Credential Stuffing Attacks: How They Work & How to Prevent Them

Introduction

You wake up to an email: "Your Netflix password has been changed." Then another: "Your Uber account order is on its way."

You panic. "How did they hack me? I still have my password! I didn't click any links!"

You weren't hacked. Adobe was hacked 4 years ago. Or LinkedIn. Or that random fitness forum you signed up for in 2018.

This is Credential Stuffing. It is the #1 cause of Account Takeovers (ATO) today, and it relies entirely on one bad habit: Password Reuse.

The Anatomy of an Attack

Here is how it works, step by step:

1. The Breach: A poorly secured site (let's call it CatForum.com) gets hacked. 1 million emails and passwords leak to the dark web.
2. The Aggregation: Hackers add these to a "Combo List"—a master database of billions of leaked credentials.
3. The Automation: The hacker feeds this list into a bot tool (like Sentry MBA or Snipr).
4. The Stuffing: The bot tries these Email/Password combinations on high-value targets like PayPal, Amazon, Netflix, and Bank of America.
5. The Success: If 0.1% of users reused their CatForum password on PayPal, the hacker now has 1,000 valid PayPal accounts.

Why It Works (The Human Flaw)

Credential Stuffing works because humans are creatures of habit.

Stat: Over 65% of people use the same password (or slight variations) across multiple sites.

If you use Password123 for your throwaway newsletter account and your Bank account, you have tied their security together. If the newsletter gets hacked, your bank account is gone.

The Economics of Hacking

Hacking is a business. Credential Stuffing is profitable because it is cheap.

• Cost: $0. (Combo lists are often free or very cheap).
• Effort: Near zero. The bots run 24/7 while the hacker sleeps.
• Reward: High. Stolen accounts sell for $1 to $50 each.

How to Detect Stuffing Attacks

For businesses, detecting these attacks is a cat-and-mouse game.

Signs you are under attack:

1. Spike in Failed Logins: You normally see 1% failure rate. Suddenly it hits 40%.
2. High Traffic Volume: A surge of traffic to /login endpoints, often from data center IP addresses (AWS, DigitalOcean) rather than residential ISPs.
3. Account Lockouts: Customer support gets flooded with calls from users saying their accounts are locked.

Prevention Strategy for Businesses

You cannot stop users from reusing passwords. But you can stop the bots.

1. MFA (Multi-Factor Authentication): The silver bullet. Even if the bot has the password, it can't login without the OTP code.
2. Rate Limiting: Block IP addresses that fail 5 logins in a row.
3. Breach Screening: When a user creates an account or logs in, check their password against a database like Have I Been Pwned. If it's compromised, force a reset immediately.

Prevention Strategy for Users

How do you protect yourself?

1. Use a Password Manager: Every single website must have a unique password. Site A gets a random string. Site B gets a different random string.
2. Enable 2FA: It stops stuffing attacks dead.
3. Check Your Status: Go to haveibeenpwned.com. Enter your email. If you have been in a breach, change that password everywhere immediately.

Conclusion

Credential Stuffing is a tax on laziness. It punishes password reuse ruthlessly.

The defense is simple but requires discipline: Never reuse a password. Not once. Not ever. Let a password manager handle the memory work, and this entire class of attacks becomes harmless to you.