Security Monitoring Principles

Effective security monitoring transforms raw data into actionable intelligence about threats to your environment. Without visibility into what's happening across your infrastructure, even sophisticated attacks can proceed undetected until damage becomes unavoidable.

Defense in Depth

No single security control stops all attacks. Defense in depth layers multiple controls so that attackers must overcome several barriers rather than just one. When one layer fails, others provide backup protection.

At the network perimeter, firewalls restrict what traffic can enter and exit. Behind this, intrusion detection systems watch for malicious patterns. Endpoint protection runs on individual systems. Application controls validate user input. Data encryption protects sensitive information even if other controls fail.

Each layer generates data useful for monitoring. Firewall logs show blocked connections. IDS alerts identify known attack signatures. Endpoint agents report process execution and file modifications. Together, these sources provide comprehensive visibility across the security stack.

Monitoring complements prevention by detecting what prevention missed. Perfect prevention is impossible, so detecting compromises quickly enough to limit damage becomes essential. The goal shifts from "prevent all attacks" to "detect and contain attacks before significant harm occurs."

Essential Log Sources

Effective monitoring requires data from the right sources. Not all logs have equal security value, and storage and processing costs mandate prioritization.

Authentication logs record who logged in where, when, and how. Failed login attempts might indicate password attacks. Successful logins from unusual locations suggest compromised credentials. These logs often provide the first indication of unauthorized access.

Network flow data shows what systems communicate with each other. While lacking the detail of full packet capture, flows reveal connections that shouldn't exist—internal systems reaching known malicious infrastructure, or lateral movement patterns spreading across your network.

Endpoint logs capture process execution, file modifications, registry changes, and user actions. Modern EDR platforms generate rich telemetry that enables detection of threats that never touch the network, like USB-delivered malware or malicious insider actions.

Application logs vary by system but often reveal attack attempts. Web server logs show injection attempts. Database logs record suspicious queries. Email system logs identify phishing campaigns.

Building Visibility

Most organizations don't immediately collect all useful data from all sources. Building visibility happens incrementally, prioritizing based on risk assessment and incident response needs.

Start with authentication and network perimeter logs. These sources cover the most common attack vectors and provide foundational visibility. Add endpoints as capability matures, focusing first on high-value systems like domain controllers, file servers, and systems with sensitive data.

Gaps in visibility should be documented and addressed over time. When incidents occur that weren't detected, ask whether better logging would have helped. Use these lessons to prioritize visibility improvements.

Log quality matters as much as quantity. Misconfigured log sources produce noise that drowns out signals. Time synchronization issues make correlation impossible. Invest in properly configuring logging rather than simply enabling everything.