How to Detect and Respond to Brute-Force Attacks Using Log Data

Brute-force attacks involve repeated attempts to guess passwords or access credentials. Detecting these attacks early and responding promptly minimizes the risk of unauthorized access.

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1. Monitor for Repeated Failed Logon Attempts

What to Look For:

• Failed Logon Events:

  • Event ID 4625: Logs failed authentication attempts.

• Patterns:

  • Multiple failed logon attempts from the same IP address or targeting the same user account within a short period.

Red Flags: High-frequency failures originating from a single IP or targeting high-privilege accounts.

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2. Check for Account Lockouts

What to Look For:

• Account Lockout Events:

  • Event ID 4740: Indicates that an account has been locked due to too many failed logon attempts.

• Patterns:

  • Repeated lockouts for the same account or across multiple accounts.

Red Flags: A surge in lockout events, especially for accounts that are critical or rarely used.

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3. Investigate Login Success Following Multiple Failures

What to Look For:

• Successful Logon After Failures:

  • Event ID 4624: Logs successful logons. Correlate with preceding Event ID 4625 logs.

• Patterns:

  • A successful logon following a series of failed attempts indicates the attacker might have guessed the correct credentials.

Red Flags: Accounts showing successful access immediately after multiple failed attempts.

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4. Examine Logs for Unusual Access Times

What to Look For:

• Access Timing:

  • Review logon attempts occurring during non-business hours or unusual times for the user.

• Patterns:

  • A sudden increase in logon attempts during weekends, holidays, or late at night.

Red Flags: Logon attempts from accounts or IPs at times when legitimate users are unlikely to be active.

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5. Review Source IPs for Geolocation

What to Look For:

• IP Analysis:

  • Investigate the origin of the source IP addresses.

  • Cross-reference IPs with geolocation data to identify unexpected or foreign locations.

• Threat Intelligence:

  • Use known threat feeds to flag IPs associated with malicious activity.

Red Flags: Login attempts originating from unfamiliar locations or countries where the organization does not operate.

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6. Immediate Mitigation

What to Do:

• Block Malicious IPs:

  • Use firewall rules or intrusion prevention systems (IPS) to block IPs exhibiting suspicious behavior.

• Enforce Account Lockout Policies:

  • Temporarily disable accounts after a set number of failed attempts.

• Implement Multi-Factor Authentication (MFA):

  • Add an additional layer of protection to prevent unauthorized access, even if credentials are compromised.

• Audit Impacted Accounts:

  • Review the activity of targeted accounts for signs of compromise or lateral movement.

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Post-Incident Actions:

• Educate Users:

  • Notify users about the attempted attack and remind them to use strong, unique passwords.

• Enhance Monitoring:

  • Deploy SIEM rules to detect and alert on similar brute-force patterns in the future.

• Implement IP Allowlists:

  • Restrict access to critical systems to trusted IP ranges or VPN connections only.

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Conclusion

Brute-force attacks are detectable through patterns of failed logon attempts, account lockouts, and unusual access activity. Combining log analysis with real-time monitoring and robust security controls ensures effective detection and mitigation of such attacks.

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