Security Logging and SIEM

When something goes wrong—a breach discovered, data exfiltrated, systems compromised—the first question is always: what happened?

Without logs, you have no answer. The past is simply gone. You find the damage but never see the hand that caused it.

With comprehensive logging, you can rewind time. Every authentication, every file access, every network connection leaves a trace. Security Information and Event Management (SIEM) systems collect these traces from across your environment, correlate them, and surface the patterns that reveal attacks in progress or reconstruct attacks already complete.

The Visibility Problem

You can't protect what you can't see.

An attacker moves through your network. They authenticate to a server using stolen credentials at 2:47 AM. They escalate privileges three minutes later. They access a database containing customer records. They exfiltrate data through an encrypted channel to an external IP.

Without logs, none of this is visible. The attacker comes and goes like a ghost. You discover the breach weeks later when stolen data surfaces on the dark web—but you have no idea how they got in, what they touched, or whether they're still there.

With logs, every step leaves evidence. The 2:47 AM authentication stands out against normal patterns. The privilege escalation triggers an alert. The database access is recorded. The outbound connection appears in firewall logs. You might catch the attack in progress. At minimum, you can reconstruct exactly what happened.

What to Log

Comprehensive security logging captures events across multiple dimensions:

Authentication events: Successful and failed logins, privilege escalations, account lockouts, password changes. Failed authentication attempts often signal attack attempts. Successful authentication at unusual times or from unusual locations might indicate compromise.

Access events: Who accessed what, when. File access, database queries, application usage. This is the audit trail that answers "did anyone touch that sensitive data?"

Network events: Firewall accepts and denies, intrusion detection alerts, VPN connections, traffic patterns. Network logs reveal reconnaissance, attack attempts, and data leaving your environment.

System events: Startups, shutdowns, service changes, software installations, configuration modifications. Unexpected changes might indicate malware or unauthorized access.

Application events: Security-relevant activities within applications, errors that might indicate exploitation, user actions that matter for security.

Missing logs from any source create blind spots. Attackers find blind spots.

The Volume Problem

Here's the challenge: large environments generate enormous log volumes. Terabytes per day. A single busy web server might produce hundreds of gigabytes daily. Multiply by dozens or hundreds of servers, add network devices, security tools, cloud services—the volume becomes overwhelming.

Buried in those terabytes are the few events that actually matter. The signal hides in noise. A meaningful security event looks identical to millions of routine entries until you know to look for it.

This is why raw logging isn't enough. You need analysis.

SIEM: Making Sense of the Flood

SIEM systems address the volume problem by centralizing collection, normalizing formats, correlating events, and surfacing what matters.

Collection pulls logs from everywhere—agents on systems, direct log forwarding, API integrations with cloud services. Everything flows to a central location.

Normalization translates logs from different sources into common formats. A Windows authentication event and a Linux authentication event look different in their native formats but mean the same thing. Normalization makes them comparable.

Correlation is where SIEM earns its value. Individual events mean little. A failed login? Happens constantly. Port scanning from an IP? Background noise. But a failed login from an IP, followed by port scanning from that same IP, followed by a successful login to a different system, followed by privilege escalation—that's a pattern. That's an attack.

Alerting notifies security teams when correlation rules detect concerning patterns. Good alerting surfaces real threats. Bad alerting generates thousands of false positives that teams learn to ignore.

Correlation Rules: Teaching the System What Matters

SIEM value comes from correlation rules—logic that detects meaningful patterns across log data:

Brute force detection: Many failed logins from a single source, especially if followed by success. Someone guessing passwords until they get one right.

Lateral movement: A user or system accessing many other systems in rapid succession. Normal users access a few systems regularly. Attackers move laterally, looking for valuable targets.

Data exfiltration: Unusual outbound data volumes, especially to unfamiliar destinations. Data leaving your network that shouldn't be leaving.

Beaconing: Regular connections to external IPs at fixed intervals. Malware checking in with command-and-control servers often follows predictable timing patterns.

Privilege escalation: A user suddenly gaining administrative access, especially shortly after authenticating. Either legitimate (and should be expected) or an attacker elevating their access.

Organizations develop rules based on their specific environment and threats. Rule development never ends—new attack techniques require new detection logic.

The Retention Question

How long do you keep logs?

Attacks often aren't discovered for weeks or months. If you only retain logs for 30 days and discover a breach on day 45, you've lost visibility into how it started.

But storage costs money. Retaining everything forever isn't practical.

Most organizations use tiered retention: detailed logs available immediately for 30-90 days, summarized or compressed logs for months to a year, cold storage or deletion after that. Compliance requirements set minimums—PCI DSS mandates three months immediately available and one year archived.

The right answer depends on your threat model, your compliance requirements, and your budget. But err toward longer retention. The cost of missing logs during an investigation exceeds the cost of storage.

Protecting the Evidence

Attackers know logs exist. One of their first actions after gaining access is often to cover their tracks—deleting or modifying logs to hide how they got in and what they did.

Protecting logs requires getting them off compromised systems quickly. Logs transmitted to a central SIEM within seconds can't be deleted by an attacker who compromises the source system minutes later.

Write-once storage makes logs immutable—even administrators can't modify historical logs. Access controls limit who can view logs (they contain sensitive information) and who can configure retention (preventing premature deletion). Integrity monitoring detects any unauthorized modifications.

The goal: even if attackers fully compromise a system, the evidence of how they did it remains intact elsewhere.

Alert Fatigue: When Too Much Becomes Nothing

SIEMs can generate overwhelming alert volumes. Thousands per day. Many false positives. Low-priority events mixed with critical ones.

Security teams facing this flood can't investigate everything. They start ignoring alerts. They develop blind spots. Important alerts get missed because they look like all the other alerts that turned out to be nothing.

This is alert fatigue, and it defeats the purpose of having a SIEM.

Addressing it requires tuning—adjusting correlation rules to reduce false positives, prioritizing alerts by actual risk, automating response to routine events, and focusing human attention on what genuinely matters.

A well-tuned SIEM generates manageable volumes with high signal-to-noise ratio. Security teams can actually investigate alerts because there aren't thousands of them.

From Detection to Response

Detecting a threat is only the beginning. You still need to respond.

Security Orchestration, Automation, and Response (SOAR) platforms extend SIEM with automated response capabilities. When SIEM detects a threat, SOAR can automatically gather additional context from other tools, take containment actions (blocking IPs, isolating systems), create incident tickets, and notify appropriate people.

Automation speeds response from hours to seconds for routine threats, freeing security teams to focus on complex investigations that require human judgment.

During an Incident

When an incident occurs, logs become the ground truth. They answer the questions that matter:

• How did attackers get in?
• When did the compromise begin?
• Which systems were affected?
• What data was accessed?
• Are they still present?

Without comprehensive logs, you're guessing. With them, you can construct a precise timeline. You can understand exactly what happened. You can scope the incident accurately rather than assuming the worst about everything.

Post-incident reviews use logs to identify what failed and what needs improvement. The logs become lessons.