Why is password uniqueness (never reusing the same password across accounts) the single most important password rule?
One breach anywhere becomes a breach everywhere — credential stuffing turns a leaked LinkedIn password into your bank login.
The attack — credential stuffing:
- Attackers download a database leak (e.g. Collection #1 — 773 million emails + passwords)
- They feed every
email:passwordpair into thousands of other sites automatically - Anywhere the user reused that password → instant account takeover
Why it works so well:
- Studies show ~65% of users reuse passwords
- Once a site is breached, that password is on cracking lists forever
- Even 5-year-old leaks are still valuable — many people never rotate
The defender's worst case:
Even if your site is perfectly secured (hashed, salted, rate-limited), if a user reuses a password that leaked from a breached forum from 2014, attackers will log in through your front door looking exactly like the legitimate user.
Solution: Password manager → unique random password per site. Then a breach is contained to that one account.
Check yourself: https://haveibeenpwned.com/ — see which of your accounts are in known breaches.
Go deeper:
OWASP Credential Stuffing Prevention Cheat Sheet — the defender's playbook: MFA, breached-password checks, and why this differs from brute force.