What is a credential stuffing attack?
A credential stuffing attack uses stolen username-password pairs – from breaches, phishing, or infostealer logs – to gain unauthorized access across multiple services through automation. SpyCloud customer data attributes ~20% of customer losses to stuffing and 80% to targeted account takeover, but stuffing’s scale makes it a constant threat to any login portal.
Where the lists come from and why defenses miss them
Stuffing is only as good as its input, and the input arrives three ways: breach dumps (often bundled into combolists), infostealer logs (fresh, plaintext, with device context), and phishing captures fed straight into campaigns. Infostealer-sourced pairs are the most dangerous – recently active and no cracking required. Defenses struggle because attackers came prepared:
- Botnets spread attempts across millions of IPs to defeat rate limiting.
- Residential proxies make automated traffic look like home users.
- Modern tooling solves CAPTCHAs and mimics human timing.
How do I check if my company’s credentials could be used in credential stuffing?
Run Check Your Exposure to see whether reused or exposed credentials tied to your domain could be used in credential stuffing. SpyCloud matches your domain against its recaptured darknet data to surface the exposed credentials attackers test against your logins.
The defense that actually works: remove the credential
You can’t reliably filter the traffic, so make the stolen credential worthless before it’s tested:
- Find exposed credentials first. Identify your users’ compromised credentials and force a reset or step-up before an attacker tries them.
- Catch the reuse pattern. SpyCloud matches exact and fuzzy/variation forms, flagging “Summer2024!” → “Summer2025!” reuse.
- Block at the moment of use. Integrate into login and registration to reject known-bad credentials in-flow.
- Starve the supply. Monitoring leaked credentials in criminal markets dries up the lists that power stuffing.
Credential stuffing vs. brute force vs. password spraying
All three are automated login attacks, but they differ in what they guess and how they evade defenses:
- Credential stuffing uses known-real username-password pairs from prior exposures, betting on reuse. High success rate, low volume per account.
- Brute force guesses passwords through exhaustive combinations against a known username. Loud and easily rate-limited.
- Password spraying tries a few common passwords across many usernames, staying under lockout thresholds to avoid detection.
Stuffing is the most effective of the three because it starts from credentials that already worked somewhere – especially fresh, plaintext infostealer-sourced pairs.
Credential stuffing works because passwords get reused.
See which exposed credentials tied to your domain attackers can test.
Frequently Asked
It’s an automated attack that tests stolen username-password pairs against many services. It works because roughly 70% of people reuse passwords, so a single exposure creates attack surface across dozens of accounts. Botnet distribution and CAPTCHA-solving tooling defeat most rules-based defenses, and 5.3 billion circulating pairs keep the supply endless.
Stuffing uses known real pairs from prior breaches against other services. Spraying tries a few common passwords against many usernames while staying under lockout thresholds. Stuffing is generally more effective because it uses previously validated credentials rather than guesses – especially fresh, plaintext infostealer-sourced pairs.
Three sources: breach dumps (often hashed, bundled into combolists), infostealer logs (plaintext, fresh, with the target URL attached), and phishing captures. Infostealer-sourced credentials increasingly dominate because of their freshness and high validity rate.