Identification of Credential Stuffing

Credential stuffing is another type of cyber attack distinct from password spraying. In credential stuffing, attackers use stolen account credentials (username-password pairs) obtained from previous data breaches to gain unauthorized access to user accounts on different platforms. This attack is predicated on the tendency of users to reuse the same password across multiple services. From a penetration tester's perspective, identifying and mitigating credential stuffing attacks involves recognizing the signs and understanding the preventive measures.


  1. Unusual Login Successes: A spike in successful logins from various geographic locations could indicate credential stuffing, especially if the logins do not align with the known locations of the users.
  2. High Volume of Login Attempts: Similar to password spraying, a high number of login attempts can be a red flag, particularly when the login attempts use different username-password combinations.
  3. Account Lockout Alerts: Multiple account lockouts might indicate an ongoing attack, though credential stuffing typically has a lower rate of account lockouts compared to password spraying due to the use of valid credential pairs.
  4. Traffic Patterns: Anomalies in traffic, such as a sudden increase in requests to the login page from varied IP addresses, can signal a credential stuffing attack.
  5. Failed Logins from Recognized Users: Multiple failed login attempts from users with a history of successful logins could suggest that attackers are using their credentials in a stuffing attack.


  • Example 1: An attacker obtains a database of usernames and passwords from a breached website and uses this information to attempt logins on a series of unrelated web applications. The attacker is betting on the probability that some users have reused their credentials.
  • Example 2: During a penetration test, the tester uses a list of known compromised credentials to test the resilience of an application against credential stuffing. The tester observes a pattern of login attempts where numerous distinct accounts are accessed successfully from the same IP address within a short period.


  1. Rate Limiting: Implement rate limiting on login attempts to slow down attackers and reduce the effectiveness of credential stuffing attacks.
  2. Multi-factor Authentication (MFA): MFA is one of the most effective defenses against credential stuffing, as it requires additional verification beyond just the username and password.
  3. Credential Monitoring and Awareness: Regularly monitor for leaked credentials and alert users when their credentials may have been compromised in third-party breaches.
  4. User Education: Educate users about the dangers of password reuse across different services and encourage the use of unique passwords.
  5. Advanced Detection Mechanisms: Employ advanced threat detection mechanisms, such as anomaly detection, to identify and respond to credential stuffing attempts.

Understanding the dynamics of credential stuffing is crucial for penetration testers to help organizations prevent unauthorized access and safeguard user data effectively.