building-detection-rules-with-sigma

What this skill does

# 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):

```yaml
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:

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

# Validate rule
sigma check rule.yml
```

Alternatively, validate with Python:

```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:**

```python
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:
```spl
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):**

```python
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:**

```python
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:

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

Track detection coverage using the ATT&CK Navigator:

```python
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": technique_id,
                        "color": "#31a354",
                        "score": 1
                    })

with open("coverage_layer.json", "w") as f:
    json.dump(layer, f, indent=2)
```

### Step 5: Test Rule Against Sample Data

Create test data and validate the rule catches the expected events:

```bash
# Use sigma test framework
sigma test rule.yml --target splunk --pipeline splunk_windows

# Or manually test in Splunk with sample data
# Upload Sysmon process_access log with known Mimikatz signature
```

Validate false positive rate by running against 7 days of production data in a non-alerting saved search.

### Step 6: Deploy to Production SIEM

Deploy the converted query as a scheduled search or correlation rule:

**Splunk ES Correlation Search:**
```spl
| tstats summariesonly=true count from datamodel=Endpoint.Processes
  where Processes.process_name="*\\lsass.exe"
  by Processes.src, Processes.user, Processes.process_name, Processes.parent_process_name
| `drop_dm_object_name(Processes)`
| where count > 0
```

**Elastic Security Rule (TOML format):**
```toml
[rule]
name = "LSASS Memory Access - Credential Dumping"
description = "Detects suspicious access to LSASS process memory"
risk_score = 73
severity = "high"
type = "eql"
query = '''
process where event.action == "access" and
  process.name == "lsass.exe" and
  not process.executable : ("*\\svchost.exe", "*\\csrss.exe")
'''

[rule.threat]
framework = "MITRE ATT&CK"
[[rule.threat.technique]]
id = "T1003"
name = "OS Credential Dumping"
```

### Step 7: Version Control and CI/CD Integration

Store rules in Git with automated testing:

```yaml
# .github/workflows/sigma-ci.yml
name: Sigma Rule CI
on: [push, pull_request]
jobs:
  validate:
    runs-on: ubuntu-latest
    steps:
      - uses: actions/checkout@v4
      - uses: actions/setup-python@v5
        with:
          python-version: '3.11'
      - run: pip install pySigma pySigma-validators-sigmaHQ
      - run: sigma check rules/
      - run: sigma convert -t splunk -p splunk_windows rules/ > /dev/null
```

## Key Concepts

| Term | Definition |
|------|-----------|
| **Sigma** | Vendor-agnostic detection rule format (YAML-based) that compiles to SIEM-specific queries via backends |
| **pySigma** | Python library replacing legacy sigmac for rule conversion, validation, and pipeline processing |
| **Backend** | pySigma plugin that translates Sigma detection logic into a target platform query language (SPL, KQL, Lucene) |
| **Pipeline** | Field mapping configuration that translates generic Sigma field names to SIEM-specific field names |
| **Logsource** | Sigma rule section defining the category (process_creation, network_connection) and product (windows, linux) of the target data |
| **Detection-as-Code** | Practice of managing detection rules in version control with CI/CD test