Credential stuffing is an automated attack where stolen username/password pairs from one breach are tested against other websites and services. Unlike brute force attacks that guess passwords, credential stuffing uses real credentials that users have actually used — just on different sites.

The attack works because people reuse passwords. Studies consistently show that over 60% of users use the same password across multiple accounts. When one site gets breached, attackers can access dozens of other accounts belonging to the same user.

The numbers are staggering. Data breaches have exposed billions of credentials, and the problem continues to grow every year.

Major breaches from companies like LinkedIn, Adobe, Dropbox, and countless others have contributed to this massive pool of exposed credentials. Even breaches from years ago remain valuable because many users never change their passwords.

A combo list (or combolist) is simply a text file containing username:password or email:password pairs, typically one per line. These lists are the ammunition for credential stuffing attacks.

An antipublic checker is a tool that compares credentials against known public databases to determine if they've been previously leaked. The term "antipublic" refers to filtering out credentials that are already publicly known, leaving only "private" (previously unknown) lines.

AntiPublic checkers maintain massive databases of known leaked credentials. When you submit a list for checking, each line is compared against this database:

After processing, an antipublic checker typically outputs two files:

AntiPublic checkers are classic dual-use tools. The same technology that helps attackers filter their stolen credentials also helps defenders protect their organizations. Understanding both perspectives is crucial.

From an attacker's perspective, antipublic checking is about efficiency and value:

For defenders, antipublic checkers serve equally important purposes:

The distinction between public and private credentials has significant implications for both attackers and defenders.

Private credentials are more valuable because:

The public/private distinction helps prioritize response:

Modern antipublic checkers don't just handle email:password combinations. URL antipublic checking compares discovered vulnerable URLs against known databases of previously exploited sites.

This is particularly useful for pentesters who want to avoid testing sites that have already been reported and patched, focusing instead on genuinely undiscovered vulnerabilities.

When dealing with combo lists containing millions or billions of lines, processing speed becomes critical. A slow checker can take days to process what a fast one handles in minutes.

At enterprise scale, this speed difference translates to significant time and cost savings. Processing a 100 million line combo list at 50 lines/second would take 23 days — at 10,000 lines/second, it takes under 3 hours.

APlus is our dedicated antipublic checker built for speed and scale. Whether you're a security researcher monitoring corporate exposure or a pentester validating credential findings, APlus processes your lists faster than any alternative.

Stop wasting hours on slow checkers. APlus processes what others take days to complete in just minutes, letting you focus on analysis rather than waiting.

Understanding credential stuffing is the first step to defending against it. Here are practical measures every organization should implement:

When conducting authorized security assessments, here's how to effectively use antipublic checking in your workflow:

This approach demonstrates real-world attack scenarios and helps clients understand their actual exposure to credential-based attacks.

Credential stuffing remains one of the most prevalent and damaging attack techniques in 2026. As long as users reuse passwords and breaches continue to occur, attackers will exploit this vulnerability.

AntiPublic checkers serve as essential tools for both sides of the security equation. For defenders, they enable proactive monitoring and breach response. For pentesters, they help demonstrate real-world attack scenarios and validate security controls.

Whether you're protecting an organization or conducting authorized security assessments, understanding how these tools work — and having access to fast, comprehensive checkers like APlus — is essential for modern security work.