Credential Stuffing Explained: How Old Leaks Become New Breaches

Billions of username and password pairs circulate freely on criminal forums, collected from years of data breaches and, more recently, from infostealer malware. Because people reuse passwords, a leak from one website becomes a key to many others.

Credential stuffing is the automated abuse of those leaked pairs against login pages at scale.

How a credential stuffing attack works

1. Acquire credential lists

Attackers buy or download combo lists containing millions of email and password pairs. Fresh infostealer logs, which often include session cookies too, are especially valuable.

2. Automate login attempts

Using purpose-built tools and large pools of residential proxies, attackers try each pair against a target login page. Spreading attempts across many IP addresses helps them avoid simple rate limits.

3. Monetize successful logins

Valid accounts are drained of stored value, used for fraud, or resold. Corporate accounts can become the initial access for a much larger intrusion.

Warning signs

Look for spikes in failed logins from many different IPs, unusually high login volume at odd hours, many accounts failing with valid usernames, and successful logins from infrastructure that doesn't match your customers' usual locations.

How to defend against it

Enforce multi-factor authentication

MFA is the single most effective control. Even when the password is correct, the attacker is stopped at the second factor. Prefer passkeys where you can.

Screen passwords against known leaks

At signup and password change, reject passwords that appear in known breach corpora. This takes the most dangerous reused passwords out of circulation.

Add bot and anomaly detection

Device fingerprinting, behavioral signals and adaptive challenges make large-scale automation much more expensive for attackers.

Monitor for your own leaked credentials

Threat intelligence monitoring can alert you when employee or customer credentials for your domains appear in new dumps, so you can force resets before they're used.