Day 19: Sigma Rules — A Universal Language for Detection Engineering

The same attacker behavior must be detected across different security platforms.

A rule written for one SIEM may not work in another because each platform uses its own query language and syntax.

This is where Sigma Rules become incredibly valuable.

Sigma provides an open, vendor-neutral format for describing detection logic that can later be converted into platform-specific queries.

Rather than rewriting the same detection multiple times, security teams can write it once and translate it into the format required by their SIEM or XDR platform.

title: Suspicious PowerShell Encoded Command
logsource:
  product: windows

detection:
  selection:
    Image: powershell.exe
    CommandLine|contains: "-EncodedCommand"
  condition: selection

title: Suspicious PowerShell Encoded Command
logsource:
  product: windows

detection:
  selection:
    Image: powershell.exe
    CommandLine|contains: "-EncodedCommand"
  condition: selection

A simple Sigma Rule describing suspicious PowerShell behavior.

What Are Sigma Rules?

Sigma is an open-source detection rule format written in YAML.

It allows detection engineers to describe suspicious activity in a standardized way without being tied to a specific vendor.

Think of Sigma as the detection equivalent of what YARA is for malware classification — except Sigma focuses on security events and attacker behavior.

Why Sigma Was Created

Every SIEM has its own syntax.

For example:

Microsoft Sentinel uses Kusto Query Language (KQL).
Splunk uses Search Processing Language (SPL).
Elastic Security uses its own query syntax.

Without Sigma, detection engineers would need to maintain separate rules for every platform.

Sigma solves this problem by providing a common detection format that can be converted into platform-specific queries using conversion tools.

Typical Structure of a Sigma Rule

A Sigma Rule usually contains:

Title
Description
Author
Date
Log source
Detection logic
False positives
Severity level
MITRE ATT&CK tags
References

This structure makes rules easier to understand, review, and share.

Example Use Cases

Sigma Rules can detect behaviors such as:

PowerShell executed with encoded commands
Suspicious use of rundll32.exe
Credential dumping attempts
Creation of local administrator accounts
Registry persistence
WMI abuse
Scheduled task creation
LOLBin abuse

These behaviors can then be translated into detections for multiple security platforms.

Sigma and MITRE ATT&CK

Many Sigma Rules include mappings to the MITRE ATT&CK Framework.

For example:

Technique: PowerShell (T1059.001)
Detection: Encoded PowerShell command execution

This helps organizations measure detection coverage and understand which attacker techniques are being monitored.

Sigma in Modern SOC Operations

Detection engineers use Sigma to:

Standardize detections
Share rules with the community
Improve collaboration
Accelerate detection development
Reduce duplicated work

Organizations often maintain internal Sigma repositories alongside their SIEM content.

Sigma vs Platform-Specific Rules

Sigma RulesPlatform-Specific RulesVendor-neutralVendor-specificYAML formatNative query languageEasy to shareLimited portabilityReusable across platformsWorks only within one platform

Sigma doesn't replace platform-specific languages — it complements them by providing a portable starting point.

Key Takeaways

Sigma is an open, vendor-neutral rule format for detection engineering.
It allows security teams to write one detection rule and adapt it for multiple SIEM and XDR platforms.
Sigma improves collaboration, consistency, and portability across cybersecurity tools.
Mapping Sigma Rules to MITRE ATT&CK strengthens threat detection and coverage.

In Day 20, we'll explore YARA Rules, the industry-standard language for identifying malware and suspicious files based on their characteristics, helping analysts detect threats beyond traditional signatures.

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