Stateful Rule Designer

Stateful Rule Designer

You are an expert in designing complex stateful Detection & Response rules in LimaCharlie. Help users create rules that correlate multiple events over time, track process tree relationships, and detect sophisticated attack patterns that require temporal or relational context.

What are Stateful Rules?

Stateful rules track and remember the state of past events to make decisions based on historical context. Unlike stateless rules that evaluate events in isolation, stateful rules can detect patterns over time and relationships between events.

When to Use Stateful Rules

Use stateful rules when you need to:

1. Track Parent-Child Relationships: Detect when a specific process spawns a particular child process
2. Monitor Process Trees: Identify malicious behavior across multiple generations of processes
3. Count Event Occurrences: Alert when an event happens N times within a timeframe
4. Correlate Related Events: Connect events that share a common ancestor or timeframe
5. Detect Multi-Stage Attacks: Identify attack chains that unfold over time

When to Use Stateless Rules

Stateless rules are simpler and more performant. Use them when:

• A single event contains all the information needed for detection
• No temporal or relational context is required
• The detection criteria can be evaluated in isolation

Performance Principle: Always prefer stateless rules unless you specifically need stateful correlation.

Core Stateful Operators

LimaCharlie provides three stateful operators, each designed for different correlation scenarios.

with_child: Matching Immediate Children

The with child operator matches events that are direct children of the initial event.

Basic Example

detect:
  event: NEW_PROCESS
  op: ends with
  path: event/FILE_PATH
  value: cmd.exe
  case sensitive: false
  with child:
    op: ends with
    event: NEW_PROCESS
    path: event/FILE_PATH
    value: calc.exe
    case sensitive: false
respond:
  - action: report
    name: CMD Spawning Calculator

Detects: cmd.exe --> calc.exe (direct child) Does NOT detect: cmd.exe --> firefox.exe --> calc.exe (grandchild)

With Counting

detect:
  event: NEW_PROCESS
  op: ends with
  path: event/FILE_PATH
  value: outlook.exe
  case sensitive: false
  with child:
    op: ends with
    event: NEW_DOCUMENT
    path: event/FILE_PATH
    value: .ps1
    case sensitive: false
    count: 5      # At least 5 PowerShell files
    within: 60    # Within 60 seconds
respond:
  - action: report
    name: Outlook Dropping Multiple PowerShell Scripts
    priority: 4

with_descendant: Matching Any Descendant

The with descendant operator matches events that are descendants at any depth (children, grandchildren, great-grandchildren, etc.).

Basic Example

detect:
  event: NEW_PROCESS
  op: ends with
  path: event/FILE_PATH
  value: cmd.exe
  case sensitive: false
  with descendant:
    op: ends with
    event: NEW_PROCESS
    path: event/FILE_PATH
    value: calc.exe
    case sensitive: false
respond:
  - action: report
    name: CMD Process Tree Contains Calculator

Detects:

• cmd.exe --> calc.exe (direct child)
• cmd.exe --> firefox.exe --> calc.exe (grandchild)
• cmd.exe --> firefox.exe --> notepad --> calc.exe (great-grandchild)

Real-World Example: Office Exploit Chain

detect:
  event: NEW_PROCESS
  op: or
  rules:
    - op: ends with
      path: event/FILE_PATH
      value: winword.exe
      case sensitive: false
    - op: ends with
      path: event/FILE_PATH
      value: excel.exe
      case sensitive: false
  with descendant:
    event: NEW_PROCESS
    op: and
    rules:
      - op: ends with
        path: event/FILE_PATH
        value: powershell.exe
        case sensitive: false
      - op: or
        rules:
          - op: contains
            path: event/COMMAND_LINE
            value: -enc
            case sensitive: false
          - op: contains
            path: event/COMMAND_LINE
            value: downloadstring
            case sensitive: false
respond:
  - action: report
    name: Office Application Process Tree Contains Encoded PowerShell
    priority: 5
  - action: isolate network

with_events: Counting Proximal Events

The with events operator detects repetition of events close together in time on the same sensor. Ideal for threshold-based detections.

Basic Example: Failed Login Attempts

detect:
  event: WEL
  op: is windows
  with events:
    event: WEL
    op: is
    path: event/EVENT/System/EventID
    value: '4625'  # Failed logon
    count: 5
    within: 60
respond:
  - action: report
    name: Multiple Failed Login Attempts
    priority: 3

Port Scanning Detection

detect:
  event: NEW_TCP4_CONNECTION
  op: is platform
  name: windows
  with events:
    event: NEW_TCP4_CONNECTION
    op: and
    rules:
      - op: exists
        path: event/DESTINATION/PORT
      - op: is private address
        path: event/DESTINATION/IP_ADDRESS
        not: true  # External IPs only
    count: 50
    within: 30
respond:
  - action: report
    name: Potential Port Scanning Activity
    priority: 3
    metadata:
      mitre: T1046

Event Selection: Choosing What to Report

By default, stateful rules report the parent event (the initial event that triggered the stateful matching). Use report latest event: true to report the child/descendant instead.

Default Behavior

detect:
  event: NEW_PROCESS
  op: ends with
  path: event/FILE_PATH
  value: outlook.exe
  case sensitive: false
  with child:
    op: ends with
    event: NEW_PROCESS
    path: event/FILE_PATH
    value: chrome.exe
    case sensitive: false
respond:
  - action: report
    name: Outlook Spawning Chrome

Reported event: The outlook.exe NEW_PROCESS event

Report Latest Event

detect:
  event: NEW_PROCESS
  op: ends with
  path: event/FILE_PATH
  value: outlook.exe
  case sensitive: false
  report latest event: true  # Report the child event
  with child:
    op: ends with
    event: NEW_PROCESS
    path: event/FILE_PATH
    value: chrome.exe
    case sensitive: false
respond:
  - action: report
    name: Outlook Spawning Chrome

Reported event: The chrome.exe NEW_PROCESS event

Important: Response actions (like task) always use the latest event in the chain, regardless of the report latest event setting.

Flipping Between Stateful and Stateless

Within a stateful context, all operators are stateful by default, meaning sub-rules can match across different events. Use is stateless: true to require that multiple conditions match the same event.

The Problem

detect:
  event: NEW_PROCESS
  op: ends with
  path: event/FILE_PATH
  value: outlook.exe
  case sensitive: false
  with child:
    op: and
    rules:
      - op: ends with
        event: NEW_PROCESS
        path: event/FILE_PATH
        value: evil.exe
        case sensitive: false
      - op: contains
        path: event/COMMAND_LINE
        value: malicious-flag
        case sensitive: false

Problem: This could match two different child events.

The Solution

detect:
  event: NEW_PROCESS
  op: ends with
  path: event/FILE_PATH
  value: outlook.exe
  case sensitive: false
  with child:
    op: and
    is stateless: true  # Both conditions must match the same child event
    rules:
      - op: ends with
        event: NEW_PROCESS
        path: event/FILE_PATH
        value: evil.exe
        case sensitive: false
      - op: contains
        path: event/COMMAND_LINE
        value: malicious-flag
        case sensitive: false
respond:
  - action: report
    name: Outlook Spawning Evil.exe with Malicious Flag

Now requires: A single child event that is both evil.exe AND has malicious-flag in the command line.

Common Detection Patterns

Pattern 1: Office Document Spawning Shells

detect:
  event: NEW_PROCESS
  op: or
  rules:
    - op: ends with
      path: event/FILE_PATH
      value: winword.exe
      case sensitive: false
    - op: ends with
      path: event/FILE_PATH
      value: excel.exe
      case sensitive: false
  with child:
    event: NEW_PROCESS
    op: or
    rules:
      - op: ends with
        path: event/FILE_PATH
        value: cmd.exe
        case sensitive: false
      - op: ends with
        path: event/FILE_PATH
        value: powershell.exe
        case sensitive: false
respond:
  - action: report
    name: Office Application Spawning Shell
    priority: 4
  - action: task
    command: deny_tree <<routing/this>>

Pattern 2: Brute Force with User Context

detect:
  event: WEL
  op: is windows
  with events:
    event: WEL
    op: and
    rules:
      - op: is
        path: event/EVENT/System/EventID
        value: '4625'
      - op: exists
        path: event/EVENT/EventData/TargetUserName
        truthy: true
    count: 10
    within: 300
respond:
  - action: report
    name: "Brute Force Attack - {{ .event.EVENT.EventData.TargetUserName }}"
    priority: 4
    metadata:
      mitre: T1110
    suppression:
      is_global: false
      keys:
        - '{{ .event.EVENT.EventData.TargetUserName }}'
        - '{{ .routing.sid }}'
      max_count: 1
      period: 1h

Pattern 3: Suspicious Process Tree

detect:
  event: NEW_PROCESS
  op: ends with
  path: event/FILE_PATH
  value: explorer.exe
  case sensitive: false
  with descendant:
    event: NETWORK_CONNECTIONS
    op: is public address
    path: event/NETWORK_ACTIVITY/SOURCE/IP_ADDRESS
    count: 10
    within: 300
respond:
  - action: report
    name: Explorer Process Tree Making Many External Connections
    priority: 3

Performance Best Practices

Stateful rules consume more resources than stateless rules because they maintain state in memory.

Key Guidelines

1. Filter Early: Put the most restrictive conditions in the parent event
2. Use Specific Event Types: Always specify the event type to avoid matching all events
3. Limit Time Windows: Keep within parameters as short as possible
4. Use Platform Filters: Filter by platform early to reduce the event set
5. Avoid Deep Nesting: Limit nesting to 2-3 levels maximum
6. Use Suppression: Always use suppression when triggering sensor commands

Example: Good vs Bad Filtering

Good: Filters to specific process immediately

detect:
  event: NEW_PROCESS
  op: ends with
  path: event/FILE_PATH
  value: outlook.exe  # Very specific
  case sensitive: false
  with child:
    event: NEW_PROCESS  # Specific event type
    op: ends with
    path: event/FILE_PATH
    value: calc.exe

Bad: Matches all processes

detect:
  event: NEW_PROCESS
  op: exists
  path: event/FILE_PATH  # Matches everything!
  with child:
    # Missing event type - will check ALL child events!
    op: ends with
    path: event/FILE_PATH
    value: outlook.exe

Key Reminders

1. Stateful rules are forward-looking only - state starts when the rule is active
2. Modifying a rule resets all state - parent processes must restart
3. Response actions always use the latest event in the chain
4. Use is stateless: true to require conditions match the same event
5. Always use suppression with sensor commands
6. Test thoroughly with unit tests before deploying
7. Keep time windows short to minimize memory usage
8. Filter early with specific parent event criteria

Navigation & Additional Resources

• REFERENCE.md: Complete operator syntax, all parameters, nested logic details
• EXAMPLES.md: 5+ complete attack detection scenarios (ransomware, credential dumping, lateral movement, etc.)
• TROUBLESHOOTING.md: Testing workflows, performance tuning, debugging tips

Quick Reference

This skill provides comprehensive guidance for creating sophisticated stateful detection rules. When helping users, always emphasize testing, performance considerations, and the importance of understanding the attack chain they're trying to detect. For complete details, examples, and troubleshooting, refer to the additional documentation files linked above.