CVE-2026-4566 identifies a stack-based buffer overflow vulnerability in the Belkin F9K1122 router firmware version 1.00.33. The vulnerability resides in the formWISP5G function, which processes requests to the /goform/formWISP5G endpoint. Specifically, the 'webpage' argument is improperly handled, allowing an attacker to supply crafted input that overflows the stack buffer. This overflow can corrupt the stack frame, potentially enabling arbitrary code execution on the device. The vulnerability is remotely exploitable over the network without requiring authentication or user interaction, making it highly accessible to attackers. The CVSS v4.0 score is 8.7, reflecting the ease of exploitation (network vector, low complexity) and the high impact on confidentiality, integrity, and availability. Although no active exploits have been observed in the wild, proof-of-concept exploit code has been published, increasing the likelihood of future attacks. The vendor has not issued a patch or responded to disclosure, leaving affected devices vulnerable. This flaw could be leveraged to gain persistent control over the router, intercept or manipulate network traffic, or launch further attacks within the network environment.

The impact of CVE-2026-4566 is significant for organizations using the Belkin F9K1122 router, particularly those with devices exposed to the internet or untrusted networks. Successful exploitation can lead to remote code execution, allowing attackers to take full control of the device. This compromises the confidentiality of network traffic, integrity of data passing through the router, and availability of network services. Attackers could use the compromised router as a foothold for lateral movement within corporate networks or as a platform for launching attacks against other targets. The lack of vendor response and patch availability prolongs exposure, increasing the risk of widespread exploitation. Small and medium enterprises, home offices, and possibly some larger organizations relying on this router model are at risk. The vulnerability also threatens the security of connected IoT devices and sensitive data traversing the network.

Given the absence of an official patch from Belkin, organizations should immediately implement compensating controls. First, isolate affected routers from untrusted networks by restricting inbound access to the /goform/formWISP5G endpoint using firewall rules or network segmentation. Disable remote management features if enabled. Monitor network traffic for unusual activity indicative of exploitation attempts targeting this vulnerability. Replace or upgrade affected devices to models with updated firmware once available. If possible, apply custom firmware or third-party patches that address the buffer overflow. Conduct regular vulnerability scanning to identify exposed devices. Educate users and administrators about the risk and signs of compromise. Maintain robust network monitoring and incident response capabilities to quickly detect and respond to exploitation attempts. Finally, advocate for vendor accountability and timely patch releases through coordinated disclosure channels.