend code, committed to Git, or leaked through error messages. Rotate everything and use environment variables.

3. Missing rate limiting

Login, signup, password reset, and API endpoints with zero rate limiting = open door for brute force, credential stuffing, and abuse.

4. Insecure direct object references (IDOR)

Similar to broken access control but specifically through predictable IDs. If your user IDs are sequential integers, an attacker is already enumerating them.

5. JWT misconfigurations

No signature verification, secrets stored client-side, tokens that never expire. JWTs are powerful but dangerous if implemented wrong.

Most of these take less than a day to fix once you know they exist. The problem is most teams don't know until it's too late.

If you're shipping fast and haven't had a security review yet, now is the time.