Credential stuffing is an automated attack that tries usernames and passwords stolen from one breach against many other websites and apps. It works because people often reuse the same password across services. Attackers use bots, proxies, and large credential lists to make many login attempts quickly while avoiding simple rate limits.
This matters because a successful login can give an attacker immediate access without exploiting software flaws. In real incidents, credential stuffing is often aimed at portals, email, account management systems, and other identity-driven services where stolen access can expose records or enable further abuse. Defenses include unique passwords, password managers, multi-factor authentication, phishing-resistant MFA where possible, login monitoring, rate limiting, bot detection, and alerts for impossible travel or unusual device fingerprints. When these controls are in place, reused passwords from past breaches are much less useful to attackers.