How to remediate – Do not scan printers (AppSocket)

1. Introduction

The vulnerability “Do not scan printers (AppSocket)” refers to a situation where network scanning tools identify devices as printers supporting the AppSocket protocol, and subsequently avoid scanning them due to potential instability issues. This matters because while it prevents crashes or excessive printing, it can leave these devices unassessed for other vulnerabilities. Network printers and multi-function devices are usually affected. A successful exploit could lead to denial of service (DoS) if a scan were attempted, but the primary risk is missed vulnerability detection.

2. Technical Explanation

The remote host appears to be running AppSocket (JetDirect), a protocol used for printer management and communication. Nessus detects this and marks the host as ‘Dead’ to prevent scanning-related issues like crashes or excessive printing. The root cause is the potential instability of printers when subjected to network scans, particularly those using the AppSocket protocol. An attacker could exploit this by intentionally deploying vulnerable printers on a network to disrupt security assessments. Affected platforms include devices running AppSocket/JetDirect services.

• Root cause: Printers react poorly to standard network scanning techniques due to the nature of the AppSocket protocol.
• Exploit mechanism: An attacker could place vulnerable printers on a network, causing scans to be skipped and leaving them unassessed for other vulnerabilities.
• Scope: Network printers and multi-function devices supporting the AppSocket (JetDirect) protocol.

3. Detection and Assessment

To confirm if a system is affected, you can check its network services or use Nessus scan results. A quick check involves identifying devices advertising the JetDirect service. Thorough assessment relies on reviewing Nessus output for hosts marked as ‘Dead’ due to AppSocket detection.

• Quick checks: Use nmap -sV <target_ip> and look for services indicating JetDirect (AppSocket).
• Scanning: Nessus scan results will flag hosts marked as ‘Dead’ due to AppSocket detection.
• Logs and evidence: Review Nessus scan logs for entries stating the host was skipped because it appears to be a printer supporting AppSocket.

nmap -sV 192.168.1.100

4. Solution / Remediation Steps

To fix this issue, enable the ‘Scan Network Printers’ setting in Nessus if you are not concerned about potential problems with scanning printers. This allows Nessus to scan these devices despite the AppSocket protocol detection.

4.1 Preparation

• Backups are generally not required for this change. No services need to be stopped.
• Change window needs: This is a low-risk change and does not typically require formal approval.

4.2 Implementation

1. Step 1: Log in to the Nessus web interface as an administrator.
2. Step 2: Navigate to Settings > Advanced settings.
3. Step 3: Search for ‘Do not scan fragile devices’.
4. Step 4: Expand the ‘Do not scan fragile devices’ block.
5. Step 5: Enable the ‘Scan Network Printers’ setting.
6. Step 6: Save the changes and re-run your scans.

4.3 Config or Code Example

Before

# Do not scan fragile devices
scan_fragile_devices = true

After

# Do not scan fragile devices
scan_fragile_devices = false
scan_network_printers = true

4.4 Security Practices Relevant to This Vulnerability

Asset discovery and regular vulnerability scanning are relevant practices for this issue. Maintaining an accurate asset inventory helps identify all network-connected devices, including printers. Regular scans ensure these devices are assessed for vulnerabilities.

• Practice 1: Asset Inventory – Knowing what is on your network allows you to assess risk appropriately.
• Practice 2: Vulnerability Scanning – Regularly scanning all assets helps identify and remediate potential weaknesses.

4.5 Automation (Optional)

Automation is not typically required for this change, as it involves a simple configuration update within the Nessus interface.

# No automation script provided - this is a UI-based setting.

5. Verification / Validation

To confirm the fix worked, re-run your Nessus scan and verify that printers are now being scanned. Check the scan results to ensure they no longer appear as ‘Dead’ due to AppSocket detection. A simple service smoke test involves confirming basic printer functionality (printing a test page).

• Post-fix check: Re-run Nessus scan and confirm that printers are now being scanned without being marked as ‘Dead’.
• Re-test: Review Nessus logs to ensure the AppSocket detection message is no longer present for these hosts.
• Smoke test: Print a test page from each printer to verify basic functionality remains intact.

# Check Nessus scan logs for 'AppSocket' - should no longer appear for scanned printers.

6. Preventive Measures and Monitoring

For example, maintaining a current asset inventory helps identify all network-connected devices. Implementing a regular vulnerability scanning schedule ensures that these devices are assessed on a consistent basis.

• Baselines: Update your security baseline to include regular scans of all network-connected devices, including printers.
• Pipelines: Integrate Nessus or other scanners into your CI/CD pipeline for automated vulnerability assessments.
• Asset and patch process: Review asset inventory regularly and schedule printer scans at least monthly.

7. Risks, Side Effects, and Roll Back

The main risk is potential instability of printers during scanning, which could lead to crashes or excessive printing. If this occurs, disable the ‘Scan Network Printers’ setting in Nessus to revert to the previous state.

• Risk or side effect 1: Printer crashes or excessive printing if a vulnerable printer is scanned. Mitigation: Disable ‘Scan Network Printers’.
• Roll back:
  1. Log in to the Nessus web interface as an administrator.
  2. Navigate to Settings > Advanced settings.
  3. Search for ‘Do not scan fragile devices’.
  4. Expand the ‘Do not scan fragile devices’ block.
  5. Disable the ‘Scan Network Printers’ setting.
  6. Save the changes.

8. References and Resources

• Vendor advisory or bulletin: Tenable Plugin Details – Do not scan printers (AppSocket)
• NVD or CVE entry: Not applicable, this is a configuration issue within Nessus.
• Product or platform documentation relevant to the fix: Tenable Nessus Advanced Settings