Credential-stuffing bots test stolen username-password pairs by automatically submitting them to website login forms. A successful login shows that the same credentials work on another account. Attackers exploit password reuse rather than discover a new password. According to OWASP's credential-stuffing definition, the goal is fraudulent access to user accounts.
Table of Contents
- Where the stolen credentials come from
- How bots run the tests
- What attackers do after a successful login
- Why blocking the bots is difficult
Where the stolen credentials come from
Attackers collect username-password pairs from data breaches, phishing, leaked dumps, or criminal markets. These lists may contain email addresses, usernames, passwords, and combinations of those identifiers. The attacker then targets unrelated services where victims may have reused their passwords.
OWASP's automated-threat description distinguishes this process from guessing: the bot validates previously stolen pairs instead of generating new password possibilities. For example, a password exposed through one website might be tested against email, shopping, or other accounts. It works only where the victim reused a matching login.
How bots run the tests
The attacker loads a credential list into automated software and directs it toward a website's login system. The bot submits each pair and records whether the site accepts or rejects it. Bots can send many login attempts at once.
They may also spread requests across different IP addresses and present them as coming from varied devices, making a simple block less effective. The basic workflow is: A failed attempt does not necessarily mean the stolen password is false. The user may have changed it, avoided reusing it, or used a different username at the target service.
- Submit a stolen username and password.
- Read the website's response.
- Record successful and unsuccessful pairs.
- Retain working credentials for account access or resale.
What attackers do after a successful login
A successful test turns an unverified credential into a confirmed login for that service. Depending on the account, an attacker may access private information, make purchases, drain stored value, send phishing messages, or resell the validated credentials. The consequences can extend beyond an account page.
In a 2023 complaint, the Federal Trade Commission alleged that insufficient rate limiting helped enable attacks against more than 55,000 U.S. Ring customers between January 2019 and March 2020. The compromised devices exposed hundreds of thousands of home videos. That case illustrates why reused credentials can create a new breach-like event at a service that did not originally lose the password.
Why blocking the bots is difficult
Rate limiting restricts repeated login attempts. A service might slow or block numerous attempts against one account or many attempts arriving from one IP address. That control has limits because attackers can distribute requests across addresses and devices.
Defenders therefore need layered controls and monitoring rather than relying on one threshold. For account holders, the practical defenses are direct: Unique passwords stop a credential exposed at one service from working elsewhere. MFA adds another requirement, so possession of the password alone is insufficient, as described in NIST's 2025 password guidance.
- Use a unique password for every account.
- Replace passwords known or suspected to have been exposed.
- Enable multifactor authentication, or MFA, where available.
- Treat unexpected account activity as a reason to secure the account promptly.
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