Custom Signatures Skill

Goal: Help users create, format, and deploy custom detection content for THOR.

Overview

THOR processes all files in the ./custom-signatures folder. The file extension and filename tags determine how each file is interpreted:

Simple IOCs

Filename tags determine IOC type. Tag is detected via regex \Wc2\W (word boundary match).

Rules

YARA Rules

Critical: The filename determines how THOR initializes the rule. See YARA Rules Reference for full details.

Common mistake: Using limit = "ScheduledTasks" without keyword in the filename. This causes the rule to be initialized as a generic rule (file/memory scanner), which won't match module output like scheduled task names.

Sigma Rules

Applied to Windows Eventlogs and log files. By default only high and critical levels shown.

STIX v2

Supports file observables (name, path, hashes, size, timestamps) and registry key observables.

THOR-Specific YARA Enhancements

Score Attribute

meta:
    score = 80  // Default is 75 if not specified

External Variables

Available in generic and meta YARA rules:

Restriction Attributes

meta:
    type = "memory"      // or "file" - restrict to memory/file only
    limit = "Mutex"      // Restrict to specific module
    nodeepdive = 1       // Exclude from DeepDive
    falsepositive = 1    // Reduce score instead of add

Reference Documentation

• Simple IOCs - Hash, filename, C2, keyword IOC formats
• YARA Rules - Generic and specific YARA rules for THOR
• Sigma Rules - Log detection with Sigma
• STIX IOCs - STIX v2 indicator format

Examples

• examples/hash-iocs.md - Hash IOC file examples
• examples/filename-iocs.md - Filename/path IOC patterns
• examples/yara-enhanced.md - YARA rules with THOR externals

Quick Reference

File Naming

# Good - tag detected
case22-c2-domains.txt       ✓ (c2 tag)
misp-export-hashes.txt      ✓ (hashes tag)
incident-filename-iocs.txt  ✓ (filename tag)

# Bad - tag not detected
myc2iocs.txt                ✗ (no word boundary)
filenameiocs.txt            ✗ (no word boundary)

Deployment

# Place files in custom-signatures folder
cp my-hashes.txt /path/to/thor/custom-signatures/

# For YARA rules, use yara subfolder
cp my-rules.yar /path/to/thor/custom-signatures/yara/

# Encrypt sensitive IOCs (optional)
thor-util encrypt --file my-c2-domains.txt
# Creates my-c2-domains.dat

Testing

# Run with custom signatures only
./thor-macosx --customonly -p /target/path

# Verify IOC loading in startup
./thor-macosx 2>&1 | grep -i "custom\|ioc\|signature"