CVE-2025-7673 is a critical buffer overflow vulnerability identified in the URL parser component of the zhttpd web server embedded within the Zyxel VMG8825-T50K router firmware versions prior to V5.50(ABOM.5)C0. The vulnerability arises from improper handling of input size during buffer copy operations, specifically a classic buffer overflow (CWE-120). An unauthenticated attacker can exploit this flaw by sending a specially crafted HTTP request to the vulnerable device. Due to the lack of authentication requirements and the network-exposed nature of the zhttpd web server, exploitation can be performed remotely over the network. Successful exploitation can lead to denial-of-service (DoS) conditions by crashing the web server or the device itself. More critically, it may allow arbitrary code execution, enabling attackers to gain control over the affected router. Given the CVSS 3.1 base score of 9.8, this vulnerability is rated as critical, reflecting its high impact on confidentiality, integrity, and availability, combined with ease of exploitation (network vector, no privileges or user interaction required). The vulnerability affects a widely deployed consumer and small business router model, Zyxel VMG8825-T50K, which is used for broadband internet access. The lack of available patches at the time of disclosure increases the urgency for mitigation. The vulnerability's exploitation could facilitate further attacks such as network traffic interception, lateral movement within corporate or home networks, or establishing persistent footholds for advanced threat actors.

For European organizations, the impact of CVE-2025-7673 can be significant. Many small and medium enterprises (SMEs) and residential users rely on Zyxel VMG8825-T50K routers for internet connectivity. Exploitation could disrupt business operations through denial-of-service, causing loss of internet access and productivity. More severe is the potential for arbitrary code execution, which could allow attackers to manipulate network traffic, intercept sensitive data, or pivot into internal networks, compromising confidentiality and integrity of organizational data. This risk is heightened in environments where these routers serve as the primary gateway without additional network segmentation or security controls. Critical infrastructure sectors, including healthcare, finance, and government entities using these devices, could face operational disruptions or data breaches. The vulnerability also poses a risk to home users, potentially turning compromised routers into botnet nodes for large-scale attacks impacting European internet infrastructure. Given the critical severity and ease of exploitation, the threat landscape for European organizations using affected Zyxel devices is substantial.

1. Immediate firmware upgrade: Organizations and users should prioritize upgrading Zyxel VMG8825-T50K devices to firmware version V5.50(ABOM.5)C0 or later once available. 2. Network segmentation: Isolate vulnerable routers from critical internal networks to limit potential lateral movement in case of compromise. 3. Disable remote management: If remote web management is enabled, disable it to reduce exposure of the vulnerable zhttpd service to external attackers. 4. Implement firewall rules: Restrict inbound HTTP/HTTPS traffic to the router’s management interface from trusted IP addresses only. 5. Monitor network traffic: Deploy intrusion detection/prevention systems (IDS/IPS) to detect anomalous HTTP requests targeting the router’s web server. 6. Vendor engagement: Engage with Zyxel support channels for early access to patches or mitigation guidance. 7. Incident response readiness: Prepare to isolate and remediate affected devices rapidly if exploitation is detected. 8. User awareness: Inform end-users about the risk and encourage prompt updates and secure configuration practices. These measures, combined, reduce the attack surface and mitigate the risk until patches are widely deployed.