CVE-2026-4566: Belkin F9K1122 Buffer Overflow Vulnerability

CVE-2026-4566 Overview

A stack-based buffer overflow vulnerability has been identified in Belkin F9K1122 WiFi Range Extender firmware version 1.00.33. The vulnerability exists in the formWISP5G function within the /goform/formWISP5G endpoint, where improper handling of the webpage argument can lead to memory corruption. This flaw allows remote authenticated attackers to potentially execute arbitrary code or cause denial of service conditions on affected devices.

Critical Impact

Remote attackers with low privileges can exploit this stack-based buffer overflow to potentially achieve code execution on vulnerable Belkin F9K1122 devices, compromising network infrastructure security.

Affected Products

• Belkin F9K1122 WiFi Range Extender firmware version 1.00.33
• Other firmware versions may also be affected (unconfirmed)

Discovery Timeline

• 2026-03-23 - CVE-2026-4566 published to NVD
• 2026-03-23 - Last updated in NVD database

Technical Details for CVE-2026-4566

Vulnerability Analysis

This vulnerability is classified as CWE-119 (Improper Restriction of Operations within the Bounds of a Memory Buffer). The flaw occurs when the formWISP5G function processes the webpage argument without proper bounds checking, allowing an attacker to supply input that exceeds the allocated stack buffer size.

When exploited, the overflow can overwrite adjacent memory on the stack, including return addresses and saved frame pointers. This memory corruption can lead to arbitrary code execution if an attacker crafts input that overwrites the return address with a controlled value, redirecting program flow to attacker-supplied shellcode or existing code gadgets.

The vulnerability is particularly concerning because it is remotely exploitable over the network by authenticated users. A proof-of-concept has been published demonstrating the exploitability of this flaw. The vendor was contacted during responsible disclosure but did not respond.

Root Cause

The root cause of this vulnerability is insufficient input validation in the formWISP5G function. The code fails to properly validate the length of the webpage parameter before copying it to a fixed-size stack buffer. Without proper bounds checking, user-supplied data can overflow the allocated buffer space, corrupting adjacent stack memory and potentially allowing attackers to hijack the control flow of the application.

Attack Vector

The attack vector is network-based, requiring the attacker to have authenticated access to the device's web management interface. The attacker can craft a malicious HTTP request to the /goform/formWISP5G endpoint with an oversized webpage parameter value. The exploitation does not require user interaction and can be performed with low privilege access.

The vulnerability has been documented with proof-of-concept details available in GitHub Belkin Vulnerability Info. For technical exploitation specifics, security researchers can reference the GitHub Belkin PoC Repository.

Detection Methods for CVE-2026-4566

Indicators of Compromise

• Unusual HTTP POST requests to /goform/formWISP5G with abnormally long webpage parameter values
• Unexpected device crashes or reboots that may indicate exploitation attempts
• Anomalous network traffic patterns from the Belkin device suggesting compromise
• Log entries showing malformed requests to the affected endpoint

Detection Strategies

• Implement network intrusion detection rules to identify oversized HTTP requests targeting /goform/formWISP5G
• Monitor for HTTP POST requests containing webpage parameters exceeding normal expected lengths
• Deploy web application firewall rules to block requests with suspicious payload sizes to affected endpoints
• Review device logs for repeated access attempts to the vulnerable endpoint

Monitoring Recommendations

• Enable logging on network devices to capture traffic to and from Belkin F9K1122 devices
• Set up alerts for unusual device behavior including unexpected reboots or connectivity issues
• Monitor for unauthorized configuration changes on the affected device
• Implement network segmentation monitoring to detect lateral movement from compromised IoT devices

How to Mitigate CVE-2026-4566

Immediate Actions Required

• Restrict network access to the Belkin F9K1122 management interface using firewall rules
• Place the affected device on an isolated network segment with limited access
• Disable remote management access if not required for operations
• Consider replacing the affected device with a supported alternative if no patch becomes available

Patch Information

At the time of publication, no vendor patch is available for this vulnerability. The vendor (Belkin) was contacted during responsible disclosure but did not respond. Organizations should monitor VulDB #352403 and the vendor's support page for any future security updates. Given the lack of vendor response, affected users should consider this device end-of-life from a security perspective.

Workarounds

• Implement network access control lists (ACLs) to restrict management interface access to trusted IP addresses only
• Deploy a network firewall or intrusion prevention system to filter malicious requests targeting the vulnerable endpoint
• Disable the web management interface if administrative access is not frequently required
• Consider deploying a VPN solution to access the management interface securely from remote locations

# Example iptables rules to restrict access to device management interface
# Replace 192.168.1.100 with your Belkin device IP
# Replace 192.168.1.0/24 with your trusted management network

# Block external access to the device management port
iptables -A FORWARD -d 192.168.1.100 -p tcp --dport 80 -j DROP
iptables -A FORWARD -d 192.168.1.100 -p tcp --dport 443 -j DROP

# Allow only trusted network to access management interface
iptables -I FORWARD -s 192.168.1.0/24 -d 192.168.1.100 -p tcp --dport 80 -j ACCEPT