CVE-2026-4565 is a remote buffer overflow vulnerability identified in the Tenda AC21 router firmware version 16.03.08.16. The flaw resides in the formSetQosBand function of the /goform/SetNetControlList endpoint, which improperly processes input arguments, leading to a buffer overflow condition. This vulnerability allows an attacker to send specially crafted requests remotely to the router without requiring authentication or user interaction, exploiting the overflow to execute arbitrary code or cause denial of service. The vulnerability has a CVSS 4.0 base score of 8.7, indicating high severity, with low attack complexity and no privileges or user interaction needed. The impact vector includes high confidentiality, integrity, and availability consequences, as successful exploitation can lead to full device compromise, enabling attackers to intercept, modify, or disrupt network traffic. Although no active exploitation in the wild has been reported, the availability of a public exploit increases the likelihood of future attacks. The lack of an official patch at the time of disclosure necessitates immediate mitigation efforts. This vulnerability is particularly critical given the widespread deployment of Tenda AC21 routers in consumer and small business environments, which often lack robust security monitoring.

The exploitation of CVE-2026-4565 can have severe consequences for organizations and individuals using Tenda AC21 routers. Attackers gaining remote code execution can take full control of the device, enabling interception and manipulation of network traffic, insertion of malicious payloads, or pivoting to internal networks. This compromises confidentiality by exposing sensitive data, integrity by altering network communications or configurations, and availability by causing device crashes or network outages. For businesses, this can lead to data breaches, operational disruptions, and reputational damage. In home environments, compromised routers can serve as entry points for broader attacks or be conscripted into botnets. The ease of exploitation without authentication or user interaction significantly raises the threat level, especially in environments where these routers are exposed to the internet. The absence of a patch at disclosure time increases exposure duration, amplifying risk.

1. Immediate mitigation should include restricting remote access to the router’s management interface by disabling WAN-side administration or applying firewall rules to block external access to the /goform/SetNetControlList endpoint. 2. Network segmentation should be employed to isolate vulnerable devices from critical assets. 3. Monitor network traffic for unusual requests targeting the QoS configuration endpoints indicative of exploitation attempts. 4. Where possible, upgrade to a firmware version that addresses this vulnerability once released by Tenda. 5. If no patch is available, consider replacing affected devices with alternative hardware from vendors with timely security updates. 6. Employ intrusion detection/prevention systems (IDS/IPS) with signatures targeting this exploit to detect and block attacks. 7. Educate users and administrators about the risks of exposing router management interfaces to the internet and enforce strong network security policies. 8. Regularly audit router configurations to ensure no unauthorized changes have been made.