Credential Stuffing: How Leaked Passwords Work
In January 2024, a security team at a mid-size SaaS company noticed something odd. Over a single weekend, their authentication logs showed 340,000 failed login attempts across employee and customer-facing portals. The attempts came from thousands of IP addresses, rotating every few requests. Buried in the noise: 47 successful logins.
None of those 47 accounts had been brute-forced. The attackers already had the correct passwords. They had purchased a batch of stolen credentials from a 2023 breach of an unrelated service, and 47 employees had used the same email and password combination for both.
This is credential stuffing. Not a sophisticated exploit. Not a zero-day. Just a bet that people reuse passwords, and that bet pays off roughly 0.1% to 2% of the time. At scale, that is enough.