CVE-2026-25818 is a vulnerability identified in HMS Networks Ewon Flexy devices with firmware versions prior to 15.0s4, and Cosy+ devices with firmware 22.xx before 22.1s6 and 23.xx before 23.0s3. The root cause is weak entropy in the generation of authentication cookies used by these devices. Authentication cookies are intended to securely maintain user sessions; however, due to insufficient randomness, an attacker who has already obtained a stolen session cookie can perform a brute-force attack against an encryption parameter embedded within the cookie. Successfully brute-forcing this parameter allows the attacker to recover the user password, effectively escalating their access privileges. This vulnerability undermines the confidentiality and integrity of authentication mechanisms, potentially enabling unauthorized remote access to industrial control and monitoring systems managed by these devices. Although no exploits have been reported in the wild, the vulnerability is publicly disclosed and documented in the CVE database. The affected devices are widely deployed in industrial automation, remote monitoring, and IoT environments, where secure authentication is critical to prevent unauthorized control or data exfiltration. The lack of a CVSS score suggests this is a newly published vulnerability, and the weakness in entropy indicates a cryptographic design flaw that can be exploited with moderate attacker capabilities. The vulnerability requires possession of a stolen session cookie, which implies some initial compromise or interception capability by the attacker. However, once the cookie is obtained, the brute-force attack to recover the password is feasible due to the weak entropy. This vulnerability highlights the importance of strong cryptographic practices in embedded device firmware and session management.

The primary impact of CVE-2026-25818 is the compromise of user credentials through brute-forcing encryption parameters in authentication cookies. This leads to unauthorized access to affected Ewon Flexy and Cosy+ devices, which are commonly used in industrial automation and remote monitoring. Unauthorized access can result in manipulation of operational technology systems, disruption of industrial processes, data theft, and potential sabotage. The confidentiality of user credentials and session data is at risk, and integrity of device control is compromised. Organizations relying on these devices for critical infrastructure monitoring and control face increased risk of operational downtime, safety incidents, and financial losses. The vulnerability also increases the attack surface for lateral movement within industrial networks. Although exploitation requires a stolen session cookie, the ease of password recovery once the cookie is obtained elevates the threat level. The lack of known exploits in the wild suggests limited current exploitation but does not diminish the potential impact if exploited. This vulnerability is particularly concerning for sectors such as manufacturing, energy, utilities, and transportation that depend on these devices for secure remote access and control.

1. Immediately update the firmware of all affected HMS Networks Ewon Flexy and Cosy+ devices to versions 15.0s4 or later for Flexy, 22.1s6 or later for Cosy+ 22.xx, and 23.0s3 or later for Cosy+ 23.xx to address the weak entropy issue. 2. Implement strict session management policies, including limiting session duration and enforcing secure cookie attributes (e.g., HttpOnly, Secure, SameSite) to reduce the risk of session cookie theft. 3. Employ network segmentation and access controls to restrict access to management interfaces of these devices, minimizing exposure to potential attackers. 4. Use VPNs or encrypted tunnels for remote access to reduce the risk of session interception. 5. Monitor network traffic and device logs for unusual authentication attempts or brute-force activity targeting these devices. 6. Educate users and administrators on the risks of session cookie theft and encourage the use of multi-factor authentication if supported. 7. Conduct regular security assessments and penetration testing focused on industrial control systems to detect similar cryptographic weaknesses. 8. If firmware updates are not immediately possible, consider temporary compensating controls such as disabling remote access or restricting it to trusted IP addresses.