CVE-2026-25823: HMS Networks Ewon RCE Vulnerability

CVE-2026-25823 Overview

HMS Networks Ewon Flexy and Cosy+ industrial gateway devices contain a stack buffer overflow vulnerability (CWE-121) in firmware versions prior to specific security updates. This vulnerability allows remote attackers to trigger a Denial of Service condition and can also be exploited to achieve unauthenticated remote code execution on affected devices.

Critical Impact

Unauthenticated remote attackers can exploit this stack buffer overflow to execute arbitrary code or cause denial of service on HMS Networks Ewon industrial gateways, potentially compromising industrial control system environments.

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

• 2026-03-13 - CVE-2026-25823 published to NVD
• 2026-03-16 - Last updated in NVD database

Technical Details for CVE-2026-25823

Vulnerability Analysis

This vulnerability is classified as CWE-121: Stack-based Buffer Overflow. The affected HMS Networks Ewon Flexy and Cosy+ devices fail to properly validate input boundaries when processing certain data, allowing an attacker to write beyond the allocated stack buffer. This memory corruption can overwrite critical stack data including return addresses, enabling attackers to redirect program execution flow.

The network-accessible nature of this vulnerability means no authentication or user interaction is required for exploitation. Attackers can remotely trigger the buffer overflow condition, making this particularly dangerous for internet-exposed industrial gateways. Successful exploitation can result in complete compromise of device confidentiality, integrity, and availability.

Root Cause

The root cause is insufficient bounds checking when handling input data on the affected devices. When the firmware processes specially crafted input that exceeds the allocated stack buffer size, the overflow corrupts adjacent memory regions on the stack. This classic stack buffer overflow pattern allows attackers to overwrite the return address and gain control of program execution.

Attack Vector

The attack vector is network-based, requiring no privileges or user interaction. An attacker can send malicious network requests to vulnerable Ewon Flexy or Cosy+ devices to trigger the stack buffer overflow. The attack can be executed remotely against any network-accessible device running vulnerable firmware versions.

The exploitation scenario involves:

1. Attacker identifies a vulnerable HMS Networks gateway exposed to the network
2. Attacker sends a crafted request containing oversized data designed to overflow the stack buffer
3. The overflow corrupts stack memory, either crashing the device (DoS) or enabling code execution
4. With precise payload construction, the attacker achieves unauthenticated remote code execution

For detailed technical information, refer to the HMS Security Advisory 2026-03-09.

Detection Methods for CVE-2026-25823

Indicators of Compromise

• Unexpected device crashes, reboots, or unresponsive behavior on Ewon Flexy or Cosy+ gateways
• Anomalous network traffic patterns targeting industrial gateway management interfaces
• Unusual outbound connections from gateway devices indicating potential compromise
• Unexpected configuration changes or new user accounts on affected devices

Detection Strategies

• Deploy network intrusion detection systems (IDS) to monitor for exploitation attempts against HMS Networks devices
• Implement firmware version auditing to identify devices running vulnerable versions (Flexy before 15.0s4, Cosy+ 22.xx before 22.1s6, Cosy+ 23.xx before 23.0s3)
• Monitor system logs on Ewon devices for crash events or service restarts that may indicate exploitation attempts
• Use network segmentation monitoring to detect lateral movement from compromised industrial gateways

Monitoring Recommendations

• Enable comprehensive logging on all HMS Networks Ewon devices and forward logs to centralized SIEM
• Establish baseline network behavior for industrial gateways and alert on deviations
• Monitor for large or malformed packets destined for Ewon device management ports
• Implement regular firmware version checks as part of vulnerability management programs

How to Mitigate CVE-2026-25823

Immediate Actions Required

• Immediately update HMS Networks Ewon Flexy devices to firmware version 15.0s4 or later
• Update HMS Networks Cosy+ devices running firmware 22.xx to version 22.1s6 or later
• Update HMS Networks Cosy+ devices running firmware 23.xx to version 23.0s3 or later
• Isolate vulnerable devices from untrusted networks until patches can be applied

Patch Information

HMS Networks has released security updates to address this vulnerability. Organizations should update to the following minimum firmware versions:

Refer to the HMS Security Advisory 2026-03-09 for complete patch details and update instructions. Additional product information is available on the HMS Product Information Page.

Workarounds

• Place vulnerable Ewon devices behind firewalls and restrict network access to trusted IP addresses only
• Disable unnecessary network services and management interfaces on affected devices
• Implement network segmentation to isolate industrial gateways from general network traffic
• Use VPN connections for remote management instead of exposing devices directly to the internet

# Network segmentation example - restrict access to Ewon management interface
# Add firewall rules to limit access to trusted management IPs only
iptables -A INPUT -p tcp --dport 443 -s 10.0.0.0/24 -j ACCEPT
iptables -A INPUT -p tcp --dport 443 -j DROP