CVE-2026-5608: Belkin F9K1122 Buffer Overflow Vulnerability

CVE-2026-5608 Overview

A stack-based buffer overflow vulnerability has been identified in the Belkin F9K1122 wireless range extender firmware version 1.00.33. The vulnerability exists within the formWlanSetup function located in the /goform/formWlanSetup endpoint. Improper handling of the webpage argument allows remote attackers to trigger a buffer overflow condition, potentially leading to arbitrary code execution or denial of service on affected devices.

Critical Impact

Remote attackers with low privileges can exploit this stack-based buffer overflow to potentially execute arbitrary code or crash the device, compromising network security for all connected devices.

Affected Products

• Belkin F9K1122 Wireless Range Extender firmware version 1.00.33
• Potentially other versions and similar Belkin devices sharing the vulnerable codebase

Discovery Timeline

• 2026-04-06 - CVE-2026-5608 published to NVD
• 2026-04-07 - Last updated in NVD database

Technical Details for CVE-2026-5608

Vulnerability Analysis

This vulnerability is classified under CWE-119 (Improper Restriction of Operations within the Bounds of a Memory Buffer). The affected function formWlanSetup fails to properly validate the length of user-supplied input passed through the webpage argument before copying it to a fixed-size stack buffer. When an attacker provides input exceeding the expected buffer size, the excess data overwrites adjacent memory on the stack, including potentially critical values such as return addresses and saved registers.

The network-accessible nature of this vulnerability significantly increases its risk profile, as the /goform/formWlanSetup endpoint is reachable by any authenticated user on the network. Combined with the low attack complexity, this vulnerability presents a serious threat to affected devices and the networks they serve.

Root Cause

The root cause of this vulnerability lies in insufficient input validation within the formWlanSetup function. The code fails to verify that the length of the webpage parameter does not exceed the allocated stack buffer size before performing memory copy operations. This classic stack-based buffer overflow pattern allows attackers to corrupt stack memory by providing oversized input through HTTP form submissions to the vulnerable endpoint.

Attack Vector

The attack vector is network-based, requiring the attacker to send a crafted HTTP POST request to the /goform/formWlanSetup endpoint on the affected device. The attacker must have low-level privileges (authenticated access to the device's web interface). By manipulating the webpage parameter with a payload exceeding expected bounds, an attacker can overwrite return addresses on the stack to redirect execution flow or cause a denial of service condition.

The vulnerability mechanism involves submitting a malformed HTTP POST request to the /goform/formWlanSetup endpoint with an oversized webpage parameter. The formWlanSetup function copies this user-controlled input into a stack-allocated buffer without proper bounds checking, leading to stack corruption. For detailed technical analysis, refer to the GitHub Vulnerability Database Entry.

Detection Methods for CVE-2026-5608

Indicators of Compromise

• Unexpected device reboots or crashes of Belkin F9K1122 range extenders
• Unusual HTTP POST requests to /goform/formWlanSetup with abnormally large webpage parameter values
• Device becoming unresponsive or exhibiting degraded network performance
• Suspicious authentication attempts or configuration changes on the device

Detection Strategies

• Monitor network traffic for HTTP POST requests to /goform/formWlanSetup containing unusually large payloads
• Implement intrusion detection rules to flag requests with webpage parameters exceeding normal operational sizes
• Deploy network segmentation to isolate IoT devices and monitor cross-segment traffic anomalies
• Review device logs for repeated crashes or authentication failures

Monitoring Recommendations

• Enable logging on network firewalls and capture traffic to and from Belkin devices
• Configure SIEM rules to alert on repeated connection attempts or unusual traffic patterns to embedded device management interfaces
• Periodically audit firmware versions across all network devices to identify vulnerable installations
• Monitor for public exploit releases targeting this vulnerability through threat intelligence feeds

How to Mitigate CVE-2026-5608

Immediate Actions Required

• Restrict network access to the Belkin F9K1122 web management interface using firewall rules
• Place the device on an isolated network segment with limited access from untrusted networks
• Disable remote management features if not required for operations
• Monitor for vendor firmware updates addressing this vulnerability
• Consider replacing affected devices if no patch becomes available

Patch Information

No official patch has been released by Belkin at this time. According to the vulnerability disclosure, the vendor was contacted but did not respond. Users should monitor VulDB Vulnerability #355399 and Belkin's official support channels for any future security updates. Given the vendor's non-response to disclosure, affected organizations should prioritize network-level mitigations and device replacement planning.

Workarounds

• Implement network access control lists (ACLs) to restrict access to the device management interface to trusted IP addresses only
• Place the Belkin F9K1122 behind a firewall that blocks external access to the /goform/ path
• Use VPN or other secure access methods if remote management is required
• Consider deploying a web application firewall (WAF) to filter oversized or malformed requests to the device

# Example iptables rules to restrict access to Belkin device management interface
# Replace 192.168.1.100 with your Belkin device IP and 192.168.1.50 with trusted admin IP

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

# Allow only trusted admin IP to access management interface
iptables -I FORWARD -s 192.168.1.50 -d 192.168.1.100 -p tcp --dport 80 -j ACCEPT