CVE-2025-6599 is a vulnerability classified under CWE-400 (Uncontrolled Resource Consumption) affecting the web server component of Zyxel DX3301-T0 firmware versions 5.50(ABVY.6.3)C0 and earlier. The flaw allows an unauthenticated remote attacker to conduct Slowloris-style denial-of-service (DoS) attacks by opening and maintaining numerous slow HTTP connections to the device's web management interface. This attack exhausts the web server's resources, preventing it from processing legitimate HTTP requests, thereby temporarily blocking administrative access to the device's management console. Importantly, this DoS condition affects only the web management interface and does not impact other networking services provided by the device. The vulnerability is remotely exploitable without any user interaction or privileges, increasing its risk profile. Despite its potential to disrupt device management, no known exploits have been reported in the wild, and Zyxel has not yet released patches addressing this issue. The CVSS v3.1 base score of 5.3 reflects a medium severity, driven by the network attack vector, low attack complexity, and lack of required privileges or user interaction, but limited impact confined to availability of the management interface only. This vulnerability highlights the risk of resource exhaustion attacks on embedded device management interfaces, which can hinder timely administration and incident response in network environments.

For European organizations, the primary impact of CVE-2025-6599 lies in the potential disruption of administrative access to Zyxel DX3301-T0 devices. This can delay or prevent network administrators from managing device configurations, applying security policies, or responding to incidents, thereby increasing operational risk. Although the attack does not affect core networking functions like routing or switching, the inability to access the management interface can complicate troubleshooting and prolong downtime during network events. Organizations relying on these devices for critical infrastructure or in environments with strict uptime requirements may experience degraded operational resilience. Additionally, the vulnerability could be leveraged as part of a broader attack strategy to distract or delay defenders. The lack of known exploits reduces immediate risk, but the ease of exploitation and remote accessibility mean that attackers could develop tools to exploit this vulnerability. European sectors such as telecommunications, government, and enterprises with Zyxel deployments should be particularly vigilant. The impact is more operational than data-centric, affecting availability of management rather than confidentiality or integrity.

Since no official patches are currently available, European organizations should implement compensating controls to mitigate the risk from CVE-2025-6599. First, restrict access to the Zyxel DX3301-T0 web management interface by limiting it to trusted IP addresses or management VLANs using firewall rules or access control lists (ACLs). Deploy network-level protections such as intrusion prevention systems (IPS) or web application firewalls (WAF) capable of detecting and blocking Slowloris-style attacks characterized by slow HTTP request patterns and excessive concurrent connections. Monitor device logs and network traffic for unusual connection behaviors indicative of resource exhaustion attempts. Consider reducing the timeout settings and maximum concurrent connections on the web server if configurable, to limit resource consumption. Where possible, disable web management interfaces on devices not requiring remote access or use alternative secure management channels such as SSH or dedicated management networks. Maintain up-to-date network device inventories to quickly identify affected devices. Finally, engage with Zyxel support channels to obtain updates on patch availability and apply firmware updates promptly once released.