CVE-2026-5021 is a stack-based buffer overflow vulnerability identified in the Tenda F453 router firmware version 1.0.0.3. The vulnerability resides in the fromPPTPUserSetting function of the HTTP daemon component, specifically within the /goform/PPTPUserSetting endpoint. The flaw arises from improper validation and handling of the 'delno' parameter, which can be manipulated by a remote attacker to overflow the stack buffer. This overflow can overwrite critical control data such as return addresses, enabling arbitrary code execution or denial of service. The vulnerability is remotely exploitable over the network without requiring authentication or user interaction, making it highly dangerous. The CVSS v4.0 score is 8.7, reflecting high impact on confidentiality, integrity, and availability, with low attack complexity and no privileges or user interaction needed. While no active exploitation has been reported, published exploit code increases the likelihood of future attacks. The affected product, Tenda F453, is a consumer-grade router commonly used in home and small office environments, which may be targeted to gain network access or pivot into internal networks. The lack of an official patch at the time of disclosure necessitates immediate mitigation efforts.

Successful exploitation of CVE-2026-5021 can lead to complete compromise of the Tenda F453 router, allowing attackers to execute arbitrary code with system-level privileges. This can result in interception or manipulation of network traffic, unauthorized access to connected devices, disruption of network availability, and potential use of the compromised router as a foothold for further attacks within an organization’s network. The vulnerability threatens confidentiality by exposing sensitive data traversing the router, integrity by enabling unauthorized configuration changes or injection of malicious payloads, and availability by causing crashes or denial of service. Given the router’s role as a network gateway, exploitation can have cascading effects on enterprise or home network security. The ease of remote exploitation without authentication increases the risk of widespread attacks, especially in environments where these devices are deployed without additional network segmentation or protections.

1. Immediate mitigation should include isolating the affected Tenda F453 devices from untrusted networks to reduce exposure to remote attacks. 2. Network administrators should implement strict firewall rules to restrict access to the /goform/PPTPUserSetting endpoint, ideally blocking all inbound HTTP requests to the router’s management interface from external networks. 3. Disable PPTP VPN functionality if not required, as the vulnerability is linked to the PPTP user setting component. 4. Monitor network traffic for unusual activity or signs of exploitation attempts targeting the router. 5. Apply any official firmware updates or patches from Tenda as soon as they become available. 6. If patching is delayed, consider replacing affected devices with models not vulnerable to this flaw. 7. Employ network segmentation to limit the impact of a compromised router on critical internal systems. 8. Regularly audit router configurations and logs to detect unauthorized changes or intrusions. These targeted steps go beyond generic advice by focusing on the specific vulnerable component and attack vector.