CVE-2024-9423

Executive Summary

CVE-2024-9423 describes a denial of service (DoS) vulnerability in certain HP LaserJet printers. When a user sends a raw JPEG file to the printer, the device displays a “JPEG Unsupported” message that may not clear, potentially blocking all queued print jobs. This effectively renders the printer unavailable for legitimate use until manual intervention (e.g., power cycling or clearing the error) is performed. The vulnerability is triggered by a simple network action—sending a JPEG file directly to the printer’s print queue—requiring no authentication or special privileges. The CVSS score is 5.3 (Medium), with a low attack complexity and no user interaction required. No public exploit code or proof-of-concept has been released, and HP has not yet issued a patch.

CVSS Breakdown & Threat Level

• Attack Vector (AV): Network (N) – The attack is performed remotely over the network.
• Attack Complexity (AC): Low (L) – No special conditions or timing required.
• Privileges Required (PR): None (N) – No authentication needed.
• User Interaction (UI): None (N) – The victim does not need to perform any action.
• Scope (S): Unchanged (U) – The vulnerability does not affect other components.
• Confidentiality (C): None (N) – No data exposure.
• Integrity (I): None (N) – No data modification.
• Availability (A): Low (L) – The printer becomes unavailable for new print jobs.

Threat Level: Medium. The attack is easy to execute remotely, but the impact is limited to denial of service (no data theft or system compromise). The lack of a patch and the potential for disruption in office environments elevate the risk.

Deep Dive & Mechanics

The vulnerability lies in the printer’s firmware handling of raw JPEG files. When a user sends a JPEG file directly to the printer (e.g., via a network print job or a direct TCP connection to port 9100), the printer’s image processing module attempts to parse the file. The firmware does not properly handle the “JPEG Unsupported” error condition. Instead of clearing the error after a timeout or allowing the user to dismiss it, the error message persists indefinitely, blocking the print queue. This means any subsequent print jobs (including legitimate ones) are queued but never processed until the printer is manually reset.

The root cause is a missing or insufficient error recovery mechanism in the printer’s firmware. The printer should either reject the unsupported file gracefully (e.g., by printing an error page and clearing the queue) or provide a timeout that automatically clears the error. Instead, the error state becomes permanent, leading to a denial of service.

Vulnerable vs. Patched Code

No patch is currently available. The vulnerable code is in the printer’s firmware image processing module. A patched version would likely include:

• A timeout mechanism that clears the “JPEG Unsupported” error after a few seconds.
• A queue management routine that discards unsupported files and continues processing valid jobs.
• A user-accessible menu option to clear the error without power cycling.

Since no patch exists, no code comparison can be provided.

Exploitation & Lab Setup

Red Team (Exploitation)

Prerequisites:

• A vulnerable HP LaserJet printer on the same network.
• Ability to send raw data to the printer’s IP address on port 9100 (or the appropriate print port).

Exploitation Steps:

1. Identify the printer’s IP address (e.g., via network scanning).
2. Create a simple JPEG file (any valid JPEG will do, as the printer will reject it).
3. Send the JPEG file directly to the printer using a tool like netcat:

   nc <printer_ip> 9100 < malicious.jpg
   

4. The printer will display “JPEG Unsupported” and become unresponsive to new print jobs.

Note: No public exploit code exists. The above is a conceptual demonstration.

Blue Team (Defense & Detection)

Mitigation Steps:

• Monitor printer logs for repeated “JPEG Unsupported” errors.
• Implement network segmentation to restrict direct printer access to authorized users only.
• Use print server software that validates file types before sending to the printer.
• Disable raw TCP printing (port 9100) if not required.
• Apply firmware updates as soon as HP releases a patch.

Detection:

• Monitor for a high volume of print jobs containing JPEG files.
• Alert on printer error messages indicating “JPEG Unsupported”.

Indicators of Compromise (IOCs)

• Network: Raw JPEG files sent to the printer’s IP address on port 9100.
• Printer Logs: Repeated “JPEG Unsupported” error messages.
• Printer State: Printer displays “JPEG Unsupported” and does not process new jobs.
• File Hashes: Any JPEG file sent to the printer (no specific hash, as any JPEG triggers the issue).

Detection Engineering

YARA Rule

rule CVE_2024_9423_JPEG_Unsupported {
    meta:
        description = "Detects JPEG files sent to HP LaserJet printers that may trigger CVE-2024-9423"
        author = "Valters IT"
        date = "2025-04-10"
        reference = "CVE-2024-9423"
    strings:
        $jpeg_header = { FF D8 FF E0 ?? ?? 4A 46 49 46 00 }  // JPEG SOI + JFIF
        $jpeg_exif = { FF E1 }  // EXIF marker
    condition:
        $jpeg_header or $jpeg_exif
}

Sigma Rule

title: CVE-2024-9423 - JPEG Unsupported DoS on HP LaserJet
id: 12345678-1234-1234-1234-123456789abc
status: experimental
description: Detects raw JPEG files sent to HP LaserJet printers on port 9100
author: Valters IT
date: 2025-04-10
logsource:
    category: network_flow
    product: zeek
detection:
    selection:
        dest_port: 9100
        mime_type: "image/jpeg"
    condition: selection
falsepositives:
    - Legitimate JPEG printing if supported by the printer model
level: medium

Suricata Rule

alert tcp any any -> any 9100 (msg:"CVE-2024-9423 - JPEG Unsupported DoS on HP LaserJet"; flow:to_server,established; content:"|FF D8 FF E0|"; offset:0; depth:4; content:"JFIF"; distance:4; within:4; classtype:attempted-dos; sid:1000001; rev:1;)

Remediation & Mitigation

Immediate Actions:

1. Disable raw TCP printing (port 9100) on the printer if not required.
2. Use a print server that validates file types before forwarding to the printer.
3. Monitor printer logs for “JPEG Unsupported” errors and respond quickly.
4. Implement network segmentation to limit access to printers.

Long-Term Fix:

• Apply firmware updates from HP when available.
• Contact HP support for a workaround or patch status.

Workaround:

• If the error occurs, power cycle the printer or use the printer’s front panel to clear the error (if available).

Valters IT