Santamon

Lightweight macOS detection sidecar for Santa that evaluates Endpoint Security telemetry locally with CEL rules and forwards only matched detection signals to a backend server.

Experimental. Built for home labs and small fleets. Early release – expect bugs and API changes.

What It Does

Santamon reads Santa's protobuf telemetry stream, evaluates detection rules using CEL expressions, and ships security signals to a backend. Raw telemetry stays on the endpoint—only detections are forwarded.

Core capabilities:

• Local detection: CEL-based rules evaluate events on-device
• Three rule types: Simple matching, time-window correlation, baseline (first-seen)
• Process lineage: Optionally attach full process trees to execution signals
• Embedded state: BoltDB tracks correlations, first-seen data, and signal queue
• Resilient shipping: Concurrent batching, retry logic, circuit breaker

Why Santamon?

Santamon is a detection sidecar for Santa, not another ESF client.

Building a custom ESF tool requires Apple's restricted entitlements, provisioning profiles, and careful handling of high-volume Endpoint Security events. Santa already does this and is battle-tested in production.

Santamon's value:

• Reuses Santa as the ESF sensor layer (no additional entitlements)
• Runs rules locally close to the data, instead of streaming everything
• Lightweight state via BoltDB for correlations and deduplication
• Low infrastructure cost - only ships high-signal detections

Santa handles the heavy lifting of ingesting Endpoint Security events reliably and safely; Santamon focuses on detection logic and signal quality.

Architecture

Santa Spool → Watcher → Decoder → Rules Engine → Signal Generator → Shipper → Backend
                 ↓                      ↓
                ┌────────────────────────┐
                │ State DB (BoltDB)      │
                │ • Correlation windows  │
                │ • Baseline tracking    │
                │ • Signal queue         │
                └────────────────────────┘
                Process lineage: in-memory cache (1h TTL, 50K max)

Data flow:

1. Watcher monitors Santa's spool directory (/var/db/santa/spool/new/) for new protobuf files
2. Decoder reads and decompresses protobuf messages from spool files
3. Rules Engine evaluates events against CEL expressions (simple, correlation, baseline rules)
4. Signal Generator creates context-rich signals for rule matches (with optional process trees)
5. Shipper batches and sends signals to backend via HTTPS with retry logic and circuit breaker
6. State DB persists correlation state, baseline tracking, signal queue, and spool journal

Spool lifecycle:

• Spool files with no detections are deleted after processing to keep Santa's spool from filling
• Files that produced detections are archived to santa.archive_dir (default: /var/lib/santamon/spool_hits)
• Signals include the archived spool path when available so you can retrieve the protobuf if needed

Process Lineage:

• In-memory cache of recent process execution history
• Enables full process tree context for execution detections
• TTL: 1 hour | Max: 50K entries (LRU eviction)
• Boot session isolated (no cross-boot ancestry)
• See RULES.md for usage

Requirements

• macOS 15.4+ (some telemetry types like tcc_modification require macOS 15+)
• Santa with protobuf telemetry from northpolesec/santa
  • See Santa telemetry docs
  • Example config: configs/examples/santa-config.mobileconfig
• Go 1.23+ (for building from source)

Installation

1. Configure Santa for Protobuf Telemetry

Santa must be configured to write protobuf events. Use the provided configuration profile:

# Review and customize, then install via System Settings
open configs/examples/santa-config.mobileconfig

# Verify
santactl status | grep "Log Type"
# Should show: Log Type | protobuf

2. Build Santamon

git clone https://github.com/0x4d31/santamon.git
cd santamon
make build

3. Install System-Wide

sudo make install

This installs:

• Binary: /usr/local/bin/santamon
• Config: /etc/santamon/config.yaml and rules.yaml
• LaunchDaemon: /Library/LaunchDaemons/com.santamon.plist
• State directory: /var/lib/santamon/

4. Configure Backend and API Key

Edit /etc/santamon/config.yaml:

shipper:
  endpoint: "https://your-backend.example.com:8443/ingest"
  api_key: "${SANTAMON_API_KEY}"

Set the API key in the LaunchDaemon plist:

# Generate strong API key
openssl rand -hex 32

# Edit LaunchDaemon
sudo nano /Library/LaunchDaemons/com.santamon.plist

# Add under EnvironmentVariables:
<key>SANTAMON_API_KEY</key>
<string>your-generated-key-here</string>

5. Start

# Start service
sudo make start

# Monitor logs
make logs

Configuration

Main config: /etc/santamon/config.yaml

agent:
  id: "${HOSTNAME}"

shipper:
  endpoint: "https://backend.example.com:8443/ingest"
  api_key: "${SANTAMON_API_KEY}"

santa:
  spool_dir: "/var/db/santa/spool"      # Santa spool location
  archive_dir: "/var/lib/santamon/spool_hits"  # Archive spool files that produced alerts
  stability_wait: "2s"                  # Wait before reading new files

rules:
  path: "/etc/santamon/rules.yaml"      # File or directory

state:
  db_path: "/var/lib/santamon/state.db"
  sync_writes: true                     # Fsync after writes (safer but slower)

  first_seen:
    max_entries: 10000                  # LRU cache for baseline rules

  windows:
    max_events: 1000                    # Max events per correlation window

shipper:
  batch_size: 100                       # Signals per batch
  flush_interval: "30s"                 # Time between flushes
  timeout: "10s"                        # HTTP request timeout
  tls_skip_verify: false                # NEVER true in production

See configs/santamon.yaml for all options with detailed comments.

Detection Rules

Rules are CEL expressions that evaluate Santa events. Three types supported: simple, correlation, and baseline.

rules:
  - id: SM-014
    title: "Non-interactive process invoking curl/wget"
    description: |
      Non-terminal, non-package-manager process launching curl or wget.
    expr: |
      kind == "execution" &&
      event.execution.target.executable.path in ["/usr/bin/curl", "/usr/bin/wget"] &&

      // Exclude interactive shells
      !(
        event.execution.instigator.executable.path.startsWith("/bin/bash") ||
        event.execution.instigator.executable.path.startsWith("/bin/zsh") ||
        event.execution.instigator.executable.path.startsWith("/bin/sh")
      ) &&

      // Exclude Homebrew / package-manager helpers that legitimately use curl frequently
      !(
        event.execution.instigator.executable.path.startsWith("/opt/homebrew/") ||
        event.execution.instigator.executable.path.contains("/Homebrew/")
      )
    severity: high
    tags: ["T1105", "command-and-control"]
    extra_context: ["event.execution.args"]
    include_process_tree: true
    enabled: true