CVE-2026-25818: HMS Networks Ewon Auth Bypass Flaw

CVE-2026-25818 Overview

HMS Networks Ewon Flexy and Cosy+ industrial remote access devices contain a critical vulnerability involving weak entropy for authentication cookies. This cryptographic weakness allows an attacker who has obtained a stolen session cookie to brute-force an encryption parameter and recover user passwords. The vulnerability affects Ewon Flexy devices with firmware before 15.0s4, Cosy+ devices with firmware 22.xx before 22.1s6, and Cosy+ devices with firmware 23.xx before 23.0s3.

Critical Impact

Attackers with access to session cookies can recover plaintext user passwords through brute-force attacks on weak encryption parameters, potentially leading to complete account takeover of industrial remote access gateways.

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 publishes security advisory
• 2026-03-13 - CVE CVE-2026-25818 published to NVD
• 2026-03-16 - Last updated in NVD database

Technical Details for CVE-2026-25818

Vulnerability Analysis

This vulnerability stems from insufficient entropy in the cryptographic implementation used to protect authentication cookies. When session cookies are generated by the affected HMS Networks devices, they use a weak or predictable encryption parameter that significantly reduces the effective keyspace an attacker must search.

Industrial remote access gateways like the Ewon Flexy and Cosy+ series are commonly deployed in operational technology (OT) environments to enable secure remote access to industrial control systems. The compromise of such devices could provide attackers with a foothold into critical infrastructure networks.

The weakness is classified under CWE-315 (Cleartext Storage of Sensitive Information in a Cookie), indicating that the cryptographic protection applied to sensitive authentication data is inadequate. An attacker who intercepts or obtains a valid session cookie through network sniffing, man-in-the-middle attacks, or other means can leverage this weakness to recover the underlying user credentials.

Root Cause

The root cause of this vulnerability is the use of insufficient entropy in the encryption parameter that protects authentication cookies. The cryptographic implementation fails to generate adequately random values, resulting in a predictable or limited keyspace that makes brute-force attacks computationally feasible.

This design flaw allows attackers to systematically test possible encryption parameter values until they successfully decrypt the cookie contents and extract the user password. The reduced complexity of this brute-force operation transforms what should be a computationally infeasible attack into a practical exploitation scenario.

Attack Vector

The attack requires the adversary to first obtain a valid session cookie from the target device. This can be accomplished through various network-based techniques:

1. Network Interception: If the device is accessed over unencrypted HTTP or if TLS is improperly configured, attackers on the same network segment can capture session cookies in transit.

2. Man-in-the-Middle Attacks: Attackers positioned between users and the device can intercept authentication traffic and extract cookies.

3. Cross-Site Scripting (XSS): If the web interface contains XSS vulnerabilities, attackers could exfiltrate session cookies to external servers.

Once a session cookie is obtained, the attacker performs an offline brute-force attack against the weak encryption parameter. Due to the insufficient entropy, this process requires significantly fewer computational resources than would be expected from properly implemented cryptographic protections. Upon successful decryption, the attacker recovers the user's password in plaintext, enabling unauthorized access to the device with the victim's credentials.

Detection Methods for CVE-2026-25818

Indicators of Compromise

• Unexpected login attempts or sessions from unfamiliar IP addresses on Ewon Flexy or Cosy+ devices
• Multiple authentication events occurring in rapid succession from the same or different sources
• Network traffic patterns indicating potential cookie interception or replay attacks
• Unauthorized configuration changes to remote access gateway settings

Detection Strategies

• Monitor authentication logs on HMS Networks devices for anomalous access patterns
• Implement network detection rules to identify potential session hijacking or cookie theft attempts
• Deploy intrusion detection systems to alert on suspicious traffic to and from industrial remote access gateways
• Analyze outbound network connections from OT networks for data exfiltration indicators

Monitoring Recommendations

• Enable comprehensive logging on all affected HMS Networks devices and forward logs to a centralized SIEM
• Implement network segmentation monitoring to detect unauthorized access attempts to industrial control system networks
• Regularly audit user accounts and active sessions on Ewon Flexy and Cosy+ devices
• Monitor for firmware version discrepancies across deployed devices to ensure consistent patch levels

How to Mitigate CVE-2026-25818

Immediate Actions Required

• Update Ewon Flexy firmware to version 15.0s4 or later immediately
• Update Cosy+ firmware 22.xx to version 22.1s6 or later
• Update Cosy+ firmware 23.xx to version 23.0s3 or later
• Force password resets for all users with existing sessions on affected devices
• Review access logs for indicators of compromise prior to patching

Patch Information

HMS Networks has released firmware updates that address this vulnerability. Organizations should obtain the latest firmware versions from HMS Networks' official channels and apply them to all affected devices. Refer to the HMS Security Advisory 2026-03-09 for detailed patching instructions and additional technical guidance.

Additional product information is available at the HMS Flexy Product Information page.

Workarounds

• Implement network segmentation to isolate affected devices from untrusted networks until patches can be applied
• Enforce HTTPS-only access to device management interfaces to reduce cookie interception risk
• Deploy additional network monitoring and intrusion detection around affected device segments
• Consider disabling remote access functionality temporarily if devices cannot be immediately patched

# Network segmentation example - restrict access to management interface
# Add firewall rules to limit access to trusted management IPs only
iptables -A INPUT -p tcp --dport 443 -s 10.0.1.0/24 -j ACCEPT
iptables -A INPUT -p tcp --dport 443 -j DROP