detecting-ransomware-encryption-behavior

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• SKILL.md
• api-reference.md · references/
• agent.py · scripts/

SKILL.md (first lines)

# Detecting Ransomware Encryption Behavior

## When to Use

- Building or tuning a behavioral detection layer for ransomware that catches unknown/zero-day variants
- Monitoring file servers and endpoints for mass encryption activity that evades signature-based detection
- Implementing entropy-based detection to identify when files are being replaced with encrypted (high-entropy) content
- Analyzing suspicious process behavior patterns: rapid sequential file opens, writes, renames, and deletes
- Validating EDR detection rules against actual ransomware encryption patterns during red team exercises

**Do not use** entropy analysis alone as the only detection signal. Compressed files (ZIP, JPEG, MP4) naturally have high entropy and will cause false positives. Always combine entropy with behavioral signals like I/O rate and file rename patterns.

## Prerequisites

- Python 3.8+ with `watchdog` and `psutil` libraries
- Administrative access for process monitoring and file system event capture
- Understanding of Shannon entropy and its application to file content analysis
- Windows: Sysmon installed for detailed process and file system event logging
- Linux: auditd configured for file access monitoring, or inotify-based watchers
- Baseline entropy values for common file types in the monitored environment

## Workflow

### Step 1: Establish Entropy Baselines

Calculate normal entropy ranges for files in the environment:

```
Entropy Baselines by File Type:
━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━
File Type       Normal Entropy    Encrypted Entropy
.docx           3.5 - 6.5        7.8 - 8.0
.xlsx           4.0 - 6.8        7.8 - 8.0
.pdf            5.0 - 7.2        7.8 - 8.0
.txt            2.0 - 5.0        7.8 - 8.0
.csv            2.0 - 5.5        7.8 - 8.0
.sql            2.5 - 5.0        7.8 - 8.0
.jpg/.png       7.0 - 7.9        7.9 - 8.0 (hard to distinguish)
.zip/.7z        7.5 - 8.0        7.9 - 8.0 (hard to distinguish)

Key insight: Text-based files show the largest entropy jump when encrypted,
making them the best candidates for entropy-based detection.
```

### Step 2: Implement Real-Time Entropy Monitoring

Monitor file writes and calculate entropy of new content:

```python
import math
from collections import Counter

def shannon_entropy(data):
    """Calculate Shannon entropy of byte data (0.0 to 8.0 scale)."""
    if not data:
        return 0.0
    freq = Counter(data)
    length = len(data)
    return -sum((c / length) * math.log2(c / length) for c in freq.values())

def is_encryption_entropy(data, threshold=7.5):
    """Check if data entropy indicates encryption."""
    entropy = shannon_entropy(data)
    return entropy >= threshold, entropy
```

### Step 3: Monitor File System I/O Patterns

Track process-level file operations for ransomware patterns:

```
Ransomware I/O Behavior Signatures:
━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━
1. Rapid sequential file modification:
   - >20 files modified per minute by single process
   - Read original → Write encrypted → Rename with new extension
   - Pattern: CreateFile → ReadFile → WriteFile → CloseHandle → MoveFile

2. File extension changes:
   - Original: report.docx → Encrypted: report.docx.locked
   - Many extensions changed within short time window

3. Ransom note creation:
   - Same text file (README.txt, DECRYPT.html) created in multiple directories
   - Created immediately after file encryption in each directory

4. Shadow copy deletion:
   - vssadmin.exe delete shadows /all /quiet
   - wmic.exe shadowcopy delete
   - PowerShell: Get-WmiObject Win32_Shadowcopy | Remove-WmiObject

5. Entropy spike pattern:
   - File read: entropy 3.5 (normal document)
   - File write: entropy 7.9 (encrypted content)
   - Delta > 3.0 is strong ransomware indicator
```

### Step 4: Implement Behavioral Scoring

Combine multiple signals into a composite ransomware score:

```python
def calculate_ransomware_score(process_metrics):
    """Score process behavior for ransomware likelihood (0-100)."""
    score = 0

    # High file modification rate
    files_per_min = process_metrics.get("files_modified_per_minute", 0)
    if files_per_min > 50:
        score += 30
    elif files_per_min > 20:
        score += 15

    # Entropy increase in written files
    avg_entropy_delta = process_metrics.get("avg_entropy_delta", 0)
    if avg_entropy_delta > 3.0:
        score += 30
    elif avg_entropy_delta > 2.0:
        score += 15

From mukul975/Anthropic-Cybersecurity-Skills, Apache-2.0.

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