CVE-2025-1731 is a high-severity vulnerability affecting Zyxel USG FLEX H series devices running uOS firmware versions from V1.20 through V1.31. The vulnerability stems from incorrect permission assignment (CWE-732) in the PostgreSQL command interface embedded within the firmware. Specifically, an authenticated local attacker with low privileges can exploit this flaw to gain unauthorized access to the underlying Linux shell environment. This is achieved by leveraging a stolen administrator token that remains valid if the administrator has not logged out. Once access is obtained, the attacker can escalate privileges and execute malicious scripts or modify critical system configurations with administrator-level rights. The vulnerability does not require user interaction but does require the attacker to have some level of authenticated access, albeit with low privileges. The CVSS v3.1 score of 7.8 reflects the high impact on confidentiality, integrity, and availability, with low attack complexity and no user interaction needed. No public exploits are currently known in the wild, and no patches have been released at the time of this report. The root cause is improper permission settings on critical PostgreSQL commands within the firmware, allowing privilege escalation and unauthorized system control. This vulnerability could be leveraged to compromise network security devices, potentially leading to network disruption, data exfiltration, or further lateral movement within an organization’s infrastructure.

For European organizations, the exploitation of CVE-2025-1731 could have severe consequences. Zyxel USG FLEX H series devices are commonly deployed as unified security gateways in enterprise and SMB networks, providing firewall, VPN, and intrusion prevention functionalities. Successful exploitation would allow attackers to gain administrator-level control over these devices, undermining network perimeter defenses. This could lead to interception or manipulation of network traffic, unauthorized access to internal systems, and disruption of critical business operations. Confidential data could be exfiltrated or altered, and availability of network services could be compromised, affecting business continuity. Given the reliance on these devices in sectors such as finance, healthcare, manufacturing, and public administration across Europe, the vulnerability poses a significant risk to data protection and regulatory compliance (e.g., GDPR). The requirement for an authenticated local attacker somewhat limits remote exploitation but does not eliminate risk, especially in environments where internal threat actors or compromised credentials exist. The persistence of valid administrator tokens further exacerbates the risk by extending the attack window.

1. Immediate mitigation should focus on enforcing strict session management policies, ensuring that administrator sessions expire promptly and tokens are invalidated upon logout or after a short inactivity period to reduce token reuse risk. 2. Network segmentation should be applied to limit access to the management interfaces of Zyxel USG FLEX devices, restricting it to trusted administrators only. 3. Implement multi-factor authentication (MFA) for all administrative access to reduce the risk of credential compromise. 4. Monitor logs for unusual PostgreSQL command usage or shell access attempts on affected devices to detect potential exploitation attempts early. 5. Until a vendor patch is available, consider deploying compensating controls such as host-based intrusion detection systems (HIDS) on the devices or network-level anomaly detection to identify suspicious activities. 6. Conduct regular audits of active sessions and tokens on these devices to identify and terminate stale or suspicious sessions. 7. Educate administrators on the importance of logging out after management tasks to invalidate tokens and reduce attack surface. 8. Engage with Zyxel support channels to obtain updates on patch releases and apply firmware updates promptly once available.