/building-detection-rules-with-sigma

name: building-detection-rules-with-sigma
description: 'Builds vendor-agnostic detection rules using the Sigma rule format for threat detection across SIEM platforms
  including Splunk, Elastic, and Microsoft Sentinel. Use when creating portable detection logic from threat intelligence,
  mapping rules to MITRE ATT&CK techniques, or converting community Sigma rules into platform-specific queries using sigmac
  or pySigma backends.

  '
domain: cybersecurity
subdomain: soc-operations
tags:
- soc
- sigma
- detection-rules
- siem
- mitre-attack
- splunk
- elastic
- sentinel
version: '1.0'
author: mahipal
license: Apache-2.0
d3fend_techniques:
- Execution Isolation
- Process Termination
- Hardware-based Process Isolation
- Web Session Access Mediation
- Process Suspension
nist_csf:
- DE.CM-01
- DE.AE-02
- RS.MA-01
- DE.AE-06

Building Detection Rules with Sigma

When to Use

Use this skill when:

• SOC engineers need to create detection rules portable across multiple SIEM platforms
• Threat intelligence reports describe TTPs requiring new detection coverage
• Existing vendor-specific rules need standardization into a shareable format
• The team adopts Sigma as a detection-as-code standard in CI/CD pipelines

**Do not use** for real-time streaming detection (Sigma is for batch/scheduled searches) or when the target SIEM has native detection features that Sigma cannot express (e.g., Splunk RBA risk scoring).

Prerequisites

• Python 3.8+ with `pySigma` and appropriate backend (`pySigma-backend-splunk`, `pySigma-backend-elasticsearch`, `pySigma-backend-microsoft365defender`)
• Sigma rule repository cloned: `git clone https://github.com/SigmaHQ/sigma.git`
• MITRE ATT&CK framework knowledge for technique mapping
• Understanding of target SIEM log source field mappings

Workflow

Step 1: Define Detection Logic from Threat Intelligence

Start with a threat report or ATT&CK technique. Example: detecting Mimikatz credential dumping (T1003.001 — LSASS Memory):

title: Mimikatz Credential Dumping via LSASS Access
id: 0d894093-71bc-43c3-8d63-bf520e73a7c5
status: stable
level: high
description: Detects process accessing lsass.exe memory, indicative of credential dumping tools like Mimikatz
references:
    - https://attack.mitre.org/techniques/T1003/001/
    - https://github.com/gentilkiwi/mimikatz
author: mahipal
date: 2024/03/15
modified: 2024/03/15
tags:
    - attack.credential_access
    - attack.t1003.001
logsource:
    category: process_access
    product: windows
detection:
    selection:
        TargetImage|endswith: '\lsass.exe'
        GrantedAccess|contains:
            - '0x1010'
            - '0x1038'
            - '0x1fffff'
            - '0x40'
    filter_main_svchost:
        SourceImage|endswith: '\svchost.exe'
    filter_main_csrss:
        SourceImage|endswith: '\csrss.exe'
    filter_main_wininit:
        SourceImage|endswith: '\wininit.exe'
    condition: selection and not 1 of filter_main_*
falsepositives:
    - Legitimate security tools accessing LSASS
    - Windows Defender scanning
    - CrowdStrike Falcon sensor

Step 2: Validate Sigma Rule Syntax

Use `sigma check` to validate the rule:

# Install pySigma and validators
pip install pySigma pySigma-validators-sigmaHQ

# Validate rule
sigma check rule.yml

Alternatively, validate with Python:

from sigma.rule import SigmaRule
from sigma.validators.core import SigmaValidator

rule = SigmaRule.from_yaml(open("rule.yml").read())
validator = SigmaValidator()
issues = validator.validate_rule(rule)
for issue in issues:
    print(f"{issue.severity}: {issue.message}")

Step 3: Convert to Target SIEM Query

**Convert to Splunk SPL:**

from sigma.rule import SigmaRule
from sigma.backends.splunk import SplunkBackend
from sigma.pipelines.splunk import splunk_windows_pipeline

pipeline = splunk_windows_pipeline()
backend = SplunkBackend(pipeline)

rule = SigmaRule.from_yaml(open("rule.yml").read())
splunk_query = backend.convert_rule(rule)
print(splunk_query[0])

Output:

TargetImage="*\\lsass.exe" (GrantedAccess="*0x1010*" OR GrantedAccess="*0x1038*"
OR GrantedAccess="*0x1fffff*" OR GrantedAccess="*0x40*")
NOT (SourceImage="*\\svchost.exe") NOT (SourceImage="*\\csrss.exe")
NOT (SourceImage="*\\wininit.exe")

**Convert to Elastic Query (Lucene):**

from sigma.backends.elasticsearch import LuceneBackend
from sigma.pipelines.elasticsearch import ecs_windows_pipeline

pipeline = ecs_windows_pipeline()
backend = LuceneBackend(pipeline)
elastic_query = backend.convert_rule(rule)
print(elastic_query[0])

**Convert to Microsoft Sentinel KQL:**

from sigma.backends.microsoft365defender import Microsoft365DefenderBackend

backend = Microsoft365DefenderBackend()
kql_query = backend.convert_rule(rule)
print(kql_query[0])

Step 4: Map to MITRE ATT&CK and Add Coverage Metadata

Tag every rule with ATT&CK technique IDs in the `tags` field:

tags:
    - attack.credential_access        # Tactic
    - attack.t1003.001                # Sub-technique
    - attack.t1003                    # Parent technique

Track detection coverage using the ATT&CK Navigator:

import json

# Generate ATT&CK Navigator layer from Sigma rules
layer = {
    "name": "SOC Detection Coverage",
    "versions": {"attack": "14", "navigator": "4.9", "layer": "4.5"},
    "domain": "enterprise-attack",
    "techniques": []
}

# Parse Sigma rules directory for technique tags
import os
from sigma.rule import SigmaRule

for root, dirs, files in os.walk("sigma/rules/windows/"):
    for f in files:
        if f.endswith(".yml"):
            rule = SigmaRule.from_yaml(open(os.path.join(root, f)).read())
            for tag in rule.tags:
                if str(tag).startswith("attack.t"):
                    technique_id = str(tag).replace("attack.", "").upper()
                    layer["techniques"].append({
                        "techniqueID": techniq