What Is Credential Stuffing? Account Takeover Attacks and How to Defend Against Them

What Is Credential Stuffing? Lessons from a KRW 12.95 Billion Data Breach Fine in South Korea

Credential stuffing has once again drawn attention as a major cause of large-scale personal data breaches.

In August 2026, South Korea’s Personal Information Protection Commission(PIPC) imposed administrative fines totaling approximately KRW 12.95 billion on companies for violations of personal data protection regulations.

Related article (Korean)
Personal data of around 1.6 million people exposed… PIPC imposes KRW 12.9 billion in fines on three companies → [Link]

Credential stuffing is an automated cyberattack in which attackers use login credentials—such as usernames and passwords—that have already been leaked from another source to repeatedly attempt to access other websites or online services.

In the recent case, an unidentified attacker successfully gained access through credential stuffing, resulting in the exposure of personal information belonging to as many as 1,581,025 individuals.

The PIPC specifically pointed out that adequate measures had not been implemented to detect and block a large volume of login attempts originating from the same IP address within a short period of time.

In other words, the issue is not simply whether account credentials have already been compromised. What also matters is whether an organization has a system in place to identify and respond to abnormal login attempts.

Characteristics of Credential Stuffing Attacks

A brute-force attack attempts to take over an account by repeatedly guessing passwords, often making thousands of login attempts. Because of this behavior, relatively clear signs of abnormal activity can often be identified.

Credential stuffing is different.

The usernames and passwords entered by the attacker may be valid credentials belonging to real accounts. This makes it much more difficult to distinguish an attacker from a legitimate user.

A single login request may appear completely normal. But when multiple requests are analyzed together, abnormal patterns can emerge.

In the recent personal data breach case, a large number of login attempts from the same IP address within a short period of time was identified as an important warning sign.

Other suspicious behaviors may include:

• Multiple login IDs repeatedly accessed from a single IP address

• A single login ID accessed through multiple IP addresses or sessions within a short period of time

• Login activity occurring at a frequency or pattern rarely seen among normal users

Ultimately, it is important to look not only at which account is logging in, but also at how the login is being performed.

How Can Credential Stuffing Be Detected and Blocked?

For relatively simple attacks, blocking repeated requests from a specific IP address may provide some level of protection.

However, automated attacks can evade basic detection by rotating IP addresses, creating multiple sessions, or adjusting request speeds to resemble legitimate user behavior.

This means that relying on a single condition is not enough.

Multiple signals—including IP address, Login ID, Session ID, access frequency, and behavioral patterns—need to be analyzed together.

Rather than applying a blanket block to every suspicious request, organizations should define multiple behavioral criteria and evaluate combinations of signals. This approach helps reduce false positives and minimizes unnecessary friction for legitimate users.

STCLab’s malicious bot and macro detection and management solution, BotManager, can detect and block automated attacks that occur during the authentication process, including credential stuffing and password spraying.

By applying policies based on multiple access-environment and behavioral signals, BotManager can identify abnormal login attempts that may indicate automated account takeover activity.

Detection windows and thresholds can be configured according to the characteristics of each service and attack type. Requests identified as suspicious can then be immediately blocked or subjected to an additional CAPTCHA challenge for further verification.

The goal is not simply to “block logins.”

It is to distinguish legitimate users from automated account takeover attempts and apply the appropriate response only to high-risk requests.

Why Login Behavior Matters

Credential stuffing is particularly dangerous because attackers may be using valid usernames and passwords.

Checking the account credentials alone is therefore not enough.

The ability to quickly identify and respond to behavior that differs from normal users can be a critical part of defending against account takeover attacks. Examples include repeated login attempts within a short period, access to multiple accounts from a single IP address, or access to one account from multiple IP addresses.

Organizations cannot control every set of credentials that may already have been leaked elsewhere.

But they can detect and respond when those credentials are used in automated attacks against their own services.

This is why organizations need a defense system that analyzes both who is logging in and how they behave, allowing automated account takeover attempts to be identified and managed during the authentication process.

Learn more about how BotManager helps defend against account takeover attacks.