Introduction:

In today's cybersecurity landscape, Security Information and Event Management (SIEM) systems play a crucial role in detecting and alerting organizations about potential threats and incidents. However, the real challenge lies in effectively investigating these alerts and deciphering the true nature of events. In this blog post, we'll explore the process of investigating SIEM alerts on Linux hosts, focusing on how to analyze logs and uncover the story they tell. By leveraging various techniques and examples, we'll empower you to become a proficient investigator in your quest for truth.

Understanding SIEM Alerts:

SIEM alerts serve as early warning signs, highlighting potential security incidents or suspicious activities on your Linux hosts. These alerts can be triggered by a multitude of events, such as failed login attempts, unauthorized access, or abnormal system behavior. When investigating an alert, it's crucial to gather as much contextual information as possible, including the time of the incident, affected hosts, and the specific event triggering the alert.

Example:

Let's say you receive a SIEM alert indicating a high number of failed SSH login attempts on one of your Linux servers. This raises concerns about a potential brute-force attack. To uncover the truth, we'll dive into the logs.

Analyzing Log Files:

Log files serve as a treasure trove of information, capturing the activities and events occurring on your Linux hosts. They are invaluable during investigations, offering insights into the sequence of events, user actions, system processes, and potential indicators of compromise (IOCs).

Identifying the Relevant Log Files:

Linux systems typically store logs in the "/var/log" directory, with various files containing different types of information. Common log files include "auth.log" for authentication-related events, "syslog" for system-wide messages, and "secure" for security-related events. To pinpoint the logs associated with the SIEM alert, consider the event's category or specific details mentioned in the alert.

Narrowing Down the Timeframe:

To focus your investigation, identify the timeframe during which the SIEM alert was triggered. By narrowing down the logs to this specific period, you can eliminate noise and concentrate on the events directly related to the incident.

Searching for Relevant Events:

Using tools like "grep" or log analysis platforms, search for events that align with the SIEM alert's description. Look for entries related to the specific event, such as failed SSH login attempts, in the respective log files. Pay attention to timestamps, source IP addresses, usernames, and any other pertinent details.

In our scenario, we navigate to the "/var/log/auth.log" file and search for entries related to SSH authentication failures within the specified timeframe. By running the command:

grep 'sshd.*Failed' /var/log/auth.log

we can identify the failed SSH login attempts and gather more insights into the event.

Unraveling the Story:

Once you've located the relevant log entries, it's time to piece together the sequence of events, understand the context, and determine the root cause of the alert. This process involves examining the log entries holistically, correlating events across multiple log files, and considering the broader system context.

Tracing Event Sequences:

Identify the chronological order of events by analyzing timestamps. Look for patterns, anomalies, or repetitions that might indicate malicious activities or misconfigurations. Follow the trail of events, examining each step and noting any deviations from normal behavior.

Cross-Referencing Log Files:

To gain a comprehensive understanding, cross-reference entries from multiple log files. For instance, if a failed login attempt is detected, correlate it with events from the "auth.log" and "syslog" files to identify any associated system-level activities or subsequent events. This allows you to establish a timeline and uncover any interconnected actions.

Investigating User Actions:

Focus on the actions performed by users or entities involved in the events. Examine the commands executed, files accessed, or privileges escalated. Look for suspicious or unauthorized activities that might indicate a potential breach or insider threat.

Example:

In our scenario, we identify a series of failed SSH login attempts from different IP addresses. By cross-referencing the "auth.log" entries with the "syslog" file, we discover that shortly after the failed attempts, a successful SSH login was made from a different IP address. This suggests a potential compromise, where an attacker gained access using valid credentials after a series of failed attempts.

Gathering Additional Evidence:

To strengthen your investigation and gain more insight into the incident, consider collecting supplementary evidence from various sources. These sources can include system artifacts, network logs, or data from intrusion detection systems (IDS) or firewall logs. By correlating data from different sources, you can create a more complete picture of the event.

Example:

In our case, we gather network logs from the firewall to identify any suspicious outgoing connections from the compromised Linux host. Analyzing the logs reveals an unusual connection to a known malicious IP address, strengthening the evidence of a successful intrusion.

Documenting and Reporting Findings:

As you progress through the investigation, maintain detailed documentation of your findings, observations, and analysis. This documentation not only helps you in revisiting the investigation if needed but also serves as a comprehensive report to share with relevant stakeholders, such as the incident response team or management. Include timestamps, relevant log entries, correlations, and any other pertinent information.

Conclusion:

Investigating SIEM alerts on Linux hosts requires a systematic and meticulous approach. By leveraging the wealth of information contained within log files, you can unravel the truth behind security incidents. Remember to analyze log entries, correlate events, and cross-reference multiple log files to establish a cohesive narrative. Gathering supplementary evidence from different sources further strengthens your investigation. With these techniques in your toolkit, you'll be well-equipped to navigate the complexities of SIEM alert investigations and ensure the security of your Linux environment. Stay vigilant, keep digging, and let the logs reveal the truth.