Log Types, Formats, and Standards: A Security Analyst’s Guide

1. Objective

Logs are essential for understanding the operation, performance, and security of a system. Their analysis helps identify errors, track activities, detect security incidents, and support investigations.

2. Main Log Types

The most common log types cover the majority of operational and security needs.

Application Logs

These logs record events related to an application, such as startup, shutdown, errors, alerts, transactions, or unexpected behavior.

Audit Logs

These logs track important actions performed on a system or application. They are especially useful for compliance, internal controls, and traceability.

Security Logs

These logs document security-related events, such as logins, authentication failures, permission changes, firewall alerts, or suspicious activities.

Server Logs

These logs group together the events generated by servers, such as access logs, error logs, system events, or hosted services.

System Logs

These logs relate to the operating system, including startup, shutdown, kernel activity, system errors, or hardware status.

Network Logs

These logs capture connections, traffic flows, packets, network access, and events related to devices or communications.

Database Logs

These logs record database activities, such as queries, modifications, transactions, or errors.

Web Server Logs

These logs track HTTP requests handled by a web server, including requested URLs, response codes, source IP addresses, HTTP methods, and related details.

3. Why Understanding Logs Matters

Understanding log types and formats is essential for:

• analyzing events effectively;
• accelerating troubleshooting;
• optimizing performance;
• improving incident detection;
• supporting incident response and threat hunting.

4. Main Log Formats

A log format defines how data is organized within a file or logging stream. Three major categories can be identified.

4.1 Semi-Structured Logs

These logs combine predictable elements with free-text content.

Examples:

• System logs and network logs are often generated in Syslog format.
• Security logs, application logs, and system logs on Windows are often stored in Windows Event Log (EVTX) format.

Advantages:

• widely used;
• relatively readable;
• suitable for many technical environments.

Limitations:

• parsing can be more complex than with strictly structured formats;
• content may vary depending on the source.

4.2 Structured Logs

These logs follow a clear and standardized schema, which makes automated processing easier.

Examples:

• Web server logs may be generated in W3C Extended Log Format (ELF), especially on Microsoft IIS.
• Application logs, security logs, or database logs may be exported in JSON, CSV, or XML.
• Some monitoring tools or collection pipelines convert logs into structured formats to simplify SIEM integration.

Advantages:

• easy to parse;
• well suited for SIEM platforms, collection pipelines, and analysis scripts;
• easier to normalize and correlate.

Limitations:

• may require a strict schema definition;
• some formats, such as XML, can be more verbose.

4.3 Unstructured Logs

These logs consist mainly of free text with little standardization.

Examples:

• Web server logs from Apache are often stored in NCSA Common Log Format (CLF).
• Web server logs from Apache or Nginx may also use Combined Log Format, with additional fields such as referrer and user-agent.
• Some legacy server logs or application logs may also be stored as free text.

Advantages:

• rich in context;
• widely used in web environments.

Limitations:

• more difficult to process at scale;
• often require regular expressions or dedicated parsers.

5. Custom Log Formats

Some organizations use log formats tailored to their applications or business requirements. This approach provides flexibility, but it often requires:

• dedicated parsers;
• custom normalization rules;
• clear documentation to ensure effective use and interpretation.

6. Difference Between Format and Standard

Log Format

A log format describes the technical structure of a log, including fields, order, separators, and encoding.

Log Standard

A log standard defines a broader framework, including which events should be logged, how logs should be transmitted, how they should be stored, how they should be protected, and how long they should be retained.

7. Main Standards and References

Common Event Expression (CEE)

Provides a common structure to simplify the generation, transmission, storage, and analysis of logs.

OWASP Logging Cheat Sheet

A best-practice guide for designing effective application logging, especially from a security perspective.

Syslog Protocol

A standard for log transmission and message handling.

NIST SP 800–92

An important reference for log management in information security.

Azure Monitor Logs

Logging guidance for Microsoft Azure environments.

Google Cloud Logging

A reference for log collection and monitoring in GCP environments.

Oracle Cloud Infrastructure Logging

A logging framework for OCI environments.

Virginia Tech Logging Standard

An example of a logging review and compliance guideline.

8. Key Best Practices

For effective log management, it is recommended to:

• centralize logs;
• normalize formats whenever possible;
• protect the integrity and confidentiality of logs;
• define an appropriate retention policy;
• log critical security and administrative events;
• document log sources and their purpose;
• regularly verify the quality and usability of collected data.

9. Conclusion

Logs are an essential source of information for operations, security, and compliance. Understanding log types, formats, and standards helps improve visibility across environments, strengthen incident analysis, and enhance detection and investigation capabilities.