What Is Credential Stuffing? How to Detect and Stop It

Learn how to detect stolen credentials before attackers use them to break into your systems.

Someone steals a password from one website. Then they try it on 50 others. That’s credential stuffing in a nutshell. And it works because most people reuse passwords across multiple accounts.

Breaches involving stolen credentials cost organizations an average of $4.67 million and take 246 days to identify and contain. This is according to IBM’s Cost of a Data Breach Report 2025. The attackers aren’t guessing passwords. They’re using real ones stolen from previous breaches.

Billions of username and password combinations circulate on dark web marketplaces. Automated tools test these credentials against login pages at massive scale. Even with a success rate below 1%, attackers still compromise millions of accounts.

This guide explains how credential stuffing works and why it’s so effective. You’ll also learn what you can do to protect your organization.

What Is Credential Stuffing?

Password reuse creates a massive security hole. When you use the same password on multiple sites, a breach at one company exposes your accounts everywhere else.

This attack differs from brute force in one critical way. Brute force attacks guess passwords through trial and error. Credential stuffing uses real passwords that actually worked somewhere else. The attacker already knows the password is valid for that user. They’re just checking if it works on other sites too. OWASP’s credential stuffing documentation provides additional technical details on how these attacks are classified.

The math makes this attack highly effective. Even with a success rate of just 0.1%, testing 1 billion credential pairs yields 1 million compromised accounts. And billions of leaked credentials are freely available on criminal marketplaces.

How Do Credential Stuffing Attacks Work?

The attack follows a predictable pattern. Understanding each step helps you defend at multiple points.

Step 1: Obtaining Credential Lists

Attackers get credentials from two main sources. The first is third-party data breaches. When companies like LinkedIn or Adobe get hacked, their user databases are leaked in criminal markets. The second is infostealer malware, which harvests credentials directly from infected computers by capturing passwords saved in browsers and credentials typed into login forms. Both sources feed into dark web combo lists, massive compilations containing billions of username and password pairs.

Collection #1-5, a famous combo list discovered in 2019, contained over 22 billion username and password combinations. New breach data gets added to these lists constantly.

Step 2: Automated Testing

Nobody tests billions of credentials by hand. Attackers use specialized tools that automate the entire process. These tools rotate through proxy servers to avoid IP-based blocking. They mimic real browser behavior to evade bot detection. They distribute requests across thousands of IP addresses.

The tools fire login attempts at target websites continuously. Some attackers run campaigns against dozens of sites simultaneously. They configure the tools to slow down when rate limiting kicks in, then resume when blocks expire.

Step 3: Account Takeover and Monetization

When credentials work, attackers move fast. They might drain financial accounts directly. They might steal stored payment information. They might access sensitive data for identity theft. Or they might sell the verified working credentials to other criminals for a premium.

Compromised streaming service accounts sell for a few dollars. Corporate email accounts with access to internal systems command much higher prices. The value depends on what the account can access.

Why Are Credential Stuffing Attacks So Effective?

Credential stuffing is one of the most common credential-based attacks organizations face. Three factors make it so successful.

Password Reuse Is Universal

Password reuse remains rampant. Security researchers analyzing 19 billion leaked passwords found that only 6% were unique. Verizon’s 2025 Data Breach Investigations Report puts it even higher - 94% of passwords are reused across two or more accounts.

People know the risk. Surveys show 89% understand password reuse is dangerous, but only 12% actually use unique passwords. Managing dozens of unique credentials feels impossible without a password manager.

Credentials Are Abundant and Cheap

The supply of stolen credentials keeps growing. Every new data breach adds more passwords to the pool. Criminal marketplaces sell fresh breach data within hours of theft. Older breach data circulates freely on forums.

Attackers don’t need to hack anything themselves. They buy credentials for pennies per account. The investment is minimal. The potential return is enormous.

Detection Is Difficult

Credential stuffing looks like normal login traffic. Each individual request comes from a different IP address. Each uses valid credentials. Rate limiting helps but dedicated attackers work around it by slowing down and spreading requests across more proxies.

Without specialized detection tools, these attacks blend into legitimate authentication attempts.

What Are the Consequences of Credential Stuffing?

The impact extends far beyond the immediate account compromise.

Account Takeover Damage

When attackers gain access, they can do whatever the legitimate user could do. They make fraudulent purchases. They steal personal information. They access connected accounts through password reset flows. They lock out the real user by changing passwords.

Financial and Regulatory Impact

Credential-based breaches cost organizations an average of $4.67 million according to IBM’s Cost of a Data Breach Report 2025. These breaches also take the longest to detect and contain, averaging 246 days. Regulatory fines compound the financial damage when personal data gets exposed.

Reputation Damage

Customers blame the company when their accounts get compromised. It doesn’t matter that the password was stolen from a completely different breach. The customer sees unauthorized activity on your platform. They hold you responsible. Trust erodes with every incident report.

How Can You Detect Credential Stuffing Attacks?

Detection requires looking for patterns that distinguish automated attacks from normal user behavior.

Monitor Login Anomalies

Watch for unusual patterns in authentication traffic. Spikes in failed login attempts often signal an active attack. Geographic anomalies matter too. If a user typically logs in from Chicago, a sudden login attempt from Eastern Europe warrants scrutiny.

Impossible travel is a strong indicator. When the same credentials attempt login from New York and Singapore within minutes, something is wrong.

Identify Bot Behavior

Automated tools leave fingerprints. They might send requests at unnaturally consistent intervals. They might lack the JavaScript execution patterns of real browsers. They might cycle through user agents in predictable sequences.

Device fingerprinting helps distinguish automated requests from legitimate users. Browsers leave dozens of identifying characteristics that bots often don’t replicate accurately.

Implement Credential Monitoring

The most direct detection method is knowing which credentials are already compromised. Compromised credential monitoring services alert you when employee or customer credentials appear in third-party breach data. You can reset those passwords before attackers test them against your systems.

This shifts detection from reactive to preventive. Instead of spotting attacks in progress, you eliminate the attack vector entirely by invalidating stolen credentials first.

How Do You Mitigate Credential Stuffing?

Effective defense combines multiple layers. No single control stops every attack.

Require Multi-Factor Authentication

MFA remains the most effective countermeasure. Even if attackers have the correct password, they can’t complete authentication without the second factor. It’s not foolproof because attackers can steal session tokens via infostealer malware to bypass MFA entirely. But it stops the majority of credential stuffing attempts. Push notifications and authenticator apps both dramatically reduce successful account takeovers. Hardware keys provide even stronger protection.

The challenge is adoption. Not all users enable MFA voluntarily. Consider requiring it for high-risk actions even if you can’t mandate it for every login.

Implement Rate Limiting and CAPTCHA

Slow down automated attacks with rate limiting. Block IPs that exceed normal login attempt thresholds. CAPTCHA challenges disrupt automated tools, though dedicated attackers use CAPTCHA-solving services to work around them.

These controls raise the cost of attacks. They don’t stop determined attackers but they do filter out opportunistic ones.

Monitor for Compromised Credentials

Don’t wait for attackers to test stolen credentials against your systems. Use dark web monitoring to identify exposed credentials from your organization. This includes leaked passwords and session tokens. Reset compromised passwords and invalidate stolen sessions before attackers can use them.

This approach addresses the root cause. Credential stuffing only works when valid credentials exist. Remove them from attacker arsenals by rotating them first.

Consider Passwordless Authentication

Eliminating passwords eliminates credential stuffing entirely. Passkeys and biometrics authenticate users without transmittable secrets. Attackers can’t stuff credentials that don’t exist. However, infostealers can still hijack authenticated sessions by stealing tokens after login. Passwordless closes one door but doesn’t eliminate account takeover risk completely.

Passwordless adoption is growing but remains incomplete. Most organizations still rely on passwords for at least some authentication scenarios.

What Are Real-World Examples of Credential Stuffing?

Recent incidents show how these attacks continue to cause real damage.

Australian Superannuation Funds (2025)

In March 2025, multiple Australian retirement funds were hit simultaneously. AustralianSuper, Rest Super, Hostplus, and Australian Retirement Trust all detected coordinated credential stuffing attempts over a single weekend. Attackers stole AUD $500,000 from AustralianSuper members. Around 8,000 Rest Super members had personal data accessed. The coordinated timing suggests attackers used the same credential lists against multiple financial targets.

Roku (2024)

Roku disclosed that 576,000 accounts were compromised through credential stuffing in March 2024. This came after an earlier incident that hit 15,000 accounts. Attackers used credentials stolen from unrelated breaches to access Roku accounts. Once inside, they changed login information and made unauthorized purchases. Roku wasn’t breached directly, but customers still blamed the platform.

The North Face (2025)

In April 2025, VF Corporation’s The North Face website was targeted by credential stuffing. Attackers accessed customer accounts and exposed personal data including names and addresses. This was VF Corporation’s fourth credential stuffing incident since 2020. Repeat attacks show how companies remain vulnerable when customers reuse passwords.

23andMe (2023)

An attacker accessed approximately 14,000 23andMe accounts using credentials stolen from other sites. The attacker then scraped genetic ancestry data from millions of connected profiles through the DNA Relatives feature. This data was sold on criminal forums. The 23andMe data breach demonstrated how credential stuffing can expose uniquely sensitive information that users can never change.

Conclusion

Credential stuffing exploits a simple weakness: people reuse passwords. Attackers take credentials stolen from one breach and test them everywhere else. Even a tiny success rate yields massive numbers of compromised accounts.

Key takeaways:

• Credential stuffing uses real stolen passwords, not guessed passwords
• Billions of credentials circulate on criminal marketplaces
• MFA blocks most attacks but isn’t foolproof alone
• Credential monitoring lets you reset exposed passwords before attackers use them
• Detection requires watching for login anomalies and bot behavior

The most effective defense combines MFA with credential monitoring. Block attacks at the authentication layer while simultaneously removing compromised credentials from attacker arsenals.

Want to know if your organization’s credentials are already exposed? Check your dark web exposure or book a demo to see how Breachsense helps security teams detect compromised credentials in real time.