trivy-offline-vulnerability-scanning

Trivy Offline Vulnerability Scanning

This skill provides guidance on using Trivy, an open-source security scanner, to discover vulnerabilities in software dependencies using offline mode.

Overview

Trivy is a comprehensive vulnerability scanner that can analyze various targets including container images, filesystems, and dependency lock files. Offline scanning is crucial for:

• Air-gapped environments without internet access
• Reproducible security audits with fixed vulnerability databases
• Faster CI/CD pipelines avoiding network latency
• Compliance requirements for controlled environments

Why Offline Mode?

Challenges with Online Scanning

• Network dependency introduces failure points
• Database updates can cause inconsistent results across runs
• Slower execution due to download times
• Security policies may restrict external connections

Benefits of Offline Scanning

• Reproducibility: Same database = same results
• Speed: No network overhead
• Reliability: No external dependencies
• Compliance: Works in restricted environments

Trivy Database Structure

Trivy's vulnerability database consists of:

• trivy.db: SQLite database containing CVE information
• metadata.json: Database version and update timestamp

Database location: <cache-dir>/db/trivy.db

Offline Scanning Workflow

Step 1: Verify Database Existence

Before scanning, ensure the offline database is available:

import os
import sys

TRIVY_CACHE_PATH = './trivy-cache'

# Check for database file
db_path = os.path.join(TRIVY_CACHE_PATH, "db", "trivy.db")
if not os.path.exists(db_path):
    print(f"[!] Error: Trivy database not found at {db_path}")
    print("    Download database first with:")
    print(f"    trivy image --download-db-only --cache-dir {TRIVY_CACHE_PATH}")
    sys.exit(1)

Step 2: Construct Trivy Command

Key flags for offline scanning:

Flag	Purpose
fs <target>	Scan filesystem/file (e.g., package-lock.json)
--format json	Output in JSON format for parsing
--output <file>	Save results to file
--scanners vuln	Scan only for vulnerabilities (not misconfigs)
--skip-db-update	Critical: Do not update database
--offline-scan	Enable offline mode
--cache-dir <path>	Path to pre-downloaded database

import subprocess

TARGET_FILE = 'package-lock.json'
OUTPUT_FILE = 'trivy_report.json'
TRIVY_CACHE_PATH = './trivy-cache'

command = [
    "trivy", "fs", TARGET_FILE,
    "--format", "json",
    "--output", OUTPUT_FILE,
    "--scanners", "vuln",
    "--skip-db-update",      # Prevent online updates
    "--offline-scan",         # Enable offline mode
    "--cache-dir", TRIVY_CACHE_PATH
]

Step 3: Execute Scan

try:
    result = subprocess.run(
        command,
        capture_output=True,
        text=True,
        check=False  # Don't raise exception on non-zero exit
    )
    
    if result.returncode != 0:
        print("[!] Trivy scan failed:")
        print(result.stderr)
        sys.exit(1)
    
    print("[*] Scan completed successfully")
    print(f"[*] Results saved to: {OUTPUT_FILE}")
    
except FileNotFoundError:
    print("[!] Error: 'trivy' command not found")
    print("    Install Trivy: https://aquasecurity.github.io/trivy/latest/getting-started/installation/")
    sys.exit(1)

Complete Example

import os
import sys
import subprocess

def run_trivy_offline_scan(target_file, output_file, cache_dir='./trivy-cache'):
    """
    Execute Trivy vulnerability scan in offline mode.
    
    Args:
        target_file: Path to file to scan (e.g., package-lock.json)
        output_file: Path to save JSON results
        cache_dir: Path to Trivy offline database
    """
    print(f"[*] Starting Trivy offline scan...")
    print(f"    Target: {target_file}")
    print(f"    Database: {cache_dir}")
    
    # Verify database exists
    db_path = os.path.join(cache_dir, "db", "trivy.db")
    if not os.path.exists(db_path):
        print(f"[!] Error: Database not found at {db_path}")
        sys.exit(1)
    
    # Build command
    command = [
        "trivy", "fs", target_file,
        "--format", "json",
        "--output", output_file,
        "--scanners", "vuln",
        "--skip-db-update",
        "--offline-scan",
        "--cache-dir", cache_dir
    ]
    
    # Execute
    try:
        result = subprocess.run(command, capture_output=True, text=True)
        
        if result.returncode != 0:
            print("[!] Scan failed:")
            print(result.stderr)
            sys.exit(1)
        
        print("[*] Scan completed successfully")
        return output_file
        
    except FileNotFoundError:
        print("[!] Trivy not found. Install from:")
        print("    https://aquasecurity.github.io/trivy/")
        sys.exit(1)

# Usage
if __name__ == "__main__":
    run_trivy_offline_scan(
        target_file='package-lock.json',
        output_file='trivy_report.json'
    )

JSON Output Structure

Trivy outputs vulnerability data in this format:

{
  "Results": [
    {
      "Target": "package-lock.json",
      "Vulnerabilities": [
        {
          "VulnerabilityID": "CVE-2021-44906",
          "PkgName": "minimist",
          "InstalledVersion": "1.2.5",
          "FixedVersion": "1.2.6",
          "Severity": "CRITICAL",
          "Title": "Prototype Pollution in minimist",
          "PrimaryURL": "https://avd.aquasec.com/nvd/cve-2021-44906",
          "CVSS": {
            "nvd": { "V3Score": 9.8 }
          }
        }
      ]
    }
  ]
}

Common Issues

Issue: "failed to initialize DB"

Cause: Database not found or corrupted
Solution: Re-download database or check --cache-dir path

Issue: Scan finds no vulnerabilities when they exist

Cause: Database is outdated
Solution: Download newer database (before going offline)

Issue: "command not found: trivy"

Cause: Trivy not installed or not in PATH
Solution: Install Trivy following official documentation

Dependencies

Required Tools

• Trivy: Version 0.40.0 or later recommended

  # Installation (example for Debian/Ubuntu)
  wget -qO - https://aquasecurity.github.io/trivy-repo/deb/public.key | apt-key add -
  echo "deb https://aquasecurity.github.io/trivy-repo/deb $(lsb_release -sc) main" | tee -a /etc/apt/sources.list.d/trivy.list
  apt-get update
  apt-get install trivy
  

Python Modules

• subprocess (standard library)
• os (standard library)
• sys (standard library)

References

• Trivy Documentation
• Trivy Offline Mode Guide
• CVE Database