What Is Credential Stuffing and How Does It Affect Your Business?

Imagine an attacker obtains a list of 10 million leaked emails and passwords from LinkedIn. What do they do with them? They automatically test them on hundreds of other services — your bank, your email, Amazon, Netflix. That's credential stuffing.

How does it work?

The attack is surprisingly simple:

1. Obtaining credentials — The attacker buys or downloads lists of leaked emails/passwords from previous breaches.
2. Automation — They use bots that test these credential pairs on hundreds of websites simultaneously.
3. Successful access — Since many people reuse passwords, a significant percentage (between 0.1% and 2%) works.
4. Exploitation — They access the account, steal data, make purchases, or sell the access.

A typical attack can test millions of combinations per hour using distributed bot networks.

Is it different from a brute force attack?

Yes, and it's important to understand the difference:

| | Brute force | Credential stuffing | |---|---|---| | Method | Guesses passwords randomly | Uses real leaked passwords | | Speed | Slow (many attempts per account) | Fast (1-2 attempts per account) | | Success rate | Very low | Relatively high (0.1-2%) | | Detection | Easy (many failed attempts) | Difficult (they look like legitimate logins) |

Credential stuffing is more dangerous precisely because it is difficult to detect: each attempt looks like a normal login.

How does it affect businesses?

For a business, credential stuffing can mean:

• Unauthorized access to customer accounts — With the legal consequences this implies under the GDPR.
• Financial fraud — Fraudulent purchases with compromised accounts.
• Reputational damage — Customers lose confidence if they perceive that "they have been hacked".
• Operational costs — Incident management, support, resetting accounts.
• Infrastructure load — Bots generate thousands of requests per minute.

And users?

If you reuse passwords (and 65% of people do), you are a perfect target:

• The password you used on a forum in 2015 can give access to your current email
• Your Netflix account can be sold for €3 on the dark web
• A compromised email account allows resetting passwords from other services

How to protect your business

1. Implement rate limiting — Limit login attempts per IP and per account.
2. Use smart CAPTCHA — Tools like Cloudflare Turnstile detect bots without bothering real users.
3. Monitor login patterns — Detect unusual spikes in failed attempts.
4. Enforce 2FA — Especially for accounts with sensitive data.
5. Check passwords against leaked lists — APIs like HIBP allow verifying if the chosen password is already compromised.

How to protect yourself as a user

1. Use unique passwords — A password manager generates and remembers them for you.
2. Enable 2FA — The second factor blocks access even if they have your password.
3. Check your email — SecuryBlack Breach Scanner tells you if your credentials have been part of a leak.
4. Change leaked passwords — If the scanner detects breaches, change those passwords immediately.

---

Credential stuffing works because we reuse passwords. The first step to protect yourself is to know if yours are leaked.