Login Flood Attack

A Login Flood is a type of application-layer Denial of Service (DDoS) attack where an attacker sends a high volume of login requests to a target’s authentication endpoint. This is typically done to exhaust system resources, degrade performance, or cause total unavailability of the login service. Unlike brute force attacks that aim to gain unauthorized access, the goal of a login flood is disruption, not access.


Attack Vector

Login Flood attacks are usually executed via automated scripts or botnets that:

  • Rapidly send repeated HTTP POST requests to the login endpoint (e.g., /login, /auth, /signin)

  • Include either random, valid, or dictionary-based usernames and passwords

  • Often bypass simple rate-limiting by rotating IP addresses, user agents, or using proxy networks (e.g., TOR, VPNs)


Impact

  • Authentication Service Disruption: Legitimate users may experience slowdowns or inability to log in.

  • Resource Exhaustion: CPU, memory, and database resources can be overwhelmed by concurrent authentication checks.

  • Downstream Service Degradation: Other services that depend on authentication may also become unavailable.

  • Security Alert Noise: Security monitoring tools may be flooded with false positives, masking real intrusion attempts.


Preventative Measures

  • Rate Limiting: Restrict the number of login attempts per IP or user in a given time window.

  • CAPTCHA Integration: Add visual or invisible CAPTCHAs to block automated scripts.

  • Account Lockouts or Throttling: Temporarily lock accounts or slow down responses after repeated failed attempts.

  • Bot Detection: Use behavior analysis and JavaScript challenges to detect non-human traffic.

  • IP Reputation Services: Block known malicious IPs or anonymous proxies using threat intelligence feeds.


Detection Techniques

  • Unusual Spike Monitoring: Alerts for abnormal volume of login attempts per second.

  • Log Analysis: Identify patterns such as repeated login requests with varying usernames or IPs.

  • User Behavior Anomaly Detection: Spot inconsistencies in typical user login behavior.


Response Actions

  • Activate WAF rules or cloud-based DDoS protection services (e.g., Cloudflare, AWS Shield)

  • Increase authentication server elasticity to handle peak loads

  • Enable real-time alerting to trigger response playbooks


by Sergei Penchuk, CISSP

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