When I was in school, almost every classroom had that one kid who forgot everything—homework, pens, their ID card, their lab coat... and eventually, even their passwords.
Back then, we all had accounts for the school's computer lab. They weren't particularly sensitive, just enough to log into the library computers, submit assignments, and check grades. One afternoon, while waiting for class to start, someone opened the last drawer of a desk near the window. Inside was a tiny spiral notebook.
The first page simply read:
Passwords
School Login
aditya123
Gmail
aditya123
Instagram
aditya123
Steam
aditya123
Nothing was encrypted. Nothing was hidden. It was simply... there.
Everyone laughed. Someone joked, "At least it's easy to remember." The notebook went back into the drawer, and within minutes everyone had forgotten about it.
A few weeks later, the notebook disappeared. Again, nobody thought much of it until strange things started happening. Someone's assignment had been submitted from another computer. Another student discovered messages sent from their Instagram account that they had never written. Someone else's gaming account suddenly had purchases they never made.
People assumed they had all been hacked separately.
But they hadn't.
One forgotten notebook had quietly unlocked everything.
The problem wasn't the notebook
That story stayed with me for years, not because someone had left a notebook lying around, but because the notebook wasn't actually the interesting part.
The interesting part was the passwords.
Not because they were weak—some weren't. Not because they were short—some weren't. The real problem was that they had been reused.
One password. Multiple accounts.
Whoever found that notebook didn't have to crack anything or figure anything out. They simply kept trying the same password everywhere until something worked.
That's almost exactly what credential stuffing is
When people hear about password attacks, they usually picture someone sitting in a dark room typing:
password123
password124
password125
over and over until they eventually guess correctly.
That's brute force.
It's noisy, slow, and relatively easy to notice.
Credential stuffing is almost the opposite.
The attacker already has a list of real usernames and passwords, usually collected from years-old data breaches involving platforms like LinkedIn, Adobe, Dropbox, Canva, gaming forums, shopping websites, or countless other online services.
Those credentials have already been stolen.
The attacker isn't trying to invent new passwords or guess yours—they're simply checking whether people are still using old ones somewhere else.
Imagine someone with millions of keys
Think about an apartment building.
Brute force is like standing outside one apartment with a giant keyring, trying every key until one eventually opens the door.
Credential stuffing is different.
Imagine someone who has already stolen thousands of real apartment keys. Instead of guessing, they're simply walking through the city trying those keys on buildings that still use the same locks.
Most doors stay closed.
Some don't.
And they only need a handful of them to open.
Why does this work so well?
Because people rarely create completely new passwords.
Instead, they recycle old ones.
Maybe they change a single character.
Maybe they add the current year.
Maybe they replace an "a" with "@".
Attackers know this—and more importantly, their software knows it too.
Modern credential stuffing tools don't just test one password. They automatically generate and try realistic variations like:
Summer2023
Summer2024
Summer@2024
Summer#24
Summer2025
The attacker isn't being clever.
The software is.
This is why the 23andMe breach happened
One of the biggest misconceptions about the 23andMe incident is that attackers somehow hacked the company's authentication system.
They didn't.
Authentication worked exactly as intended.
The usernames and passwords came from previous breaches on completely different websites. Some customers had reused those same credentials, so when attackers tried them against 23andMe, the login succeeded because everything was technically correct.
The system wasn't fooled.
The user had simply carried an old compromise into a new service.
That's why roughly 14,000 customer accounts were accessed, eventually exposing information connected to around 6.9 million individuals through the platform's shared family relationship features.
The breach wasn't created during login.
It actually began years earlier, the moment someone reused a password on another website.
So where does brute force fit in?
Credential stuffing and brute force often get grouped together, but they're solving completely different problems.
Brute force asks:
"What if I keep guessing until I'm right?"
Credential stuffing asks:
"What if someone has already guessed for me?"
One attacks uncertainty.
The other exploits certainty.
That's why credential stuffing succeeds far more often than most people realize.
So how do organizations defend against it?
Strong passwords help.
Password managers help.
Multi-factor authentication helps.
Rate limiting helps.
CAPTCHAs help.
Detecting impossible login velocity helps.
Checking passwords against known breach databases helps.
Together, these controls make credential stuffing significantly more difficult.
But here's something interesting.
Even after you stop 99% of these attacks, one valid login can still get through.
And if that attacker behaves like a perfectly normal user after logging in, traditional security often has nothing more to say.
That's why post-login monitoring is becoming just as important as login protection.
Stopping bad logins is only half the problem.
Understanding what happens after a successful login is the other half.
The notebook, revisited
I still think about that forgotten notebook from time to time.
Nobody cracked those passwords.
Nobody guessed them.
Nobody performed some Hollywood-style hack.
Someone simply found information that already existed.
Credential stuffing works the same way.
Attackers don't wake up every morning hoping they'll invent some brand-new way into your systems.
More often than not, they're betting that someone, somewhere, is still using yesterday's password.
And surprisingly often, they're right.
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