Credential Stuffing: how to stay safe

Table of contents

• What is Credential Stuffing
• How a Credential Stuffing attack works
• How to prevent a Credential Stuffing attack
• How to react after a Credential Stuffing attack

Protecting personal information has become an absolute priority. However, cyber threats are constantly evolving, and among the most widespread and dangerous is Credential Stuffing.

This is a technique of attack that is increasingly used by hackers, which exploits a very common habit: the reuse of usernames and passwords across multiple platforms.

In this article, we will explore in detail what Credential Stuffing means, how this type of attack works, what are the best strategies to prevent it, and what to do if you discover that you have been a victim.

What is Credential Stuffing

Credential Stuffing is an automated cyberattack that uses lists of compromised credentials, usually obtained from previous data breaches, to attempt unauthorized access to user accounts across multiple platforms.

The core idea behind this attack is quite simple but highly effective: many people reuse the same username and password combination on several online services, making it easier for hackers to succeed.

Unlike Brute Force attacks, where random combinations of characters are tested until the correct one is found, Credential Stuffing relies on real, previously stolen data. These credential lists are often sold or traded on the dark web, in illegal forums or specialized marketplaces.

The success of these attacks is rooted in the widespread habit of poor password hygiene. According to the Verizon Data Breach Investigations Report, over 80% of credential-based attacks use data from previous data breaches.

The consequences can be severe: unauthorized access to bank accounts, emails, social networks, e-commerce platforms, and even corporate portals, resulting in financial loss, identity theft, and reputational damage.

How a Credential Stuffing attack works

Credential Stuffing attacks rely on the brute force of automation and the vast availability of millions of username and password pairs leaked over the years following massive data breaches.

Platforms like LinkedIn, Adobe, Dropbox, Facebook, and even government portals have all suffered security incidents in which user credentials were exposed and later sold or distributed illegally.

How an attack is prepared

Everything starts with the acquisition of compromised credentials. Hackers purchase or download these lists for free on the dark web or in underground forums.

A typical combo list looks like this:

user1@email.com:password123

user2@gmail.com:qwerty2020

user3@yahoo.com:123456

Once in possession of this data, attackers use automated tools such as Sentry MBA, SNIPR, OpenBullet, or customized software.

These tools are designed to attempt access to a specific platform by automatically and massively entering every pair of credentials.

These tools are highly configurable: they allow hackers to set proxy servers,, randomize user agents, simulate human behaviors, integrate bypasses for CAPTCHA, and exploit target lists (for example, e-commerce portals or banking services).

A practical example: OpenBullet

One of the most popular tools for Credential Stuffing is OpenBullet, because it offers an intuitive interface and allows users to create actual config files that describe how to interact with a target site.

A typical attack with OpenBullet involves:

1. Preparing a combo list with millions of credentials.
2. Configuring the software with the login behavior of the target platform (login URL, required parameters, cookie handling).
3. Setting up hundreds or thousands of proxy servers to avoid IP blocking.
4. Launching the attack, which can execute thousands of login attempts per minute.

Even if only 0.1% of the credential pairs are valid, attackers can compromise hundreds or thousands of accounts.

Real-world attack examples

A well-known case is the Disney+ attack: just hours after the platform’s global launch, hundreds of user accounts were hijacked.

Disney’s systems were not breached; instead, cybercriminals used existing leaked credentials, tested them on Disney+ login portals, and successfully accessed accounts of users who reused old passwords.

Another significant attack occurred in 2020 against Spotify, where over 300,000 accounts were compromised through Credential Stuffing, based on credentials stolen from previous data leaks.

Attack workflow: simplified diagram

The typical workflow of a Credential Stuffing attack follows these steps:

1. Data collection
   Gathering leaked credentials from previous breaches.
2. Tool preparation
   Configuring OpenBullet or similar software with the combo list, proxies, and target site parameters.
3. Execution
   Automating mass login attempts.
4. Verification
   Logging successful credential matches.
5. Monetization
   Selling the credentials, stealing sensitive data, or committing fraud.

Educational code example: how Credential Stuffing works
For educational purposes only, here’s a simplified Python script that demonstrates the basic logic behind a Credential Stuffing attack (this code is for learning purposes and cannot be used to attack real systems).

import requests

# Simplified combo list

combo_list = [

    {"username": "user1@email.com", "password": "password123"},

    {"username": "user2@gmail.com", "password": "qwerty2020"},

    {"username": "user3@yahoo.com", "password": "123456"},

]

# Target website (example)

login_url = "https://www.targetsite.com/login"

for creds in combo_list:

    payload = {

        "email": creds["username"],

        "password": creds["password"]

    }

    try:

        response = requests.post(login_url, data=payload)

        if "Welcome" in response.text or response.status_code == 200:

            print(f"[SUCCESS] {creds['username']}:{creds['password']}")

        else:

            print(f"[FAILED] {creds['username']}")

    except Exception as e:

        print(f"Error with {creds['username']}: {e}")

This is obviously an educational code and does not represent the actual complexity of an attack, but it serves to demonstrate how the basic logic works: taking a list of credentials and attempting to authenticate in an automated way.

Techniques used to bypass security controls

To maximize the success rate, cybercriminals adopt several techniques to evade detection and blocking systems:

• Proxy or VPN usage to mask IP addresses and avoid rate limits.
• User-Agent rotation to simulate various browsers and devices.
• Random delays between login attempts to avoid detection as a bot.
• CAPTCHA bypass through OCR plugins or third-party solving services.
• Targeting unprotected APIs instead of the traditional web login interface.

What attackers do with compromised accounts

Once they successfully access an account, attackers can monetize or exploit it in several ways:

• Selling verified credential packs on underground marketplaces.
• Making unauthorized purchases using saved payment methods.
• Selling access to streaming or e-commerce accounts.
• Using accounts for phishing campaigns or to spread spam.
• Stealing sensitive personal data for identity theft.

According to a 2020 Akamai report, over 100 billion Credential Stuffing attempts were recorded globally between 2018 and 2020, highlighting how common and profitable this attack vector has become.

How to prevent a Credential Stuffing attack

Prevention is, without a doubt, the most effective defense against Credential Stuffing attacks. These attacks do not exploit technical vulnerabilities in systems but rather behavioral weaknesses of users and misconfigured security measures.

For this reason, defense strategies focus on adopting best practices, deploying the right tools, and implementing solid security policies—both for individual users and organizations.

Unique and complex passwords: the golden rule

The first and most important step in reducing the risk of Credential Stuffing is to never reuse the same password across multiple accounts.

This is the main vulnerability exploited by attackers. When a data breach occurs on a website, the stolen credentials are collected and tested on other platforms.

If a user has used the same password for their email, bank account, and social media, a single compromised service can trigger a domino effect.

Cyber Security experts recommend using passwords that are:

• at least 12-16 characters long;
• composed of uppercase and lowercase letters, numbers, and special characters;
• unique for each account.

Additionally, using passphrases is highly recommended. A passphrase is a combination of unrelated words and characters, easy to remember but difficult to guess.For example:
Nature$Fox-Rain-2025!

The importance of Multi-Factor Authentication (MFA)

Even the strongest password can be exposed. That’s why enabling Multi-Factor Authentication (MFA) is essential.

MFA requires a second layer of verification in addition to the password, such as:

• a temporary code generated by apps like Google Authenticator, Authy, or Microsoft Authenticator;
• a hardware token like YubiKey or Feitian ePass;
• a code sent via SMS (less secure, but still effective);
• biometric verification (fingerprint, face recognition).

According to a Microsoft Security Report, enabling MFA can block over 99% of automated attacks that use stolen credentials.

Advanced login monitoring

Organizations can further protect themselves from Credential Stuffing attacks by implementing access monitoring and anomaly detection systems.

These systems analyze login behavior in real-time and flag suspicious patterns such as:

• a large number of failed login attempts from the same IP address;
• login attempts from unusual geographic locations;
• simultaneous logins from multiple devices;
• abnormal speed and frequency of login requests.

Security Information and Event Management (SIEM) solutions like Splunk, IBM QRadar, or Elastic Security can aggregate and analyze this data, enabling security teams to respond promptly to suspicious activities.

Additionally, many platforms enforce rate limiting policies, which restrict or slow down the number of login attempts from a single IP address, making Credential Stuffing attacks much more difficult to carry out.

Use of advanced CAPTCHA systems

Another effective way to counteract the automation typical of these attacks is the implementation of advanced CAPTCHA systems. Traditional CAPTCHA methods based on character recognition are outdated and can be bypassed using OCR software.

Modern solutions like Google reCAPTCHA v3, hCaptcha, or proprietary behavioral CAPTCHA systems analyze various parameters to distinguish humans from bots, such as:

• mouse movement patterns;
• typing speed;
• geographic location and IP reputation.

These systems significantly reduce the success rate of automated credential stuffing attempts.

Password Managers: an essential tool for users

One of the main obstacles for users is managing dozens of unique and complex passwords. A practical and highly recommended solution is the use of a reliable Password Manager.

Tools like 1Password, Bitwarden, Dashlane, or LastPass (business version) allow users to:

• generate strong, complex passwords;
• store them securely in an encrypted vault;
• synchronize credentials across devices;
• autofill login forms safely.

For corporate environments, enterprise-grade Password Managers offer additional features such as shared credential management, access auditing, and team-based permission policies.

Advanced corporate strategies

For businesses managing large volumes of user accounts, additional protection measures are essential:

• Device fingerprinting
  Tracking device-specific information to identify legitimate or suspicious access attempts.
• Behavioral analytics
  Analyzing typical user behavior to detect deviations that may indicate malicious activity.
• Bot management platforms
  Solutions like Cloudflare Bot Management, Akamai Bot Manager, or PerimeterX help detect and block malicious automated traffic.
• IP reputation filtering
  Blocking IP addresses known for malicious activities.

A multilayered approach

In summary, preventing Credential Stuffing requires a multilayered approach that includes:

• User best practices (unique passwords, MFA enabled);
• Advanced security technologies (rate limiting, CAPTCHA, login behavior monitoring);
• Continuous education and security awareness programs.

As emphasized by the National Institute of Standards and Technology (NIST) in its digital identity guidelines, credential security is a shared responsibility between users, developers, and system administrators.

How to react after a Credential Stuffing attack

If you discover you have been the victim of a Credential Stuffing attack, it is essential to act quickly to contain the damage. The first step is to conduct a thorough analysis of your access logs to detect suspicious activity, such as logins from unknown devices or unusual locations.

Next, force a password reset for all compromised accounts and immediately notify the affected users. Transparency in these situations is crucial to prevent further misuse and maintain user trust.

Simultaneously, strengthen your continuous monitoring systems to detect additional attack attempts and update your security policies, including specific measures against Credential Stuffing.

Finally, it is advisable to cooperate with the relevant authorities and data protection agencies to report the incident and adopt all necessary measures under applicable regulations, such as the GDPR in Europe.

Conclusion: why Credential Stuffing should not be underestimated

Credential Stuffing is a concrete and constantly evolving threat, fueled by risky digital behavior and poor attention to account security.

Despite the simplicity of the technique, the effects of such an attack can be devastating for both individuals and companies, leading to data breaches, identity theft, and reputational damage.

For this reason, adopting a proactive approach to Cyber Security, investing in defense tools, and promoting a culture of digital protection is more important than ever.

Questions and answers

1. What is a Credential Stuffing attack?
   It is a cyberattack where hackers use stolen credential lists to attempt unauthorized access to accounts on various online platforms.
2. How is Credential Stuffing different from Brute Force attacks?
   In Brute Force, random password combinations are tried. In Credential Stuffing, real credentials from previous data breaches are used.
3. Where do the credentials used in these attacks come from?
   They originate from data breaches and leaked databases, often sold on the dark web.
4. How can I tell if I am a victim of Credential Stuffing?
   Signs include suspicious login activity, notifications of failed login attempts, or password reset requests you didn’t initiate.
5. What should I do after discovering a Credential Stuffing attack?
   Reset all compromised passwords, enable Multi-Factor Authentication, and closely monitor your accounts.
6. Which businesses are most at risk of Credential Stuffing?
   E-commerce sites, banks, social networks, and companies managing sensitive data are common targets.
7. How can I protect my accounts from these attacks?
   By using strong, unique passwords, activating MFA, and relying on a trusted Password Manager.
8. What is Multi-Factor Authentication and how does it help?
   MFA requires an additional verification method beyond the password, making unauthorized access much harder.
9. Is it safe to use a Password Manager?
   Yes, reputable Password Managers use advanced encryption to securely store credentials.
10. What happens to data stolen through Credential Stuffing?
    It is used for fraud, identity theft, or sold on illegal dark web forums.