CVE-2026-2870 is a stack-based buffer overflow vulnerability identified in the Tenda A21 router firmware version 1.0.0.0. The vulnerability resides in the set_qosMib_list function, which is invoked via the /goform/formSetQosBand endpoint. This function improperly processes input arguments, allowing an attacker to craft malicious requests that overflow the stack buffer. The flaw is exploitable remotely without requiring authentication or user interaction, making it highly accessible to attackers. The buffer overflow can lead to arbitrary code execution on the device, enabling attackers to gain control over the router. This could allow them to manipulate network traffic, intercept sensitive data, or launch further attacks within the network. The CVSS 4.0 base score is 8.7, reflecting the high impact on confidentiality, integrity, and availability, combined with the ease of exploitation. Although no active exploitation has been reported, a public exploit is available, increasing the urgency for mitigation. The vulnerability affects only firmware version 1.0.0.0 of the Tenda A21, and no official patches have been linked yet. Given the critical role of routers in network infrastructure, this vulnerability represents a significant security risk.

The impact of CVE-2026-2870 is substantial for organizations relying on Tenda A21 routers. Successful exploitation can lead to complete compromise of the affected device, allowing attackers to execute arbitrary code remotely. This can result in unauthorized access to internal networks, interception or manipulation of network traffic, disruption of network services, and potential pivoting to other internal systems. The confidentiality of sensitive data traversing the router can be compromised, integrity of network configurations and data can be altered, and availability of network services can be disrupted. For enterprises and service providers using these routers, this could lead to data breaches, operational downtime, and reputational damage. The public availability of an exploit increases the likelihood of attacks, especially from opportunistic threat actors. The lack of patches at the time of disclosure further exacerbates the risk, making timely mitigation critical.

To mitigate CVE-2026-2870, organizations should first verify if they are using Tenda A21 routers running firmware version 1.0.0.0. If so, immediate actions include isolating these devices from untrusted networks and restricting access to the /goform/formSetQosBand endpoint through network segmentation or firewall rules. Employing network intrusion detection systems (NIDS) to monitor for suspicious requests targeting this endpoint can help detect exploitation attempts. Until an official patch is released, consider disabling QoS features or the vulnerable service if feasible to reduce the attack surface. Regularly check Tenda’s official channels for firmware updates addressing this vulnerability and apply them promptly once available. Additionally, implement strong network perimeter defenses and ensure that management interfaces are not exposed to the internet. Conduct internal audits to identify any signs of compromise and maintain up-to-date backups to facilitate recovery if needed.