CVE-2025-5839 is a critical buffer overflow vulnerability identified in the Tenda AC9 router, specifically in firmware version 15.03.02.13. The vulnerability resides in the POST request handler component, within the function fromadvsetlanip located at /goform/AdvSetLanip. The flaw is triggered by manipulating the 'lanMask' argument in the POST request, which leads to a buffer overflow condition. This type of vulnerability allows an attacker to overwrite memory beyond the intended buffer boundaries, potentially enabling arbitrary code execution, denial of service, or system compromise. The vulnerability is remotely exploitable without requiring user interaction or authentication, as indicated by the CVSS vector (AV:N/AC:L/AT:N/UI:N/PR:L). The CVSS 4.0 base score is 8.7, reflecting a high severity due to the ease of exploitation and the significant impact on confidentiality, integrity, and availability. Although no known exploits are currently reported in the wild, the public disclosure of the exploit code increases the risk of active exploitation. The vulnerability affects a widely deployed consumer-grade router model, which is commonly used in home and small office environments, making it a significant threat vector for network perimeter compromise and lateral movement within affected networks.

For European organizations, the impact of this vulnerability can be substantial. Many small and medium-sized enterprises (SMEs) and home offices in Europe utilize Tenda AC9 routers due to their affordability and feature set. Exploitation of this buffer overflow can lead to unauthorized access to internal networks, interception or manipulation of network traffic, and potential deployment of malware or ransomware. This can compromise sensitive corporate data, disrupt business operations, and lead to regulatory non-compliance, especially under GDPR requirements for data protection. Additionally, compromised routers can be leveraged as entry points for broader attacks against enterprise infrastructure or as part of botnets for distributed denial-of-service (DDoS) attacks, impacting service availability. The lack of authentication requirement and remote exploitability heighten the risk, as attackers can target vulnerable devices directly over the internet without needing prior access or user interaction.

To mitigate this vulnerability, European organizations should prioritize the following actions: 1) Immediate firmware upgrade: Check for and apply any official patches or firmware updates from Tenda addressing CVE-2025-5839. If no patch is available, consider temporary mitigations such as disabling remote management interfaces or restricting access to the router's web interface via firewall rules. 2) Network segmentation: Isolate vulnerable routers from critical internal networks to limit potential lateral movement if compromised. 3) Access control: Restrict management access to trusted IP addresses only, and disable unnecessary services or ports on the router. 4) Monitoring and detection: Implement network monitoring to detect unusual traffic patterns or signs of exploitation attempts targeting the /goform/AdvSetLanip endpoint. 5) Device replacement: For high-security environments, consider replacing affected Tenda AC9 devices with routers from vendors with a stronger security track record and timely patching practices. 6) User awareness: Educate users about the risks of using outdated firmware and the importance of timely updates. These measures go beyond generic advice by focusing on practical steps tailored to the specific vulnerability and device context.