Detection Engineer

Transform malware analysis findings into production-ready detection rules, hunting queries, and operationalized IOCs.

Note: YARA rules are authored in the malware-report-writer skill, not here. This skill covers Sigma rules, Suricata/Snort rules, and hunting queries.

Execution Model

• Start from the evidence, not from the user's memory. Read analysis_state.md, procmon_summary.txt, sysmon_summary.txt, and the tshark exports yourself; every rule below is derived from a specific observed behavior or network artifact, and you cite it in the rule's description/reference.
• Locate skill files. Scripts and reference files ship in this skill's directory. Set R="${CLAUDE_PLUGIN_ROOT:-<dir containing this SKILL.md>}" once (when installed as a plugin $CLAUDE_PLUGIN_ROOT is set; otherwise it is this skill folder). Your working directory is the user's analysis workspace, so prefix every script path below with $R, e.g. python3 "$R"/scripts/ioc_extract.py.
• Write rules to files: detections/sigma/<name>.yml, detections/suricata/<name>.rules, detections/hunting/<platform>.txt, detections/iocs.csv|.json. Create the directories.
• Test what you can, say what you couldn't. Run sigma check and suricata -T (below) when installed; otherwise write status: experimental and note "untested" in the state file. Never claim a rule is validated without output to show.
• Defang with the bundled script: python3 scripts/ioc_extract.py <evidence files> (repo root) produces the deduplicated, defanged list; --format csv|json feeds the export formats; --refang restores live values for rule bodies.
• UUIDs: python3 -c "import uuid; print(uuid.uuid4())" per Sigma rule. SIDs: 1000000+ and unique across the engagement.
• Ask the user only for: target SIEM/EDR platforms, deployment constraints (noise tolerance, log sources actually collected), and sharing scope (TLP).

When to Use This Skill

Use this skill when you need to:

• Write Sigma rules for SIEM detection (Splunk, Elastic, QRadar)
• Create Suricata/Snort rules for network IDS/IPS
• Generate hunting queries for EDR platforms
• Defang IOCs for safe documentation and sharing
• Convert IOCs to standard formats (STIX, OpenIOC, CSV)
• Assess IOC confidence levels and volatility
• Create detection logic from behavioral analysis
• Write threat hunting hypotheses

IOC Management & Defanging

Why Defang IOCs?

Problem: Live IOCs in reports can be:

• Accidentally clicked (execute malware)
• Automatically crawled by bots
• Trigger security tools (email filters, DLP)

Solution: Defang (neutralize) IOCs for safe sharing.

Defanging Patterns

# URLs
http://malicious.com/payload.exe
→ hxxp://malicious[.]com/payload[.]exe

https://evil.tk/login
→ hxxps://evil[.]tk/login

# Domains
malicious.com
→ malicious[.]com

c2-server.example.org
→ c2-server[.]example[.]org

# IPs
192.168.1.100
→ 192[.]168[.]1[.]100

10.0.0.50
→ 10[.]0[.]0[.]50

# Email addresses
attacker@evil.com
→ attacker[@]evil[.]com

phishing@malware.tk
→ phishing[@]malware[.]tk

# File paths (optional)
C:\Windows\System32\malware.exe
→ C:\Windows\System32\malware[.]exe

Automated Defanging

Bundled script (preferred — no install):

python3 scripts/ioc_extract.py evidence/*.txt evidence/*_summary.txt          # defanged, deduplicated, typed
python3 scripts/ioc_extract.py --format csv evidence/*.txt > detections/iocs.csv
python3 scripts/ioc_extract.py --refang detections/iocs_defanged.txt          # live values for Suricata/Sigma bodies

Tool: ioc-fanger (Python)

# Install
pip install ioc-fanger

# Defang
echo "http://malicious.com" | fanger --defang
# Output: hxxp://malicious[.]com

# Refang (restore for testing)
echo "hxxp://malicious[.]com" | fanger --fang
# Output: http://malicious.com

Manual sed/awk:

# Defang URLs and domains
echo "http://malicious.com/payload.exe" | sed 's/http:/hxxp:/g; s/\./[.]/g'

# Defang IPs
echo "192.168.1.100" | sed 's/\./[.]/g'

# Defang emails
echo "attacker@evil.com" | sed 's/@/[@]/g; s/\./[.]/g'

IOC Confidence & Volatility Assessment

Label IOCs appropriately:

### Network Indicators (Medium Confidence - Dynamic)
- Domain: malicious[.]com (C2 server - may rotate)
- IP: 192[.]168[.]1[.]100 (C2 IP - may change)

### Host Indicators (High Confidence - Static)
- Mutex: Global\UniqueMalwareMutex
- Registry: HKCU\Software\Microsoft\Windows\CurrentVersion\Run\Malware
- File Hash: abc123... (SHA256)

Sigma Rule Creation (SIEM Detection)

What is Sigma?

Sigma is a generic signature format for SIEM systems. Write once, convert to Splunk/Elastic/QRadar/ArcSight queries.

Official Repo: https://github.com/SigmaHQ/sigma

Sigma Rule Structure

title: Short Descriptive Title
id: unique-uuid-for-this-rule
status: experimental | test | stable
description: Detailed description of what this detects
references:
    - https://attack.mitre.org/techniques/T1059/001/
author: Your Name
date: 2025-10-26
tags:
    - attack.execution
    - attack.t1059.001
logsource:
    category: process_creation  # or network_connection, file_event, etc.
    product: windows
detection:
    selection:
        Image|endswith: '\powershell.exe'
        CommandLine|contains|all:
            - 'DownloadString'
            - 'Invoke-Expression'
    condition: selection
falsepositives:
    - Legitimate administrative scripts
level: high  # informational, low, medium, high, critical

Common Sigma Logsources

Sigma Modifiers

String Matching:

• |contains - String contains value
• |startswith - String starts with value
• |endswith - String ends with value
• |all - All values must be present
• |re - Regular expression match

Examples:

# Contains any
CommandLine|contains:
    - 'powershell'
    - 'cmd.exe'

# Contains all
CommandLine|contains|all:
    - 'Invoke-WebRequest'
    - '-OutFile'

# Ends with
Image|endswith: '\rundll32.exe'

# Starts with
CommandLine|startswith: 'C:\Windows\System32\'

# Regex
CommandLine|re: '.*\\\\AppData\\\\Local\\\\Temp\\\\[a-z]{8}\.exe'

Example 1: PowerShell Download Cradle

Generate unique UUID: python3 -c "import uuid; print(uuid.uuid4())"

title: Suspicious PowerShell Download and Execute
id: a6e0ee39-d2cb-477d-9223-7b9e6090613a
status: experimental
description: Detects PowerShell downloading content and executing it via Invoke-Expression
references:
    - https://attack.mitre.org/techniques/T1059/001/
    - https://attack.mitre.org/techniques/T1105/
author: Analyst Name
date: 2025-10-26
tags:
    - attack.execution
    - attack.t1059.001
    - attack.command_and_control
    - attack.t1105
logsource:
    category: process_creation
    product: windows
detection:
    selection:
        Image|endswith:
            - '\powershell.exe'
            - '\pwsh.exe'
        CommandLine|contains|all:
            - 'DownloadString'
            - 'IEX'
    condition: selection
falsepositives:
    - Legitimate software deployment scripts
    - Administrative automation
level: high

Example 2: Suspicious Registry Run Key

title: Malware Persistence via Registry Run Key
id: 8ff9a2f5-f996-4405-a5f1-d79d680cb5e6
status: stable
description: Detects creation of registry Run key pointing to suspicious locations
references:
    - https://attack.mitre.org/techniques/T1547/001/
author: Analyst Name
date: 2025-10-26
tags:
    - attack.persistence
    - attack.t1547.001
logsource:
    category: registry_set
    product: windows
detection:
    selection:
        TargetObject|contains: '\Software\Microsoft\Windows\CurrentVersion\Run\'
        Details|contains:
            - '\AppData\Local\Temp\'
            - '\Users\Public\'
            - '\ProgramData\'
            - '%TEMP%'
    condition: selection
falsepositives:
    - Legitimate software installations
level: medium

Example 3: Network Connection to Malicious IP

title: Network Connection to Known C2 Server
id: 54502807-546d-4152-abd4-7c12ac7f9833
status: experimental
description: Detects network connection to known malware C2 IP address
references:
    - Internal malware analysis report
author: Analyst Name
date: 2025-10-26
tags:
    - attack.command_and_control
    - attack.t1071
logsource:
    category: network_connection
    product: windows
detection:
    selection:
        DestinationIp:
            - '192.168.56.101'  # Replace with actual C2 IP
            - '10.0.0.50'
        DestinationPort:
            - 443
            - 8080
    condition: selection
falsepositives:
    - Rare, should be investigated
level: high

Example 4: Suspicious File Creation

title: Malware Dropping Files to Suspicious Location
id: 37ec0b15-debf-46d3-8132-ff7aa637a0b0
status: experimental
description: Detects file creation in common malware drop locations
references:
    - https://attack.mitre.org/techniques/T1105/
author: Analyst Name
date: 2025-10-26
tags:
    - attack.defense_evasion
    - attack.t1105
logsource:
    category: file_event
    product: windows
detection:
    selection:
        TargetFilename|contains:
            - '\AppData\Local\Temp\'
            - '\Users\Public\'
        TargetFilename|endswith:
            - '.exe'
            - '.dll'
            - '.bat'
            - '.vbs'
    condition: selection
falsepositives:
    - Software installations
    - Temporary file creation by legitimate apps
level: low

Convert Sigma to SIEM Queries

Using sigma-cli (modern — replaces legacy sigmac):

# Install sigma-cli plus the backend plugin(s) you need (backends are not bundled)
pip install sigma-cli
sigma plugin install splunk          # also: elasticsearch, qradar, sentinel (azure-monitor), sqlite ...

# Validate syntax and logsource fields first
sigma check rule.yml

# Convert — pass a processing pipeline matching your log source (Sysmon field names differ from raw 4688)
sigma convert -t splunk -p sysmon rule.yml
sigma convert -t elasticsearch -p ecs_windows rule.yml
sigma convert -t qradar -p sysmon rule.yml
sigma convert -t microsoft365defender rule.yml   # no pipeline needed

# List installed backends / pipelines
sigma list targets
sigma list pipelines splunk

Example Conversions:

Splunk:

index=windows EventCode=1
(Image="*\\powershell.exe" OR Image="*\\pwsh.exe")
CommandLine="*DownloadString*" CommandLine="*IEX*"

Elastic:

{
  "query": {
    "bool": {
      "must": [
        {"wildcard": {"process.executable": "*\\\\powershell.exe"}},
        {"wildcard": {"process.command_line": "*DownloadString*"}},
        {"wildcard": {"process.command_line": "*IEX*"}}
      ]
    }
  }
}

Sigma Rule Best Practices

Do:

• Use unique UUIDs (generate with uuidgen or online)
• Include MITRE ATT&CK tags
• List realistic false positives
• Test on real data before deployment
• Use specific conditions (avoid over-matching)
• Document references and context
• Set appropriate severity levels

Don't:

• Use overly broad conditions
• Forget false positive analysis
• Skip testing
• Hardcode environment-specific values
• Ignore performance impact

Suricata Rule Creation (Network IDS)

Suricata Rule Structure

action protocol src_ip src_port -> dest_ip dest_port (rule_options)

Components:

• Action: alert, drop, reject, pass
• Protocol: tcp, udp, icmp, http, dns, tls
• Src/Dest: IP ranges, ports, $variables
• Rule Options: Keywords that define detection logic

Example 1: HTTP C2 Traffic

alert http $HOME_NET any -> $EXTERNAL_NET any (
    msg:"CUSTOM MALWARE Suspicious C2 Checkin";
    flow:established,to_server;
    http.method; content:"POST";
    http.uri; content:"/api/checkin";
    http.user_agent; content:"Mozilla/4.0 (compatible|3b| MSIE 6.0)";
    sid:1000001;
    rev:1;
    metadata:created_at 2025_10_26;
)

Breakdown:

• alert http - Alert on HTTP traffic
• $HOME_NET any -> $EXTERNAL_NET any - Outbound traffic
• flow:established,to_server - Established connection to server
• http.method; content:"POST" - HTTP POST request (sticky buffer)
• http.uri; content:"/api/checkin" - Specific URI path (sticky buffer)
• http.user_agent; content:"..." - Specific User-Agent (sticky buffer)
• sid:1000001 - Signature ID (use 1000000+ for custom rules)
• rev:1 - Revision number

Example 2: DNS C2 Communication

alert dns $HOME_NET any -> any 53 (
    msg:"CUSTOM MALWARE Suspicious DGA Domain Query";
    dns.query; content:".tk"; nocase;
    sid:1000002;
    rev:1;
    metadata:created_at 2025_10_26;
)

Example 3: TLS C2 with SNI

alert tls $HOME_NET any -> $EXTERNAL_NET 443 (
    msg:"CUSTOM MALWARE Known C2 Server Certificate";
    tls.sni; content:"malicious.com";
    tls.cert_subject; content:"CN=Evil Corp";
    sid:1000003;
    rev:1;
    metadata:created_at 2025_10_26;
)

Example 4: Malware Download

alert http $HOME_NET any -> $EXTERNAL_NET any (
    msg:"CUSTOM MALWARE Executable Download from Suspicious TLD";
    flow:established,to_server;
    http.uri; content:".exe"; endswith;
    http.host; content:".tk"; endswith;
    sid:1000004;
    rev:1;
    metadata:created_at 2025_10_26;
)

Suricata HTTP Keywords

• http.method - GET, POST, PUT, etc.
• http.uri - Request URI path
• http.host - Host header
• http.user_agent - User-Agent string
• http.request_body - POST data
• http.response_body - Response content
• http.header - Any HTTP header
• http.stat_code - Response code (200, 404, etc.)

Suricata DNS Keywords

• dns.query - DNS query name
• dns.opcode - DNS operation code
• dns.rcode - DNS response code

Suricata TLS Keywords

• tls.sni - Server Name Indication
• tls.cert_subject - Certificate subject
• tls.cert_issuer - Certificate issuer
• tls.cert_serial - Certificate serial number
• tls.version - TLS version

Testing Suricata Rules

# Test rule syntax
suricata -T -c /etc/suricata/suricata.yaml -S custom.rules

# Run on PCAP
suricata -r sample_traffic.pcapng -S custom.rules -l /var/log/suricata/

# Check alerts
cat /var/log/suricata/fast.log

Suricata Best Practices

Do:

• Use flow keywords (established, to_server, to_client)
• Anchor strings with content modifiers (startswith, endswith)
• Use fast_pattern for performance
• Test against PCAPs before deployment
• Use metadata for rule management
• Include revision tracking

Don't:

• Write overly broad rules (high false positive rate)
• Use regex unless necessary (performance impact)
• Forget to test on benign traffic
• Use conflicting SIDs (must be unique)
• Skip documentation in msg field

Hunting Queries

Splunk Hunting Queries

Hunt for PowerShell Download Cradles:

index=windows EventCode=1
(Image="*\\powershell.exe" OR Image="*\\pwsh.exe")
(CommandLine="*DownloadString*" OR CommandLine="*DownloadFile*" OR CommandLine="*Invoke-WebRequest*")
| table _time, ComputerName, User, CommandLine
| sort -_time

Hunt for Suspicious Registry Run Keys:

index=windows EventCode=13
TargetObject="*\\Software\\Microsoft\\Windows\\CurrentVersion\\Run*"
(Details="*\\AppData\\Local\\Temp\\*" OR Details="*\\Users\\Public\\*" OR Details="*\\ProgramData\\*")
| table _time, ComputerName, TargetObject, Details
| sort -_time

Hunt for Outbound Connections to Rare Destinations:

index=network
| stats count by dest_ip
| where count < 5
| join dest_ip [search index=network]
| table _time, src_ip, dest_ip, dest_port, bytes_out

Elastic (KQL) Hunting Queries

Hunt for Process Injection:

event.code:8 AND
winlog.event_data.TargetImage:(*\\explorer.exe OR *\\svchost.exe) AND
NOT winlog.event_data.SourceImage:C\\:\\Windows\\System32\\*

Hunt for Suspicious File Creations:

event.code:11 AND
file.path:(*\\AppData\\Local\\Temp\\*.exe OR *\\Users\\Public\\*.exe) AND
NOT process.executable:(*\\Windows\\System32\\* OR *\\Program Files\\*)

EDR Hunting (Generic Pseudocode)

Hunt for Credential Access:

Process = "lsass.exe" AND
AccessMask IN (0x1010, 0x1410, 0x1438) AND
SourceImage NOT IN (known_good_processes)

Hunt for Lateral Movement:

Process = "psexec.exe" OR
Process = "wmic.exe" OR
(Process = "powershell.exe" AND CommandLine CONTAINS "Invoke-Command")

IOC Formats & Standards

STIX (Structured Threat Information Expression)

STIX 2.1 Example:

{
  "type": "indicator",
  "spec_version": "2.1",
  "id": "indicator--91ec6a8e-61ab-4c6a-b8f5-95240110b203",
  "created": "2025-10-26T12:00:00.000Z",
  "modified": "2025-10-26T12:00:00.000Z",
  "name": "Malicious Domain: malicious.com",
  "description": "C2 domain for Malware Family X",
  "pattern": "[domain-name:value = 'malicious.com']",
  "pattern_type": "stix",
  "valid_from": "2025-10-26T12:00:00.000Z",
  "labels": ["malicious-activity"]
}

CSV Format (Simple)

ioc_type,ioc_value,confidence,description,first_seen
domain,malicious.com,high,C2 server,2025-10-26
ip,192.168.1.100,medium,C2 IP address,2025-10-26
sha256,abc123...,high,Malware sample hash,2025-10-26
mutex,Global\M12345,high,Mutex name,2025-10-26
registry,HKCU\Software\...\Run,high,Persistence key,2025-10-26

OpenIOC Format

<?xml version="1.0" encoding="UTF-8"?>
<ioc xmlns="http://schemas.mandiant.com/2010/ioc">
  <short_description>Malware Family X IOCs</short_description>
  <description>IOCs from analysis of Malware Family X</description>
  <authored_by>Analyst Name</authored_by>
  <authored_date>2025-10-26T12:00:00</authored_date>
  <definition>
    <Indicator operator="OR">
      <IndicatorItem>
        <Context document="FileItem" search="FileItem/Md5sum"/>
        <Content type="md5">abc123...</Content>
      </IndicatorItem>
      <IndicatorItem>
        <Context document="Network" search="Network/DNS"/>
        <Content type="string">malicious.com</Content>
      </IndicatorItem>
    </Indicator>
  </definition>
</ioc>

Detection Logic Development

From Analysis to Detection

Step 1: Identify Unique Behaviors From dynamic analysis, extract behaviors that are:

• Uncommon in legitimate software
• Hard for attackers to change
• Observable in logs/network traffic

Step 2: Map to Data Sources

Step 3: Write Detection Rule

Choose appropriate rule type:

• Host-based → Sigma rule (SIEM/EDR)
• Network-based → Suricata rule (IDS/IPS)
• File-based → YARA rule (scanning)

Step 4: Test & Validate

• Test on malware sample (must alert)
• Test on benign samples (must not alert)
• Adjust thresholds/conditions
• Document false positive scenarios

Step 5: Deploy & Tune

• Deploy to pilot environment
• Monitor alert volume
• Investigate false positives
• Tune rule based on feedback
• Document tuning changes

Quality Checklist

Before finalizing detection content:

Sigma Rules:

• Unique UUID assigned
• MITRE ATT&CK tags included
• Tested on sample data
• False positives documented
• Appropriate severity level set
• References included
• Logsource correctly specified

Suricata Rules:

• Unique SID assigned (1000000+)
• Tested on PCAP
• Flow keywords used (performance)
• Metadata included
• No syntax errors (suricata -T)
• Tested on benign traffic
• Message clearly describes detection

IOCs:

• All IOCs defanged properly
• Confidence levels assigned
• Volatility assessed
• Context provided for each IOC
• No environment-specific artifacts
• Timestamps included (UTC)
• Format standardized (CSV/STIX/OpenIOC)

Hunting Queries:

• Query tested and returns results
• Performance acceptable (<30s)
• Results actionable
• False positive rate acceptable
• Query documented (purpose, expected results)

Integration with Malware Reports

Detection content appears in multiple report sections:

IOCs Section:

• Defanged IOCs grouped by type
• Confidence ratings
• Context for each indicator

Detection Rules Section:

• YARA rules (from malware-report-writer skill)
• Sigma rules (from this skill)
• Suricata rules (from this skill)

Remediation Section:

• Hunting queries for IR teams
• Detection deployment guidance
• IOC search instructions

Appendix:

• IOC export files (CSV, STIX)
• Sigma rule files (.yml)
• Suricata rule files (.rules)

Tool Quick Reference

Example Usage

User request: "Create detection rules for the ransomware"

What you do:

1. Read analysis_state.md and the dynamic summaries; list the detectable behaviors (vssadmin shadow deletion, mass rename to .locked, ransom-note drop, Run key, C2 POST with fixed UA/URI).
2. Write one Sigma rule per behavior (process_creation, file_event, registry_set) with ATT&CK tags and a real UUID; sigma check them.
3. Write Suricata rules for the C2 HTTP pattern and DNS name with SIDs ≥ 1000000; suricata -T them.
4. Write Splunk/KQL hunting queries for the same behaviors.
5. ioc_extract.py --format csv → detections/iocs.csv; STIX if requested.
6. Record rule paths + test results in analysis_state.md; recommend malware-report-writer next.