Credential stuffing is the practice of using stolen login information from one account to gain access to accounts on a number of sites through automated login. The exploit can allow hackers and those buying stolen credentials to access not just the accounts from the sites they are stolen from but any account where the victim uses the same password.
After obtaining credentials for a number of sites, a hacker may sell a list of user IDs, passwords and email addresses. It is common for these lists to be sold on underground or on the dark web. Whether lists are sold or used by the hacker themselves, it is likely the holder of the account details will want to get as much value from the accounts as possible.
Even though a single account was breached, it can lead to the simultaneous compromise of other sites used by the victim because the credentials are input for multiple sites using automated logins. The attacker often compromises multiple accounts before the user is aware that their account for any site has been breached.
Credential stuffing is a serious threat to both consumers and businesses, which both stand to lose money, either directly or indirectly. In retail in the United Kingdom, it is claimed that over 90 percent of logins come from credential stuffing attacks rather than authentic users. Eliminating these logins could make a significant impact on decreasing credential stuffing.
From an end-user perspective, it's recommended to create different and sufficiently strong passwords for each site. For the site or service provider, tools such as Shape Security's Blackfish or Fortinet's Fortiguard can help fight credential stuffing.