Credential stuffing is an automated attack in which criminals take username and password pairs exposed in one breach and try them against many other services. It works because people reuse the same password across accounts.

How does credential stuffing work?

1. Attackers obtain lists of leaked credentials from earlier breaches.
2. Bots try each pair against login pages and APIs, at scale.
3. Pairs that work give access to real accounts.
4. The attacker uses, sells or builds on that access.

The attempts are often spread across many IP addresses and slowed down, so that they do not trigger simple blocks.

How is it different from brute force and password spraying?

Brute force tries many passwords against one account. Password spraying tries a few common passwords against many accounts. Credential stuffing tries real pairs that already worked somewhere else, so it needs fewer attempts per account and succeeds more often.

Why is credential stuffing hard to spot?

Every attempt is a normal login request, and the successful ones use valid credentials. Nothing is broken and no malware is involved. The signs appear only in the pattern, across many requests.

What are the signs of credential stuffing?

• Sudden spikes in failed logins.
• Many accounts targeted from the same sources, or one account from many sources.
• Successful logins from unfamiliar devices or locations right after failures.
• Logins in bursts or at unusual times.
• New activity on dormant accounts.

What does credential stuffing look like in practice?

An employee uses the same password for a shopping site and for work email. The shopping site is breached and the pair ends up on a list. Bots try it against email services and find that it works. The attacker reads the inbox, finds password reset links for other services and widens the access.

Why does password reuse make it work?

Attackers rely on people, not on technical flaws. One leaked pair can unlock every account where the same password is used. The more services someone uses, the more likely it is that at least one of them was already breached.

What happens after a successful login?

The attacker holds a valid identity. They may read data, change account details, use the account to reach connected services or look for privileged access. Because the login was legitimate, the rest of the session looks normal unless behaviour is being watched.

How does credential stuffing lead to account takeover?

Stuffing is the method and account takeover is the result. Once the attacker controls an account, they can steal data, commit fraud, send phishing from a trusted address or use the account to move towards more valuable systems.

Does MFA stop credential stuffing?

It stops most attempts, because a valid password alone is no longer enough. Attackers adapt, with MFA fatigue, stolen session tokens and phishing, so phishing-resistant methods such as passkeys and hardware keys are stronger than codes or push approvals.

Who is most at risk?

Any service with a login and customers or staff who reuse passwords, but some accounts matter more. Privileged accounts, service accounts without MFA and accounts that connect to critical applications give the attacker far more for the same effort.

How do you defend against credential stuffing?

• Require MFA, preferably phishing-resistant, on important accounts.
• Encourage unique passwords and check new ones against lists of known breached passwords.
• Limit login rates and detect bots on login endpoints.
• Monitor login patterns and challenge risky attempts.
• Do not forget privileged and service accounts.

Stopping every attempt is unrealistic. Sharelock watches what identities do after login, so a valid session that starts behaving unlike its owner stands out even when the credentials were correct.

‍