Credential stuffing is an account takeover technique that relies on password reuse. Attackers will obtain large lists of email addresses and passwords from previous data breaches, and then use automated tools to test those same credentials against login pages for unrelated services.
This attack method works because many users will often reuse the same password across multiple sites, such as their email account and a social media platform. If a user’s credentials were exposed in a breach of one service, then those same credentials may still work against corporate SaaS platforms, e-commerce sites, customer portals, or internal applications exposed to the internet. Unlike password spraying, which is where an attacker will guess common passwords, credential stuffing uses real credential pairs that have already worked somewhere else.
Attackers typically automate this process at scale using proxies, browser emulation, and distributed infrastructure to avoid rate limits and detection. A low success rate can still be commercially worthwhile because the cost of testing millions of credential pairs is small. Once valid accounts are found then they may be used for fraud, data theft, or as the starting point for further compromise.
Credential stuffing matters because the organisation being attacked may have done nothing directly wrong with its own password storage and the exposure comes from users reusing credentials that were compromised elsewhere. That makes the risk partly external and partly behavioural, which is why multifactor authentication and user education is so important.
This issue is especially relevant for internet-facing web applications and identity systems, and frequently appears in web application penetration testing and API penetration testing scoping discussions.