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Introduction:
In the high-stakes world of penetration testing, noisy network scanning is often accepted as a necessary evil. However, a simple oversight regarding network-connected printers can transform a routine vulnerability assessment from a digital exercise into a physical incident. The Raw Port 9100 printing protocol, designed in an era before cybersecurity was a concern, lacks basic authentication and treats any incoming TCP traffic as a print job. This article dissects the mechanics of this often-ignored attack surface, providing a technical deep dive into how to identify, exploit, and mitigate this vector, ensuring your next internal scan doesn’t end with a floor full of paper and a very angry facilities manager.
Learning Objectives:
- Understand the inherent security flaws of the Raw Port 9100 (JetDirect) printing protocol.
- Master network discovery techniques to identify printers and open port 9100 using Nmap and other tools.
- Learn how to safely exclude printer IP addresses from automated scanners like Nuclei to prevent denial-of-service (or denial-of-paper).
- Explore command-line techniques for sending test print jobs and assessing printer vulnerabilities.
- Implement network segmentation and access control measures to harden printing infrastructure.
You Should Know:
1. Network Reconnaissance: Identifying the Offenders
Before you can exclude printers, you must find them. The most common mistake is launching a full-port scan (-p-) against an entire subnet without knowing what lives there. Port 9100 is the default for raw printing (HP JetDirect, TCP/IP printing in Windows). Here’s how to identify these devices without triggering a printout.
Step‑by‑step guide to Printer Discovery:
- Passive Discovery (Recommended First Step): Check DHCP logs or your ARP cache to identify vendors. Printers often have identifiable MAC addresses (e.g., HP, Canon, Brother).
- Active Scanning with Nmap: Instead of blasting the network, perform a targeted service scan.
Scan a subnet specifically for port 9100 open and grab the banner nmap -p 9100 --open -sV 192.168.1.0/24
– -p 9100: Only scans the printing port.
– --open: Shows only hosts with the port open.
– -sV: Version detection. This often reveals the printer make and model (e.g., “HP LaserJet”, “Xerox WorkCentre”).
3. Confirming the Device: Once you have an IP, you can verify it’s a printer by checking its web interface (usually ports 80, 443, or 631 for IPP). Opening `http://
- The “Paper DoS”: Exploiting the Protocol (or Avoiding It)
The core issue is that the protocol on port 9100 accepts raw TCP data. To a printer listening on this port, a GET request from an Nmap script or a Nuclei template looks just like a PCL (Printer Command Language) or PostScript file to be rendered.
Step‑by‑step guide to understanding the risk:
- The Unintentional Print: When Nuclei sends a payload to test for a vulnerability (e.g., a CVE check), the printer doesn’t understand the HTTP syntax. It attempts to interpret it as a print job, resulting in pages filled with code.
- Simulating a Print Job (The Right Way to Test): If you need to test if a printer is actually vulnerable to accepting external jobs, use a direct socket connection. Do not use automated scanners for this.
Linux: Send a simple text string to the printer echo "Pentest Test Page - Please Ignore" | nc <printer_ip> 9100 Or send an actual PostScript/PDF file cat testpage.pdf | nc <printer_ip> 9100
Windows PowerShell: Send a text string "Pentest Test Page - Please Ignore" | Out-File -Encoding ASCII -FilePath .\temp.txt; gc .\temp.txt | % { [System.Net.Sockets.TcpClient]::new("<printer_ip>", 9100).GetStream().Write([System.Text.Encoding]::ASCII.GetBytes($_ + "<code>r</code>n"), 0, ($_ + "<code>r</code>n").Length) } - Abusing the Protocol for Persistence: Beyond paper waste, attackers can use port 9100 to change printer settings, upload malicious firmware, or even use the printer’s storage as a persistence mechanism by storing files in print queues.
3. Tool Configuration: Excluding Printers from Nuclei
To prevent the “Paper DoS,” you must configure your scanning tools to ignore these IPs. Nuclei, a powerful vulnerability scanner, allows for several exclusion methods.
Step‑by‑step guide to configuring Nuclei exclusions:
- Create an Exclusion File: List all printer IP addresses you discovered, one per line, in a text file (e.g.,
printer_excludes.txt).192.168.1.50 192.168.1.51 192.168.1.52
- Run Nuclei with the Exclusion Flag: Use the `-exclude-hosts` flag to point to your file.
nuclei -l target_list.txt -t cves/ -exclude-hosts printer_excludes.txt
This command will run Nuclei against your main target list but will skip any host listed in the exclusion file.
- Network Range Exclusion: If printers are in a specific subnet (e.g.,
192.168.100.0/24), you can add the entire range to your exclusion file or exclude it from your target scope at the beginning of the engagement.
4. Windows Configuration: Securing the Print Spooler
While port 9100 is a network protocol on the printer itself, Windows servers often act as print servers. A related risk is the Windows Print Spooler service (RPC, not 9100), which has been the source of critical vulnerabilities like PrintNightmare.
Step‑by‑step guide to hardening Windows Print Server interaction:
- Disable Internet Printing Client: If not needed, disable the Windows Feature “Internet Printing Client” to reduce attack surface.
PowerShell (Admin) Disable-WindowsOptionalFeature -Online -FeatureName Printing-InternetPrinting-Client
2. Group Policy Settings:
- Navigate to:
Computer Configuration -> Administrative Templates -> Printers. - Enable: “Allow Print Spooler to accept client connections” (Set this to only trusted clients if necessary).
- Enable: “Point and Print Restrictions” to restrict driver installation warnings and elevation prompts.
- Network Isolation: Ensure that print servers are not directly accessible from untrusted network segments. Use Windows Firewall rules to restrict inbound traffic to port 9100 (if the server is acting as a raw print server) to only known client subnets.
5. Mitigation: Hardening the Printers Themselves
The ultimate fix lies in the configuration of the printers. The 2016 Bangladesh Bank heist, mentioned in the comments, highlights how disabling logging (or in this case, physical printouts) can mask fraudulent activity. Conversely, a misconfiguration here can expose you.
Step‑by‑step guide to Printer Hardening:
- Access Control: If possible, restrict access to port 9100 via an Access Control List (ACL) on the printer’s network settings or the switch port it’s connected to. Only allow the IP addresses of legitimate print servers.
- Disable Unused Protocols: Log into the printer’s web interface and disable Raw Port 9100 printing if IPP (port 631) or LPD (port 515) is sufficient and provides better authentication/encryption.
- SNMP Hardening: Printers often use SNMP for management. If using SNMPv1 or v2c, change the read/write community strings from the default “public” and “private”.
Example: Using snmpset to change a community string (conceptual) snmpset -v 2c -c public <printer_ip> ... .1.3.6.1.4.1.11.2.3.9.1.1.13.0 s "NewSecureString"
- Firmware Updates: Ensure printer firmware is up-to-date to patch vulnerabilities like remote code execution flaws that have been found in various printer models over the years.
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The Red Team Angle: Lateral Movement via Printers
For a red teamer, printers are a goldmine. They are often forgotten, unmonitored, and reside on the internal network.
Step‑by‑step guide to Post-Exploitation Printer Recon:
- Gather Information: Once on a compromised machine, use `arp -a` to identify likely printer IPs. Use `nmap` from the compromised host to scan for port 9100.
- Extract Data: If you can write to port 9100, you can send a PostScript file that instructs the printer to store a copy of every scanned document on an FTP server, or to retain a copy of every print job in memory for later retrieval.
PostScript Code Snippet (Conceptual):
%!PS % Instruct printer to save all subsequent jobs to internal storage % This is highly manufacturer-specific, but demonstrates the concept. << /JobLog /LogToFile true >> setdeviceparams
3. Maintain Access: Some printers have writable file systems. If you can upload a malicious firmware file, you can create a persistent backdoor inside the network that lives outside the standard Windows/Linux monitoring scope.
What Undercode Say:
- The Obsolete Protocol Trap: The incident highlights a classic cybersecurity pitfall: insecure legacy protocols (like port 9100) that remain operational by default in modern devices, creating a hidden attack surface.
- Physical Impact of Digital Actions: This serves as a critical reminder that cybersecurity vulnerabilities can have direct physical consequences (paper waste, equipment damage), bridging the gap between IT and operational technology (OT) security.
- Context is King in Pentesting: Effective penetration testing isn’t just about running tools; it’s about understanding the environment. A junior pentester sees an open port; a senior pentester sees a printer and adjusts their methodology to prevent collateral damage.
Prediction:
We will see an increase in printer-focused attacks as part of “living off the land” strategies. Attackers will move beyond simple paper jams to exploit printers as a persistent foothold within air-gapped networks. As the market for printer-specific CVEs grows, and as firmware complexity increases, these devices will become a primary target for initial access and long-term data exfiltration, forcing organizations to finally apply the same patching cadence and segmentation policies to printers as they do to servers.
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