CVE-2026-1637 is a stack-based buffer overflow vulnerability identified in the Tenda AC21 router firmware version 16.03.08.16. The vulnerability resides in the function fromAdvSetMacMtuWan within the /goform/AdvSetMacMtuWan endpoint. This function improperly handles input parameters, allowing an attacker to overflow a stack buffer remotely. The overflow can overwrite the return address or other control data on the stack, enabling arbitrary code execution with the privileges of the affected process. The vulnerability requires no user interaction and no prior authentication, making it remotely exploitable over the network. The CVSS 4.0 score of 8.7 reflects the high impact on confidentiality, integrity, and availability, combined with low attack complexity and no privileges required. Although no active exploitation has been reported, a public exploit is available, increasing the risk of exploitation by threat actors. The vulnerability can lead to full device compromise, allowing attackers to intercept or manipulate network traffic, pivot into internal networks, or cause denial of service. The affected product, Tenda AC21, is a widely used consumer and small business router, often deployed in home and office environments. The lack of a vendor patch at the time of publication necessitates immediate mitigation actions to reduce exposure.

For European organizations, this vulnerability poses significant risks. Compromise of Tenda AC21 routers can lead to interception and manipulation of sensitive data traversing the network, undermining confidentiality. Attackers gaining control of routers can alter routing, inject malicious payloads, or disrupt network availability, impacting business operations. Small and medium enterprises, as well as home office setups relying on Tenda AC21 devices, are particularly vulnerable due to limited network segmentation and security controls. Critical infrastructure entities using these routers may face elevated risks of targeted attacks aiming to disrupt services or exfiltrate data. The remote, unauthenticated nature of the exploit increases the attack surface, especially if management interfaces are exposed to the internet or poorly segmented internal networks. The availability of a public exploit lowers the barrier for attackers, potentially leading to widespread compromise if mitigations are not applied promptly.

1. Immediate action should be taken to check for firmware updates from Tenda addressing this vulnerability; if available, apply them without delay. 2. If no patch is available, restrict access to the router’s management interfaces by implementing network segmentation and firewall rules to block unauthorized inbound connections, especially from the internet. 3. Employ network intrusion detection and prevention systems (IDS/IPS) to monitor and block exploit attempts targeting the /goform/AdvSetMacMtuWan endpoint. 4. Disable remote management features if not strictly necessary, or restrict them to trusted IP addresses only. 5. Conduct thorough network audits to identify all Tenda AC21 devices and replace or isolate those that cannot be patched. 6. Educate IT staff and users about the risks and signs of router compromise. 7. Monitor threat intelligence feeds for updates on exploit activity and vendor patches. 8. Consider deploying network-level mitigations such as web application firewalls (WAF) that can detect and block buffer overflow attack patterns targeting this vulnerability.