Sigma writing

SIGMA Rule Writing Guide

SIGMA rules are vendor-agnostic detection rules written in YAML. They can be converted to platform-specific query languages (Splunk SPL, Elastic KQL, Microsoft Sentinel KQL, etc.).

Rule Structure

title: Descriptive title of the detection
id: <UUID>                          # Generate a unique UUID
status: experimental|test|stable
description: >
    Detailed description of what this rule detects and why.
references:
    - https://reference-url.com     # Source intelligence, blog posts, CVE
author: CTI Platform
date: YYYY/MM/DD
modified: YYYY/MM/DD
tags:
    - attack.initial_access         # ATT&CK tactic (lowercase, dots)
    - attack.t1566.001              # ATT&CK technique
    - cve.2024.12345                # CVE if applicable
logsource:
    category: process_creation      # Log category
    product: windows                # OS/product
    service:                        # Optional: specific service
detection:
    selection:
        FieldName|modifier:
            - 'value1'
            - 'value2'
    filter_known_good:
        FieldName: 'legitimate_value'
    condition: selection and not filter_known_good
falsepositives:
    - Description of known false positive scenario
level: critical|high|medium|low|informational

Log Source Categories

Category	Description	Common fields
process_creation	New process started	Image, CommandLine, ParentImage, User
network_connection	Network connection initiated	DestinationIp, DestinationPort, SourceIp
file_event	File created/modified/deleted	TargetFilename, Image
registry_event	Registry key/value change	TargetObject, Details
dns_query	DNS resolution	QueryName, QueryType
image_load	DLL/module loaded	ImageLoaded, Image
pipe_created	Named pipe created	PipeName
ps_script	PowerShell script execution	ScriptBlockText
webserver	Web server access logs	cs-uri-query, c-ip
firewall	Firewall logs	src-ip, dst-ip, dst-port, action

Field Modifiers

Modifier	Description	Example
contains	Substring match	CommandLine|contains: '-enc'
startswith	Starts with	Image|startswith: 'C:\Temp'
endswith	Ends with	Image|endswith: '\powershell.exe'
re	Regex match	CommandLine|re: '.*-e(nc)?.*'
base64offset	Base64 encoded content	CommandLine|base64offset: 'IEX'
all	All values must match	CommandLine|contains|all:
cidr	CIDR range match	DestinationIp|cidr: '10.0.0.0/8'

Detection Logic

Condition operators

• and — All conditions must match
• or — Any condition must match
• not — Negate a condition
• 1 of selection* — Any of the named selections matching selection*
• all of selection* — All named selections must match

Common patterns

Process execution with specific arguments:

detection:
    selection:
        Image|endswith: '\cmd.exe'
        CommandLine|contains:
            - '/c whoami'
            - '/c ipconfig'
            - '/c net user'
    condition: selection

Suspicious parent-child relationship:

detection:
    selection:
        ParentImage|endswith: '\outlook.exe'
        Image|endswith:
            - '\cmd.exe'
            - '\powershell.exe'
            - '\wscript.exe'
    condition: selection

Network connection to suspicious destination:

detection:
    selection:
        DestinationIp|cidr:
            - '185.220.0.0/16'
    filter_internal:
        SourceIp|cidr: '10.0.0.0/8'
    condition: selection and not filter_internal

MITRE ATT&CK Tag Format

tags:
    - attack.tactic_name           # e.g., attack.initial_access
    - attack.tXXXX                 # e.g., attack.t1566
    - attack.tXXXX.XXX            # e.g., attack.t1566.001 (sub-technique)

Common tactic tags: attack.initial_access, attack.execution, attack.persistence, attack.privilege_escalation, attack.defense_evasion, attack.credential_access, attack.discovery, attack.lateral_movement, attack.collection, attack.command_and_control, attack.exfiltration, attack.impact

Quality Checklist

• Unique UUID generated for id field
• ATT&CK technique(s) mapped in tags
• False positives documented
• Level accurately reflects severity
• Log source correctly specified
• Detection logic tested against known-good and known-bad scenarios
• References link to source intelligence
• Description explains WHAT is detected and WHY it matters

Output Location

Write SIGMA rules to: data/detection-rules/sigma/<technique-id>-<slug>.yml