Credential Stuffing is a common practice in cyber crime where a hacker or cyber criminal gains access to a user’s email addresses and password, and proceeds to try that password against other accounts/services belonging to that individual. This is performed based on the knowledge that users often reuse the same passwords between different accounts/services, albeit sometimes with slight variations.
This is a highly effective means of attack, as users may change passwords for services that they are aware have been breached, but may not think to change that password where it is in use on other accounts. Credential stuffing is also commonly used when attempting to commit identity theft against a user.
It is highly recommended that all users do not reuse passwords between services, and to use a password manager if required to help them remember distinct, secure passwords.