Detecting a Brute-Force Attack Using an Intrusion Detection System (IDS)

Case Study Overview

An organization hosts a public-facing web application and SSH service for remote administration. The security team has deployed a Network Intrusion Detection System (NIDS) to monitor inbound and outbound traffic. The IDS is configured with signature-based and anomaly-based detection rules.

During routine monitoring, the IDS begins generating alerts indicating suspicious authentication activity.

Incident Background

• Environment: Corporate network with public web and SSH access
• Security Control: Network-based IDS (e.g., Snort / Suricata)
• Objective: Detect unauthorized access attempts before compromise

Attack Scenario

An external attacker initiates a brute-force login attack against the organization’s SSH service.

Attack Flow

1. The attacker scans the network to identify open ports.
2. Port 22 (SSH) is discovered and targeted.
3. Multiple failed login attempts are sent from a single IP address.
4. The attacker rotates usernames to evade simple lockout controls.

IDS Detection

The IDS detects:

• High volume of failed SSH login attempts
• Repeated authentication requests in a short time window
• Abnormal traffic patterns compared to baseline behavior

Sample IDS Alert

[ALERT] SSH Brute Force Attempt Detected

Source IP:185.XX.XX.12

Destination Port:22

Severity:High

Security Impact

If undetected, the attack could lead to:

• Unauthorized system access
• Privilege escalation
• Lateral movement within the network
• Data exfiltration or service disruption

Response Actions

1. SOC analyst investigates IDS alerts
2. Source IP is blocked at the firewall
3. SSH access is restricted to trusted IPs
4. Multi-factor authentication (MFA) is enforced
5. Logs are reviewed for successful login attempts