Credential Stuffing: What It Is and How To Prevent It

What is credential stuffing?

Credential stuffing is a cyberattack in which criminals use stolen username-and-password combinations from past data breaches to access accounts on other platforms. First coined in 2011 by Sumit Agarwal after observing waves of login attempts using leaked credentials, it’s a common method in account takeovers because it allows attackers to use real leaked credentials rather than guessing.

Credential stuffing is often confused with brute-force attacks, but it’s a more precise approach. Brute-force attacks refer to any method of trying passwords repeatedly until one works, usually revolving around guessing. Credential stuffing skips the guessing — attackers already have large lists of stolen credentials and use automated tools to test them across websites, sending waves of login attempts to see where they succeed.

Password reuse is the main reason why credential stuffing is so effective. When people use the same login details across multiple sites, a single set of credentials being exposed can unlock email, shopping accounts, banking apps, and workplace platforms. Because the credentials are valid, these logins can appear legitimate, making the attack harder to detect and stop.

How does credential stuffing work?

Credential stuffing works by pairing large datasets of real login credentials leaked in previous data breaches with automated tools to test the credentials across many different websites and services.

These databases — typically containing email or username and password pairs — are widely traded on cybercrime forums and dark web marketplaces. Criminals may also obtain credentials themselves if they successfully infiltrate a company’s systems or databases through phishing attempts or man-in-the-middle attacks.

Once attackers have these lists, the “stuffing” process sends large waves of login attempts to platforms such as banking apps, e-commerce sites, social media accounts, and workplace tools. Once an account is successfully compromised, hackers then try to reuse those credentials to log into the user’s other accounts.

To fully understand how credential stuffing attacks work, here’s the anatomy of a typical attack:

1. Credentials are collected: Attackers gather leaked usernames and passwords from past data breaches or other means, usually bundled into massive lists and traded or sold online.

2. Attack tools are configured: Automated tools such as Sentry MBA, OpenBullet, or SNIPR are configured to test those credentials on specific websites. These tools are widely known in security circles and mentioned here for awareness, not endorsement.

3. Automation does the heavy lifting: Bots fire off thousands of login attempts in quick succession, checking whether the stolen credentials work on other online services too.

4. Defenses are quietly evaded: To avoid being spotted, attackers constantly change their IP address so the traffic doesn’t appear to be coming from a single location. They may route traffic through proxy networks or botnets made up of compromised devices. Many tools also mimic normal browser behavior to avoid triggering security filters.

5. Successful logins are harvested and exploited: Any account that is successfully accessed is flagged and often exploited further. This could be to commit fraud, steal an identity, or resell verified details on underground marketplaces.

After breaching a database, an attacker tests stolen credentials on other websites. Password reuse allows them to access multiple accounts across different services.

If passwords are unique to each account, and properly hashed and salted, they are far harder for attackers to use. But when credentials are stored in plaintext — or protected with weak hashing methods — attackers gain immediate access to usable login data. Timing plays a role as well: newly leaked credentials are the most valuable because many users haven’t changed their passwords yet.

Credential stuffing vs. brute force attacks

Credential stuffing is often categorized along with brute force attacks as part of a broader set of password cracking techniques. The Open Web Application Security Project (OWASP), for example, classifies credential stuffing as a type of brute force attack because both rely on repeated login attempts to break into accounts.

In practice, though, the two attacks work quite differently:

• Traditional brute force attacks rely on guessing. Attackers try thousands or millions of possible passwords — random character combinations or common choices like Password123. Because most guesses fail, these attacks generate a huge number of login attempts. That makes them relatively slow and easier for modern security systems to detect and block.

• Credential stuffing is far more efficient. Instead of guessing passwords, attackers use login details exposed in previous data breaches. Automated tools test these real username-password combinations across many websites and services. Because people tend to reuse credentials across accounts, credential stuffing is faster, more cost-effective, and harder to stop.

Credential stuffing vs. brute force: key differences

Two related password-cracking techniques are password spraying and dictionary attacks. Password spraying tests a single common password across many accounts, helping attackers avoid lockouts. Dictionary attacks use automated tools to try large lists of common words, phrases, and previously leaked passwords, exploiting predictable human habits when creating passwords.

Credential stuffing vs. password spraying

Password spraying is often confused with credential stuffing because both rely on stolen or weak credentials rather than breaking into systems directly. But credential stuffing tests many known username-and-password pairs across multiple websites, attempting to reuse credentials leaked in earlier breaches. Password spraying, by contrast, tries a single common password across many accounts.

Both methods avoid the repeated guessing that can trigger security alerts. Instead, they exploit weak password practices — especially predictable or reused credentials — but approach the attack from different angles.

Why is credential stuffing a growing threat?

Credential stuffing isn’t spreading because attackers have suddenly become more sophisticated. The modern internet simply provides ideal conditions for the attack to scale: a constant flow of leaked credentials, widespread password reuse, and powerful automation. At the same time, people are managing more online accounts than ever, expanding the potential attack surface.

Recent data underscores the scale of the problem. According to the Verizon Data Breach Investigations Report n(DBIR) 2025:

• Compromised credentials initiated 22% of analyzed breaches.

• Only 49% of user passwords were unique across services, meaning more than half were reused.

• Credential stuffing accounted for a median of 19% of daily authentication attempts, rising to 25% in enterprise environments.

Here’s why credential stuffing keeps growing:

• An endless stream of stolen logins: Major data breaches regularly spill billions of usernames and passwords, giving attackers fresh material to reuse elsewhere. In 2025, the threat-intelligence firm Synthient aggregated 2 billion unique email addresses from credential-stuffing lists found online.

• Re-used passwords: When the same credentials protect multiple accounts — email, shopping platforms, social media, or banking apps — a single breach can act like a master key, unlocking several services at once.

• Advanced automation tools: Modern attack tools automate nearly every step of the process. Bots rotate IP addresses, mimic real browser behavior, and sometimes bypass CAPTCHAs, allowing attackers to test massive credential lists while avoiding simple detection systems.

• More accounts to target: Work, banking, entertainment, and communication increasingly rely on a diverse array of online platforms. Remote work and app-based services mean individuals and organizations maintain more accounts than ever — expanding the pool of potential targets.

• High profits, even with low success rates: Credential stuffing doesn’t require a high success rate to be profitable. When millions of login attempts are made, even a small percentage of successful logins can yield valuable accounts, enabling fraud, identity theft, or the resale of verified credentials on underground marketplaces.

Real-world examples of credential stuffing attacks

Credential stuffing isn’t just a theoretical risk. It affects everyday users and major companies alike. Even large organizations with significant security resources have fallen victim, often exposing sensitive data, damaging reputations, and triggering regulatory penalties or lawsuits.

23andMe (2023)

Attackers reused credentials from unrelated breaches to access user accounts on the genetic testing platform. Through features like “DNA Relatives,” they scraped sensitive profile information — including ancestry and health-related data — affecting roughly 7 million 23andMe users.

The incident led to regulatory scrutiny, and the company was fined £2.31 million for failing to protect the genetic data of its UK users. It also demonstrated how credential stuffing can expose deeply personal information even when the company’s core systems aren’t directly breached.

General Motors (2022)

General Motors experienced a credential stuffing attack in which hackers accessed customer accounts and redeemed accumulated reward points. The credentials were likely obtained from breaches at unrelated services. GM responded by notifying affected users and advising them to reset their passwords and review their financial statements.

Dunkin’ Donuts (2018-2019)

The company suffered two credential stuffing attacks within three months, targeting customer DD Perks rewards accounts. Attackers used credentials leaked from other services to access accounts containing usernames and email addresses.

Following allegations that earlier incidents had not been fully disclosed, Dunkin’ Donuts agreed to strengthen its cybersecurity measures and paid $650,000 in fines. One automation script used in the attacks, SNIPR, was reportedly developed specifically to target Dunkin’ Donuts accounts.

Uber (2016)

A major breach exposed data belonging to 57 million riders and 7 million drivers. Attackers gained access after developers accidentally uploaded credentials to a GitHub repository, which hackers discovered and used to access internal systems. Uber later admitted it had paid the attackers $100,000 to delete the stolen data rather than immediately disclosing the breach.

The consequences of credential stuffing attacks

Account lockouts, takeovers, drained bank accounts, and identity theft… welcome to the dreaded domino effect of credential stuffing. A cracked password can have serious consequences that extend well beyond that single breach. Businesses risk operational and reputational damage — not to mention serious fines and GDPR problems.

How credential stuffing impacts individuals

Credential stuffing doesn’t end when the attacker logs out. For individual victims, the impact often lingers — hitting our wallets, privacy, and peace of mind long after the initial breach.

Financial impact

• Theft from bank accounts and digital wallets.

• Unauthorized purchases on your behalf via compromised shopping accounts and saved payment methods.

• Identity theft, including fraudulent loans, applying for new accounts, or tax fraud.

• Stolen loyalty points and balances from airlines, retailers, and apps.

Personal impact

• Account takeovers that lock users out after passwords are changed.

• Loss of personal data if the hacker deletes photos, emails, contacts, and cloud files.

• Reselling of your credentials on dark web markets.

• Privacy invasion if the hackers access private messages and sensitive documents.

• Reputational damage if your hijacked accounts are used to scam or spam others.

Emotional impact

• Stress and anxiety from losing control of your personal information and possibly finances.

• Tedious recovery efforts as you try to contain the damage and regain control.

• Eroded trust in online services and constant fear of another attack.

How credential stuffing impacts businesses

For businesses, the impact of a data breach can be severe. If customer accounts are taken over, they become platforms for fraud and unauthorized purchases, which can be particularly disastrous for financial services and e-commerce.

Companies face steep remediation costs, including refunds to customers and internal security investigations. There’s also the inconvenience of forced password resets and security upgrades. In the IBM Cost of a Data Breach Report 2025, the global average cost of a data breach is 4.4 million US dollars.

And reputational, operational, and regulatory exposure can be just as costly. Public security incidents erode customer trust, increase churn, and weaken long-term brand value, while large waves of automated login attempts can strain infrastructure, slowing systems for legitimate users and employees. Organizations may face penalties under data protection laws such as GDPR, CCPA, or HIPAA if they fail to adequately safeguard user accounts.

There’s also the ever-present danger of lateral movement, as security experts call it. Once an attacker gains a foothold in a system, who knows what they will access next or unleash?

How to detect and respond to credential stuffing attacks

If you notice any of the following warning signs, stay alert. They may indicate that someone is testing your credentials and attempting a credential stuffing attack. Recognizing the signals early can help you act quickly to secure your accounts and limit potential damage.

Steps for individuals

The following warning signs may indicate a credential stuffing attack: repeated account lockouts, slow website performance during login attempts, or a surge of failed login alerts.

You may also notice unauthorized activity — such as purchases or account changes you didn’t make — or login attempts from unusual locations, which can signal automated bots operating across global networks.

Here’s what to do if you suspect a credential stuffing attack:

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  Change and strengthen passwords: Create a new, strong, and unique password for the affected account, then update any other accounts that use the same credentials. Ensure each password is complex and never reused across services.

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  Enable Multi-Factor Authentication (MFA): MFA adds a critical second layer of security. Even if someone has your password, they won’t be able to access the account without the additional verification step, such as a code sent to your phone or email.

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  Check for data exposure: Search breach-monitoring services like Avast Hack Check to see whether your email address or other credentials have appeared in known data leaks.

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  Use breach monitoring tools: Go even further with automated alerts from tools like Avast BreachGuard, which continuously monitors the web and even the dark web for signs that your personal data was involved in a security breach. The service also includes smart privacy advice and expert support if anything goes wrong.

Steps for businesses

For organizations, early detection and swift action can mean the difference between a contained incident and a large-scale breach. The following steps help identify and limit credential stuffing attacks.

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  Watch for failed login spikes: A sudden surge in failed login attempts is often one of the earliest indicators of a credential stuffing campaign.

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  Track abnormal login behaviour: Logins at unusual hours, from unfamiliar devices, or with irregular patterns can signal unauthorized access attempts.

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  Monitor unusual IP activity: Repeated login attempts from numerous IP addresses, rapid IP rotation, or traffic originating from unexpected regions may indicate automated bot activity.

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  Implement breach detection: Use automated monitoring systems that detect when employee or customer credentials appear in known data breaches and trigger alerts or forced password resets.

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  Rate-limit and temporarily block activity: Restrict login attempts from suspicious sources while investigating. Rate limiting and temporary blocks can slow automated attacks and reduce potential damage.

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  Respond quickly with account protections: Immediately lock affected accounts, reset compromised credentials, and notify users so they can secure any related accounts. Prompt communication helps prevent further exploitation.

How to help prevent credential stuffing attacks

Fortunately, credential stuffing is highly preventable. These attacks rely primarily on password reuse and large-scale automation. Strong password practices and a few key security measures can block most attempts before they succeed.

Prevention tips for individuals

Credential stuffing thrives on reused passwords and weak security habits. Strengthening your login practices is one of the most effective defenses.

• Use strong, unique passwords: Avoid reusing passwords and steer clear of personal details such as pet names or birth dates. Current guidance from NIST emphasizes longer passwords — up to 64 characters — because length significantly increases password strength. If a password is easy to remember, it’s often easy to guess.

• Use a password manager: Password managers generate and securely store strong, unique credentials for every account. This eliminates the need to remember dozens of complex passwords while preventing reuse across services.

• Enable multifactor authentication (2FA or MFA): Adding a second verification step (or more), like an app or text code, helps stop attackers even if they’ve swiped your password. Find out more about how two-factor authentication works.

• Use passwordless authentication if it’s available: Technologies such as passkeys or biometric logins remove passwords entirely, cutting off the primary mechanism that credential stuffing attacks rely on.

• Adopt continuous authentication: Continuous authentication systems monitor user behavior and context — such as typing patterns, device type, and location. If something suddenly looks unusual, the system can flag the activity or block further access even after login.

• Turn on breached password protection: Breach monitoring services like Avast BreachGuard notify you if your credentials appear in known data leaks, allowing you to quickly reset affected passwords and secure your accounts.

Prevention tips for businesses

There’s no single fix for credential stuffing. Effective protection requires a layered approach that secures login systems while combining user awareness with automated detection tools.

These practical steps can help businesses stop credential stuffing attacks:

• Enable multi-factor authentication (MFA): Stolen usernames and passwords are far less useful when an additional verification step is required.

• Adopt passwordless authentication: Replacing traditional passwords with biometrics, hardware tokens, or passkeys eliminates the primary mechanism that credential stuffing attacks rely on. It can also simplify the login experience for users.

• Use single sign-on (SSO): SSO allows users to authenticate once and securely access multiple applications. Centralized authentication can improve both usability and security when properly implemented.

• Block breached passwords: Check user passwords against databases of known compromised credentials and require resets when matches are found. This prevents users from choosing passwords already exposed in previous breaches.

• Use adaptive authentication and monitor login activity: Implement systems that detect suspicious login behavior, such as attempts from unfamiliar devices, locations, or IP addresses. Sudden spikes in failed login attempts can signal an ongoing attack.

• Apply rate limiting and IP controls: Limit the number of login attempts allowed within a specific timeframe and block abusive IP addresses. These measures help disrupt large-scale automated attacks.

• Detect and block bots: Bot-detection tools and CAPTCHAs help distinguish real users from automated login attempts.

• Educate employees on security best practices: Even strong technical controls can fail if users unknowingly expose credentials. Encourage unique passwords, teach staff how to recognize phishing attempts, and establish clear procedures for reporting suspicious activity.

• Maintain a tested incident response and recovery plan: Preparation matters. A well-defined response plan allows organizations to contain breaches quickly and minimize operational, financial, and reputational damage.

It’s also prudent to be prepared for every eventuality with a disaster recovery plan, so you can act fast after a data breach, cyberattack (or even a flooded basement). This helps minimize the operational, reputational, and financial impact on the business.

You could also consider a convenient, single solution like the Avast Business Hub for unified and layered cybersecurity. It lets you monitor and manage threats from one intuitive platform.

Help protect your accounts from credential stuffing attacks

Credential stuffing often only succeeds because attackers exploit common mistakes like using the same password across multiple accounts or taking too long to change a password you know has been breached. Strengthening your defenses starts with awareness — but it shouldn’t stop there.

Avast BreachGuard helps monitor the web for exposed credentials and alerts you if your information appears in a breach. With dark web monitoring and tools to identify weak or reused passwords, it helps you secure your accounts before attackers can take advantage. Don’t wait for a wake-up call in the form of a compromised account — take control of your digital security today.