CVE-2026-25819: HMS Networks Ewon DoS Vulnerability

CVE-2026-25819 Overview

HMS Networks Ewon Flexy and Cosy+ industrial remote access devices are vulnerable to an unauthenticated Denial of Service (DoS) attack. The vulnerability exists in devices running vulnerable firmware versions and can be exploited by sending specially crafted HTTP requests to the device's web-based GUI, causing the device to reboot unexpectedly.

This vulnerability affects critical industrial communication infrastructure commonly deployed in operational technology (OT) environments for remote monitoring and maintenance of industrial equipment.

Critical Impact

Unauthenticated attackers with network access to the device GUI can cause repeated device reboots, disrupting industrial remote access connectivity and potentially impacting operational continuity.

Affected Products

• HMS Networks Ewon Flexy with firmware before 15.0s4
• HMS Networks Cosy+ with firmware 22.xx before 22.1s6
• HMS Networks Cosy+ with firmware 23.xx before 23.0s3

Discovery Timeline

• 2026-03-09 - HMS Networks releases Security Advisory
• 2026-03-13 - CVE-2026-25819 published to NVD
• 2026-03-16 - Last updated in NVD database

Technical Details for CVE-2026-25819

Vulnerability Analysis

This vulnerability is classified as CWE-400 (Uncontrolled Resource Consumption), indicating that the affected devices fail to properly handle malformed HTTP requests. When an attacker sends a specially crafted HTTP request to the device's web-based graphical user interface, the device is unable to process it correctly, leading to resource exhaustion and an automatic reboot.

The attack does not require authentication, meaning any attacker with network access to the device's GUI interface can trigger the condition. This is particularly concerning for devices that may be exposed to less-trusted network segments or inadvertently accessible from the internet.

Root Cause

The root cause is improper input validation and resource management in the HTTP request handler of the affected firmware versions. The device fails to adequately sanitize or limit incoming HTTP requests, allowing maliciously crafted payloads to consume excessive resources or trigger an unhandled exception that results in a device reboot.

Attack Vector

The attack is network-based and requires the attacker to have access to the device's web GUI interface. The exploitation involves:

1. Identifying a vulnerable Ewon Flexy or Cosy+ device on the network
2. Crafting a malicious HTTP request designed to exploit the resource consumption flaw
3. Sending the request to the device's GUI endpoint
4. The device processes the malformed request, exhausts resources or encounters an error, and reboots

The vulnerability requires no user interaction and can be exploited by unauthenticated attackers, making it highly accessible for malicious actors who have network visibility to the target device.

Detection Methods for CVE-2026-25819

Indicators of Compromise

• Unexpected or repeated device reboots with no corresponding administrative action
• Anomalous HTTP requests in web server logs targeting the device GUI
• Network traffic patterns showing malformed or unusually large HTTP requests to Ewon devices
• Service disruption alerts from monitoring systems indicating intermittent connectivity to Flexy or Cosy+ devices

Detection Strategies

• Monitor network traffic for unusual HTTP request patterns targeting Ewon Flexy and Cosy+ management interfaces
• Implement intrusion detection rules to identify potential DoS attack signatures against industrial remote access devices
• Configure alerting for device reboot events outside of scheduled maintenance windows
• Review web server access logs on affected devices for suspicious request patterns

Monitoring Recommendations

• Deploy network segmentation to isolate industrial remote access devices and monitor cross-segment traffic
• Implement uptime monitoring for all Ewon Flexy and Cosy+ devices to detect unexpected reboots
• Enable logging on network firewalls and application-layer gateways protecting these devices
• Consider deploying SentinelOne Singularity for OT environments to provide visibility into industrial network anomalies

How to Mitigate CVE-2026-25819

Immediate Actions Required

• Identify all HMS Networks Ewon Flexy and Cosy+ devices in your environment and verify their firmware versions
• Restrict network access to device management interfaces using firewall rules or network segmentation
• Monitor affected devices for signs of exploitation while planning firmware updates
• Review network architecture to ensure industrial remote access devices are not directly exposed to untrusted networks

Patch Information

HMS Networks has released firmware updates to address this vulnerability. Affected organizations should update to the following versions or later:

Firmware updates and detailed patching instructions are available from HMS Networks Security Advisory. Additional product information is available on the HMS Flexy 20500 Product Page.

Workarounds

• Implement strict network access controls to limit connectivity to device management interfaces to authorized IP addresses only
• Place affected devices behind a VPN or firewall that requires authentication before allowing access to the GUI
• Deploy a web application firewall (WAF) or reverse proxy to filter potentially malicious HTTP requests
• Disable unnecessary network services on affected devices if supported by the firmware

# Example firewall rule to restrict access to Ewon device management interface
# Replace 192.168.1.100 with your device IP and 10.0.0.0/24 with authorized management subnet
iptables -A INPUT -d 192.168.1.100 -p tcp --dport 80 -s 10.0.0.0/24 -j ACCEPT
iptables -A INPUT -d 192.168.1.100 -p tcp --dport 80 -j DROP
iptables -A INPUT -d 192.168.1.100 -p tcp --dport 443 -s 10.0.0.0/24 -j ACCEPT
iptables -A INPUT -d 192.168.1.100 -p tcp --dport 443 -j DROP