Anatomy of a Credential Stuffing Attack

What is Credential Stuffing?

Credential stuffing is a type of cyberattack where attackers use large lists of stolen username/password combinations (credentials) to gain unauthorized access to user accounts on other websites. The attack's success relies on the common human behavior of password reuse—using the same password across multiple online services.

If a user's credentials are leaked in a data breach from website-A.com, attackers will "stuff" those same credentials into the login forms of website-B.com, website-C.com, and so on, hoping to find a match. This is a highly automated process carried out by sophisticated bots.

Here is a step-by-step breakdown of how a typical credential stuffing attack unfolds.

Step 1: Acquiring Credential Lists

The first step for an attacker is to obtain lists of stolen credentials. These are readily available on the dark web and criminal forums.

• Data Breaches: Large-scale data breaches from companies are the primary source. These breaches can expose millions of email addresses, usernames, and passwords.
• Phishing and Malware: Credentials are also harvested through phishing campaigns and keylogging malware.
• Combolists: Attackers compile and trade "combolists," which are massive text files containing millions or even billions of username/password pairs, often aggregated from numerous breaches.

Step 2: Setting Up the Attack Infrastructure

Credential stuffing is a numbers game that requires a massive volume of login attempts. To avoid being detected and blocked, attackers use a sophisticated infrastructure.

• Botnets: They use a network of compromised computers (a botnet) or cloud servers to launch the attack.
• Proxy Networks: To hide their true origin and bypass IP-based blocking, attackers route their traffic through large proxy networks.
  • Datacenter Proxies: These are cheap and fast but are often easier to detect as their IP addresses belong to hosting providers.
  • Residential Proxies: These are more expensive and effective. They use the IP addresses of real, everyday internet users (often through malware or unethical free VPN/proxy apps), making the bot traffic appear indistinguishable from legitimate user traffic.

Step 3: Reconnaissance and Tooling

Before launching the attack, attackers study the target website's login process to configure their tools.

• Analyzing the Login Page: They identify the login API endpoint, the required parameters (e.g., username, password, csrf_token), and any basic anti-bot defenses.
• Configuring the Attack Tool: They use specialized software (e.g., Sentry MBA, OpenBullet) designed for credential stuffing. This software is configured to mimic a real browser, sending the correct HTTP headers and handling cookies and CSRF tokens.

Step 4: Launching the Attack

The attack is launched at a massive scale, with bots attempting to log in using the credentials from the combolists.

• Low-and-Slow Attacks: To evade rate limiting and detection, bots often distribute their login attempts over long periods and across thousands of IP addresses. This "low-and-slow" approach might involve only a few login attempts per IP address per hour, making it difficult to distinguish from normal traffic patterns.
• Mimicking Human Behavior: Sophisticated bots use headless browsers (like Puppeteer or Selenium) to simulate real user actions, such as mouse movements and keystrokes, to defeat behavioral bot detection systems.
• Bypassing Defenses: If a CAPTCHA is encountered, the bot can forward the challenge to a CAPTCHA-solving service, where it is solved by a human or an AI for a small fee.

Step 5: Identifying and Monetizing Successful Logins

The attack tool logs all successful login attempts. These compromised accounts are known as "hits."

• Account Takeover (ATO): Once an attacker has access to an account, they can exploit it in various ways:
  • Financial Fraud: Stealing stored payment information or draining funds.
  • Data Theft: Accessing and exfiltrating sensitive personal information.
  • Spam and Phishing: Using the compromised account to send malicious messages to other users.
  • Resale: Selling the credentials for the compromised account on dark web marketplaces. The value of an account depends on the service (e.g., a financial or e-commerce account is more valuable than a forum account).

How to Defend Against Credential Stuffing

• Multi-Factor Authentication (MFA): The single most effective defense. Even if an attacker has a valid password, they cannot log in without the second factor.
• Bot Management Solution: Use an advanced bot detection service that can identify and block automated traffic using techniques like device fingerprinting, behavioral analysis, and reputation tracking.
• Password Policies: Enforce the use of strong, unique passwords and check user passwords against lists of known breached credentials.
• Monitor for Failed Logins: A high rate of failed login attempts is a strong indicator of a credential stuffing attack.