Trivy Offline Vulnerability Scanning Skill

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)

Agent Skills: Trivy Offline Vulnerability Scanning

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