What is Credential Stuffing? How Hackers Exploit Reused Passwords

Credential stuffing is a cyberattack where hackers use stolen usernames and passwords to break into accounts across different websites. It takes advantage of password reuse, making it easy for attackers to succeed with minimal effort.

Verizon’s 2024 Data Breach Investigations Report found that stolen credentials were the starting point in 24% of breaches. Weak password habits make the problem worse—Bitwarden’s survey shows 25% of people reuse passwords across 11 or more sites, and 36% use personal info in their passwords.

In this blog, we’ll break down how credential stuffing works, how it differs from brute force attacks, the top targets, ways to detect and prevent it, and what companies can do to stop it.

What is Credential Stuffing?

Credential stuffing is a cyberattack where hackers use stolen username and password pairs to try logging into multiple websites. They rely on automated tools to quickly test thousands of credentials, hoping that users have reused the same login details across different platforms. When a match is found, attackers gain access to personal or corporate accounts—often without triggering any security alerts.

To dive deeper into how credentials are stolen in the first place, check out Keepnet’s article on Understanding and Preventing Credential Theft.

How Does a Credential Stuffing Attack Work?

A credential stuffing attack starts with a list of stolen usernames and passwords—often leaked in previous data breaches. Hackers load these credentials into automated tools, which rapidly test them across multiple websites. If a user has reused the same login on more than one site, the attacker can gain access.

These attacks usually happen at scale and are hard to detect, as they mimic normal login behavior. Once inside, attackers can steal sensitive data, make unauthorized purchases, or move laterally within a network.

How Hackers Obtain Credentials

Hackers typically get login credentials from large-scale data breaches, where usernames and passwords are leaked or stolen. These credentials are then sold or shared on the dark web, often in bulk.

Phishing attacks, malware, and social engineering are also common tactics to trick users into giving up their login details. Once hackers have these credentials, they add them to databases used in credential stuffing attacks, hoping users have reused the same passwords on other sites.

Data Breaches and Leaked Password Databases

When companies suffer data breaches, millions of usernames and passwords can be exposed. These stolen credentials are collected into massive databases and shared or sold online, often on dark web forums.

Hackers use these databases as the foundation for credential stuffing attacks, testing the leaked logins across various websites to find reused credentials.

For instance, in March 2025, several major Australian superannuation funds—including AustralianSuper and Rest Super—were targeted in a large-scale credential stuffing campaign. Attackers used previously stolen credentials to access member accounts, affecting hundreds of users. In AustralianSuper’s case alone, four members lost a combined total of $500,000 due to unauthorized access. (Source)

Similarly, in June 2024, clothing giant Levi’s reported a credential stuffing attack that compromised over 72,000 customer accounts. Cybercriminals exploited credentials from earlier breaches to access order histories, personal information, and partial credit card data. (Source)

These incidents demonstrate how leaked credentials from past breaches continue to fuel new attacks—emphasizing the need for better password hygiene and multi-factor authentication.

The Role of the Dark Web in Credential Distribution

The dark web plays a major role in spreading stolen credentials. After a data breach, hackers often upload usernames and passwords to underground marketplaces or forums where they are sold or shared.

These platforms make it easy for cybercriminals to access millions of login credentials, which are then used in credential stuffing attacks. Some even offer subscription services that give continuous access to updated credential dumps, making attacks more frequent and effective.

This hidden ecosystem fuels the cycle of breaches, allowing attackers to strike repeatedly using easily available stolen data.

Differences Between Credential Stuffing and Brute Force Attacks

While both credential stuffing and brute force attacks aim to gain unauthorized access to accounts, they differ significantly:

• Credential Stuffing: Utilizes known username-password pairs, relying on users' tendency to reuse credentials across multiple sites. This method has a higher success rate due to the validity of the credentials used.
• Brute Force Attacks: Involve systematically guessing passwords without prior knowledge, often using random combinations or common password lists. This approach is generally less efficient and more time-consuming.

Common Targets of Credential Stuffing Attacks

Hackers often aim credential stuffing attacks at platforms where users store valuable data or conduct financial transactions. These targets are chosen for their high user volume and potential rewards. Let’s dive into the key targets attackers frequently go after.

E-commerce Websites

E-commerce platforms are prime targets for credential stuffing attacks because they combine convenience with valuable digital assets. These sites often store credit card details, personal contact information, order histories, and even loyalty rewards—making them a goldmine for cybercriminals.

Unlike banking systems, many e-commerce sites prioritize user convenience over strict security, which can mean fewer login protections like multi-factor authentication. If attackers successfully access an account, they can:

• Make unauthorized purchases using saved payment methods.
• Change shipping addresses to redirect deliveries.
• Exploit loyalty points or store credits.
• Harvest personal information for future scams or fraud.

The high volume of daily transactions and large user bases also make it easier for malicious activity to blend in unnoticed, giving attackers more time to exploit compromised accounts before detection.

Social Media and Email Accounts

Social media and email accounts are valuable targets for credential stuffing attacks because they offer access to personal communications, contacts, and private information. Once compromised, attackers can impersonate users, spread phishing links, reset passwords for other services, or gather sensitive data for identity theft. These accounts are also often used to verify logins on other platforms, making them key entry points for broader attacks.

To better understand the threats lurking on social platforms, read Keepnet’s article on Most Common Social Media Phishing Scams.

How to Detect Credential Stuffing?

Detecting a credential stuffing attack requires careful monitoring of login activity and spotting unusual patterns. Key signs include:

• Spike in failed login attempts: A sudden increase may indicate bots testing large numbers of credentials.
• Logins from unfamiliar IP addresses: Repeated access from unknown or foreign locations can be a red flag.
• Multiple logins in a short time: Attempts to access many accounts quickly suggest automated attacks.
• Unusual traffic on login pages: A sharp rise in visits to the login portal can signal bot activity.
• Increased account lockouts: Many users getting locked out at once may indicate widespread credential testing.

Tools and Strategies to Detect Credential Stuffing

Detecting credential stuffing attacks requires more than just monitoring login attempts—it demands the right tools and smart strategies. Organizations need to distinguish between legitimate users and automated threats in real time. Below are key methods and technologies that can help identify and stop these attacks before damage is done.

Using CAPTCHA to Thwart Automated Attacks

CAPTCHA is a simple but effective tool for blocking bots in credential stuffing attacks. It forces users to complete challenges that are easy for humans but difficult for automated scripts. By adding CAPTCHA to login pages, organizations can disrupt large-scale credential testing and reduce the success rate of automated attacks.

Implementing IP and Device-Based Anomaly Detection

IP and device-based anomaly detection helps identify suspicious login activity by monitoring where and how users access their accounts. If a login attempt comes from an unusual IP address or an unrecognized device, it can trigger alerts or additional security checks. This method adds a strong layer of defense by detecting patterns that don’t match a user’s typical behavior, helping to block credential stuffing attacks before they succeed.

How to Prevent Credential Stuffing?

Preventing credential stuffing attacks starts with strong user habits and smart security controls. Organizations should enforce the use of unique, complex passwords and implement multi-factor authentication to block unauthorized access. On the technical side, using rate limiting, CAPTCHA, and anomaly detection can significantly reduce the risk of automated login attempts. Regular user education is also key to minimizing password reuse and raising awareness of credential-based threats.

The Importance of Strong and Unique Passwords

Strong and unique passwords are the first line of defense against credential stuffing attacks. When users reuse passwords across multiple sites, a single breach can expose all their accounts. Encouraging the use of complex passwords—and storing them with a password manager—helps limit the damage if one site is compromised. This simple step can significantly reduce the success rate of automated attacks.

For practical tips, check out Keepnet’s guide on Password Security Best Practices.

How Multi-Factor Authentication Can Protect You

Multi-Factor Authentication (MFA) adds an extra layer of security by requiring users to verify their identity using something beyond just a password—like a code sent to their phone or a biometric scan. Even if attackers have the correct login credentials, they can’t access the account without the second factor. Enabling MFA greatly reduces the chances of a successful credential stuffing attack, making it one of the most effective defenses available.

The Role of Companies in Combating Credential Stuffing

Preventing credential stuffing attacks isn’t just the user’s responsibility—organizations that manage user data and online services must take an active role in defending against these threats. By implementing the right mix of technical controls, user education, and continuous monitoring, they can significantly lower the chances of successful attacks. In the section that follow, we’ll explore how businesses can secure login systems, detect suspicious activity, and empower users to adopt safer authentication practices.

How Organizations Can Secure User Data

To protect users from credential stuffing attacks, organizations must go beyond basic security. Enforcing strong password policies, enabling multi-factor authentication, and monitoring login behavior are essential first steps.

To strengthen the human layer of security, organizations can use tools like the Phishing Simulator and Security Awareness Training. These tools train employees to identify suspicious login attempts, phishing links, and social engineering tactics—key techniques used to steal credentials in the first place.

The Keepnet Human Risk Management Platform adds another layer of protection by continuously assessing employee risk levels, tracking behavioral patterns, and automating tailored awareness interventions. This ensures that high-risk users receive targeted support before they become an entry point for attackers.

For deeper threat visibility and faster incident handling, Incident Responder and Threat Intelligence enable real-time detection of exposed credentials and ongoing monitoring of threat actors. These advanced tools allow security teams to act quickly and stop breaches before they escalate.

Future Trends in Credential Stuffing and Cybersecurity

Credential stuffing attacks are becoming more advanced, and organizations must evolve their defenses to keep up. Looking ahead, several key trends are expected to shape how these threats are managed:

• AI-Powered Attacks: Hackers are using AI to create smarter bots that can bypass security controls and behave like real users.
• Advanced Detection Tools: Security systems will increasingly use AI and machine learning to spot unusual login behavior and stop attacks in real time.
• Passwordless Authentication: More companies will move toward login methods like biometrics and passkeys, reducing reliance on vulnerable passwords.
• Zero Trust Security Models: Businesses will shift to stricter access control, verifying every user and device before granting access.
• Dark Web Monitoring: Automated tools will continuously scan the dark web for stolen credentials linked to company domains.
• Adaptive User Training: Awareness programs will become more personalized, using behavior data to target high-risk users with relevant training.

These trends show a shift from reactive to proactive security, where stopping credential stuffing attacks requires both intelligent technology and informed users.

To strengthen your organization’s security culture in line with these trends—and build healthy security habits that reduce the risk of credential-based attacks—explore Keepnet’s guide on Building a Security-Conscious Corporate Culture.