CHFI (312-49) Digital Forensics Simulation
Investigation Scenario
Your forensic unit has been called to investigate a suspected unauthorized access incident on a public-facing Linux web server (`prod-web-01`). The system administrator noticed suspicious network traffic and immediately preserved the system logs before taking the server offline for imaging.
You are examining the extracted `secure` authentication logs to determine exactly how the threat actor gained access to the system and what methodology was employed.
Evidence Collected
Review the following chronological extract from `/var/log/secure`:
Question
Expert Analysis
1. What the Evidence Shows
The system authentication log clearly displays high-frequency, sequential SSH login attempts originating from a single external IP address (`203.0.113.88`). The attacker is cycling through a dictionary of common account names (`root`, `admin`, `test`, `user1`, `backup`). The final entry confirms the attack was successful on the `backup` account.
2. Forensic Stage
Analysis. The investigator is correlating artifacts collected during the preservation phase (log files) to reconstruct the timeline and methodology of the breach.
3. Why the Correct Answer is Correct (A)
Brute Force attack is the correct classification. Specifically, this evidence demonstrates a "horizontal brute force" (or password spraying) attack. In a standard brute force, an attacker might try many passwords against one account. In a horizontal brute force, the attacker tries a few passwords across many different usernames to evade standard account lockout policies. The automated, rapid nature of the log entries confirms this methodology.
4. Why Others Are Wrong
B. Phishing: Phishing relies on deceptive emails or websites to trick a user into handing over credentials. It does not generate high-volume, automated system-level authentication failures from a single remote IP.
C. Man-in-the-Middle (MitM): A MitM attack involves intercepting communication between two parties (e.g., ARP spoofing or rogue access points). It is generally a passive interception technique, not an active barrage of authentication requests.
D. Social Engineering: This involves psychological manipulation of individuals (e.g., calling the helpdesk pretending to be the CEO). There is no human manipulation evident in these automated, direct protocol attacks.
5. Real-World Forensic Action
As an investigator, identifying this pattern initiates specific scoping actions. The examiner will pivot to analyzing the actions of the compromised `backup` account (UID/GID) after the 02:15:15 timestamp. Did they spawn a shell? Did they initiate an outbound connection (C2)? The forensic team will carve bash history, search for dropped malicious binaries, and examine network connections established by the `backup` user.
MINI LESSON: Log Anomalies & Automated Tools
During the Analysis phase, differentiating between human error and automated attacks is crucial:
- Timing Artifacts: Human typing cannot generate SSH connection and authentication sequences at precise 2-second intervals. This strict cadence indicates a tool like Hydra, Ncrack, or Metasploit.
- Horizontal vs. Vertical: Modern attackers prefer horizontal brute forcing (changing usernames, keeping passwords constant) because traditional security controls (like Fail2Ban) often trigger lockouts based on repeated failures against a single account.
- Log Integrity: Always verify if the successful login was followed by commands to wipe the `/var/log/secure` or `~/.bash_history` files, which is standard anti-forensics behavior.
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