Performing Authenticated Vulnerability Scan

Core Concepts

Why Authenticated Scanning

Unauthenticated scanning can only assess externally visible services and banners, often leading to:

• Missed vulnerabilities in locally installed software
• Inaccurate version detection from banner changes
• Inability to check patch levels, configurations, or local policies
• Higher false positive rates due to inference-based detection

Authenticated scanning resolves these by directly querying the target OS.

Credential Types by Platform

Linux/Unix Systems

• SSH Key Authentication: RSA/Ed25519 key pairs (recommended)
• SSH Username/Password: Fallback for systems without key-based auth
• Sudo/Su Elevation: Non-root user with sudo privileges
• Certificate-based SSH: X.509 certificates for enterprise environments

Windows Systems

• SMB (Windows): Domain or local admin credentials
• WMI: Windows Management Instrumentation queries
• WinRM: Windows Remote Management (HTTPS preferred)
• Kerberos: Domain authentication with service tickets

Network Devices

• SNMP v3: USM with authentication and privacy (AES-256)
• SSH: For Cisco IOS, Juniper JunOS, Palo Alto PAN-OS
• API Tokens: REST API for modern network platforms

Databases

• Oracle: SYS/SYSDBA credentials or TNS connection
• Microsoft SQL Server: Windows auth or SQL auth
• PostgreSQL: Role-based authentication
• MySQL: User/password with SELECT privileges

Workflow

Step 1: Create Dedicated Service Accounts

# Linux: Create scan service account
sudo useradd -m -s /bin/bash -c "Vulnerability Scanner Service Account" nessus_svc
sudo usermod -aG sudo nessus_svc

# Configure sudo for passwordless specific commands
echo 'nessus_svc ALL=(ALL) NOPASSWD: /usr/bin/dpkg -l, /usr/bin/rpm -qa, \
/bin/cat /etc/shadow, /usr/sbin/dmidecode, /usr/bin/find' | sudo tee /etc/sudoers.d/nessus_svc

# Generate SSH key pair
sudo -u nessus_svc ssh-keygen -t ed25519 -f /home/nessus_svc/.ssh/id_ed25519 -N ""

# Distribute public key to targets
for host in $(cat target_hosts.txt); do
    ssh-copy-id -i /home/nessus_svc/.ssh/id_ed25519.pub nessus_svc@$host
done

# Windows: Create scan service account via PowerShell
New-ADUser -Name "SVC_VulnScan" `
    -SamAccountName "SVC_VulnScan" `
    -UserPrincipalName "[email protected]" `
    -Description "Vulnerability Scanner Service Account" `
    -PasswordNeverExpires $true `
    -CannotChangePassword $true `
    -Enabled $true `
    -AccountPassword (Read-Host -AsSecureString "Enter Password")

# Add to local Administrators group on targets via GPO or:
Add-ADGroupMember -Identity "Domain Admins" -Members "SVC_VulnScan"
# For least privilege, use a dedicated GPO for local admin rights instead

# Enable WinRM on targets
Enable-PSRemoting -Force
Set-Item WSMan:\localhost\Service\AllowRemote -Value $true
winrm set winrm/config/service '@{AllowUnencrypted="false"}'

Step 2: Configure Scanner Credentials

Nessus Configuration

{
  "credentials": {
    "add": {
      "Host": {
        "SSH": [{
          "auth_method": "public key",
          "username": "nessus_svc",
          "private_key": "/path/to/id_ed25519",
          "elevate_privileges_with": "sudo",
          "escalation_account": "root"
        }],
        "Windows": [{
          "auth_method": "Password",
          "username": "DOMAIN\\SVC_VulnScan",
          "password": "stored_in_vault",
          "domain": "domain.local"
        }],
        "SNMPv3": [{
          "username": "nessus_snmpv3",
          "security_level": "authPriv",
          "auth_algorithm": "SHA-256",
          "auth_password": "stored_in_vault",
          "priv_algorithm": "AES-256",
          "priv_password": "stored_in_vault"
        }]
      }
    }
  }
}

Step 3: Validate Credential Access

# Test SSH connectivity
ssh -i /path/to/key -o ConnectTimeout=10 nessus_svc@target_host "uname -a && sudo dpkg -l | head -5"

# Test WinRM connectivity
python3 -c "
import winrm
s = winrm.Session('target_host', auth=('DOMAIN\\\\SVC_VulnScan', 'password'), transport='ntlm')
r = s.run_cmd('systeminfo')
print(r.std_out.decode())
"

# Test SNMP v3 connectivity
snmpwalk -v3 -u nessus_snmpv3 -l authPriv -a SHA-256 -A authpass -x AES-256 -X privpass target_host sysDescr.0

Step 4: Run Authenticated Scan

Configure and launch the scan using the Nessus API:

# Create scan with credentials
curl -k -X POST https://nessus:8834/scans \
  -H "X-Cookie: token=$TOKEN" \
  -H "Content-Type: application/json" \
  -d '{
    "uuid": "'$TEMPLATE_UUID'",
    "settings": {
      "name": "Authenticated Scan - Production",
      "text_targets": "192.168.1.0/24",
      "launch": "ON_DEMAND"
    },
    "credentials": {
      "add": {
        "Host": {
          "SSH": [{"auth_method": "public key", "username": "nessus_svc", "private_key": "/keys/id_ed25519"}],
          "Windows": [{"auth_method": "Password", "username": "DOMAIN\\SVC_VulnScan", "password": "vault_ref"}]
        }
      }
    }
  }'

Step 5: Verify Credential Success

After scan completion, check credential verification results:

• Plugin 19506 (Nessus Scan Information): Shows credential status
• Plugin 21745 (OS Security Patch Assessment): Confirms local checks
• Plugin 117887 (Local Security Checks): Credential verification
• Plugin 110385 (Nessus Credentialed Check): Target-level auth status

Credential Security Best Practices

1. Use a secrets vault (HashiCorp Vault, CyberArk, AWS Secrets Manager) for credential storage
2. Rotate credentials every 90 days or after personnel changes
3. Principle of least privilege - only grant minimum required access
4. Audit credential usage - monitor service account login events
5. Encrypt in transit - use SSH keys over passwords, WinRM over HTTPS
6. Separate accounts per scanner - never share credentials across tools
7. Disable interactive login for scan service accounts where possible
8. Log all authentication events for scan accounts in SIEM

Common Pitfalls

• Using domain admin accounts instead of least-privilege service accounts
• Storing credentials in plaintext scan configurations
• Not testing credentials before scan launch (leads to wasted scan windows)
• Forgetting to configure sudo/elevation for Linux targets
• Windows UAC blocking remote credentialed checks
• Firewall rules blocking WMI/WinRM/SSH between scanner and targets
• Credential lockout from multiple failed authentication attempts

Related Skills

• scanning-infrastructure-with-nessus
• performing-network-vulnerability-assessment
• implementing-continuous-vulnerability-monitoring