CVE-2025-65221 identifies a buffer overflow vulnerability in the Tenda AC21 router firmware version V16.03.08.16. The flaw exists in the handling of the 'list' parameter within the /goform/setPptpUserList endpoint, which is likely part of the router's PPTP VPN user configuration interface. Buffer overflow vulnerabilities occur when input data exceeds the allocated memory buffer, potentially overwriting adjacent memory and enabling arbitrary code execution or system crashes. An attacker can exploit this vulnerability by sending a specially crafted HTTP request to the router's management interface, targeting the vulnerable parameter. Successful exploitation could allow remote code execution with the privileges of the router's firmware process, leading to full compromise of the device. This could enable attackers to intercept or manipulate network traffic, pivot into internal networks, or disrupt network availability. No public exploits or patches are currently available, and the vulnerability was published on November 20, 2025, with no CVSS score assigned yet. The lack of authentication requirement is inferred given the nature of the endpoint, but network access to the router's management interface is necessary. The vulnerability is significant due to the widespread use of Tenda AC21 routers in consumer and small business environments, which often lack robust network segmentation or monitoring.

For European organizations, exploitation of this vulnerability could lead to severe consequences including unauthorized access to internal networks, interception of sensitive communications, and disruption of network services. Small and medium enterprises (SMEs) using Tenda AC21 routers for VPN connectivity or network routing are particularly at risk, as compromise of these devices can serve as a foothold for further attacks. Confidentiality could be breached through traffic interception or data exfiltration, integrity compromised by manipulation of network traffic or device configurations, and availability impacted by denial of service or device crashes. The absence of known exploits reduces immediate risk, but the public disclosure increases the likelihood of future exploitation attempts. Organizations in Europe with limited IT security resources may be more vulnerable due to delayed patching or lack of network access controls. The threat also extends to home office environments increasingly common in Europe, where such routers are deployed without enterprise-grade protections.

Immediate mitigation should focus on restricting access to the router's management interface, ideally limiting it to trusted internal networks and disabling remote management features if not required. Network segmentation should be enforced to isolate vulnerable devices from critical infrastructure. Monitoring network traffic for unusual requests to the /goform/setPptpUserList endpoint can help detect exploitation attempts. Organizations should engage with Tenda to obtain firmware updates addressing this vulnerability and apply them promptly once available. In the interim, consider replacing vulnerable devices with models from vendors with more robust security track records. Employ network-level protections such as firewalls and intrusion detection/prevention systems configured to detect anomalous HTTP requests targeting router management interfaces. Educate users and administrators about the risks of using outdated firmware and the importance of timely updates. Finally, maintain regular backups of router configurations to enable rapid recovery if devices are compromised.