CVE-2026-3678 is a stack-based buffer overflow vulnerability identified in the Tenda FH451 router firmware version 1.0.0.9. The vulnerability resides in the function sub_3C434 within the /goform/AdvSetWan endpoint, which processes WAN configuration parameters such as wanmode and PPPOEPassword. Improper validation or sanitization of these input parameters allows an attacker to overflow the stack buffer, potentially overwriting the return address or other control data on the stack. This condition can be triggered remotely over the network without requiring authentication or user interaction, making it highly exploitable. The vulnerability has a CVSS 4.0 base score of 8.7, reflecting its high severity due to network attack vector, low complexity, no privileges or user interaction required, and high impact on confidentiality, integrity, and availability. Exploitation could allow an attacker to execute arbitrary code with elevated privileges on the router, leading to full device compromise. Although no known exploits are currently active in the wild, the public disclosure of exploit details increases the likelihood of future attacks. The affected product, Tenda FH451, is a consumer and small business router commonly used in various global markets. The lack of an official patch at the time of disclosure necessitates immediate mitigation efforts to reduce exposure.

The exploitation of CVE-2026-3678 can have severe consequences for organizations and individuals using the Tenda FH451 router. Successful exploitation may lead to remote code execution, allowing attackers to gain full control over the device. This can result in interception or manipulation of network traffic, unauthorized access to internal networks, disruption of internet connectivity, and potential pivoting to other systems within the network. Confidential information passing through the router could be exposed or altered, compromising data integrity and privacy. The availability of the network could also be impacted by denial-of-service conditions caused by device instability or malicious reconfiguration. Given the router’s role as a network gateway, the vulnerability poses a significant risk to the security posture of affected environments, particularly in small businesses and home networks where such devices are often deployed with minimal additional security controls.

Until an official patch is released by Tenda, organizations should implement the following mitigations: 1) Restrict remote access to the router’s management interface by disabling WAN-side administration or limiting access via firewall rules to trusted IP addresses only. 2) Change default credentials and ensure strong, unique passwords are used for device administration to reduce the risk of unauthorized access. 3) Monitor network traffic for unusual or suspicious activity targeting the /goform/AdvSetWan endpoint or attempts to exploit buffer overflow patterns. 4) Segment the network to isolate the router management interface from general user traffic where possible. 5) Regularly check for firmware updates from Tenda and apply patches promptly once available. 6) Consider deploying intrusion detection/prevention systems (IDS/IPS) capable of detecting buffer overflow exploit attempts against known vulnerable endpoints. 7) Educate users about the risks and encourage vigilance regarding network device security. These targeted actions go beyond generic advice by focusing on access control, monitoring, and network segmentation specific to the vulnerability’s attack vector.