CVE-2026-1637 is a stack-based buffer overflow vulnerability identified in the Tenda AC21 router firmware version 16.03.08.16. The vulnerability exists in the fromAdvSetMacMtuWan function within the /goform/AdvSetMacMtuWan endpoint. This function improperly handles input data, allowing an attacker to overflow a stack buffer remotely. The flaw does not require authentication or user interaction, making it highly exploitable over the network. Exploiting this vulnerability can lead to arbitrary code execution with elevated privileges on the router, enabling attackers to manipulate device configurations, intercept or redirect traffic, or launch further attacks within the network. The CVSS v4.0 base score is 8.7, reflecting its high impact on confidentiality, integrity, and availability, combined with ease of exploitation. Although no active exploitation has been reported, a public exploit is available, increasing the urgency for mitigation. The vulnerability affects a widely deployed consumer and small business router model, which is often used as a gateway device, making the potential attack surface significant. No official patches or updates have been linked yet, so organizations must implement interim controls to reduce exposure.

The impact of CVE-2026-1637 is substantial for organizations using Tenda AC21 routers. Successful exploitation can lead to full compromise of the router, allowing attackers to intercept sensitive data, manipulate network traffic, and establish persistent footholds within internal networks. This can result in data breaches, disruption of network services, and facilitation of lateral movement to other critical systems. The vulnerability’s remote, unauthenticated nature increases the risk of widespread exploitation, especially in environments where these routers are exposed to untrusted networks or the internet. Small and medium businesses, home offices, and branch offices relying on Tenda AC21 devices are particularly vulnerable. The compromise of such gateway devices can undermine overall network security, potentially affecting confidentiality, integrity, and availability of organizational resources.

To mitigate CVE-2026-1637, organizations should immediately restrict remote access to the /goform/AdvSetMacMtuWan endpoint by implementing firewall rules or access control lists that limit management interface exposure to trusted networks only. Network segmentation should be employed to isolate vulnerable devices from critical infrastructure. Monitoring network traffic for unusual requests targeting the vulnerable function can help detect exploitation attempts. If possible, disable remote management features or change default management ports to reduce attack surface. Organizations should contact Tenda for official firmware updates or patches and apply them promptly once available. In the absence of patches, consider replacing vulnerable devices with models not affected by this vulnerability. Regularly update device firmware and maintain an inventory of network devices to ensure timely vulnerability management. Employ intrusion detection/prevention systems capable of recognizing exploit attempts targeting this vulnerability.