CVE-2025-65221 identifies a buffer overflow vulnerability in the Tenda AC21 router firmware version V16.03.08.16. The vulnerability exists in the handling of the 'list' parameter within the /goform/setPptpUserList HTTP endpoint, which is used to configure PPTP VPN user lists. A buffer overflow occurs when the input exceeds the allocated buffer size, leading to memory corruption. This can cause the device to crash or reboot, resulting in a denial of service (DoS) condition. The vulnerability does not require any privileges, user interaction, or authentication, and can be exploited remotely by an attacker with network access to the router's management interface. The CVSS v3.1 score is 4.3 (medium severity), reflecting the low complexity of the attack vector (adjacent network), no privileges required, no user interaction, and impact limited to availability (no confidentiality or integrity impact). No patches or fixes have been published at the time of disclosure, and no known exploits are reported in the wild. The weakness is classified under CWE-121 (Stack-based Buffer Overflow), a common and well-understood vulnerability type. Exploitation could disrupt network connectivity for users relying on the affected router, impacting business operations or home network availability.

For European organizations, especially small and medium enterprises (SMEs) and home office users relying on Tenda AC21 routers, this vulnerability poses a risk of network downtime due to device crashes. While it does not compromise data confidentiality or integrity, the denial of service impact can interrupt business continuity, remote work capabilities, and access to critical network resources. In sectors where continuous connectivity is essential, such as finance, healthcare, or critical infrastructure, even temporary outages can have operational and reputational consequences. The vulnerability's exploitation requires network access to the router's management interface, which may be exposed in poorly secured environments or through compromised internal networks. Given the lack of known exploits, the immediate risk is moderate, but the absence of patches necessitates proactive mitigation. Disruption of VPN services configured on the device could also affect secure remote access, further impacting organizational security posture.

1. Immediately restrict access to the router's management interface by limiting it to trusted internal networks and disabling remote management if not required. 2. Implement network segmentation to isolate router management traffic from general user traffic, reducing exposure to potential attackers. 3. Monitor router logs and network traffic for unusual requests targeting the /goform/setPptpUserList endpoint or signs of device instability. 4. Where possible, replace affected Tenda AC21 routers with models from vendors providing timely security updates and patches. 5. If replacement is not feasible, consider deploying firewall rules to block unauthorized access to the vulnerable endpoint. 6. Educate users and IT staff about the vulnerability and encourage vigilance for signs of network disruption. 7. Regularly check for firmware updates from Tenda and apply patches promptly once available. 8. Employ intrusion detection/prevention systems (IDS/IPS) to detect exploitation attempts targeting this vulnerability. 9. Review VPN configurations and consider alternative secure remote access solutions until the vulnerability is resolved.