CVE-2026-4535: Tenda FH451 Buffer Overflow Vulnerability

CVE-2026-4535 Overview

A stack-based buffer overflow vulnerability has been discovered in Tenda FH451 firmware version 1.0.0.9. This vulnerability affects the WrlclientSet function within the /goform/WrlclientSet file, where improper handling of the GO argument allows attackers to trigger a buffer overflow condition. The vulnerability can be exploited remotely over the network, making it particularly concerning for organizations with exposed Tenda devices.

Critical Impact

Remote attackers can exploit this stack-based buffer overflow to potentially achieve arbitrary code execution on vulnerable Tenda FH451 routers, potentially compromising network infrastructure and gaining persistent access to affected environments.

Affected Products

• Tenda FH451 firmware version 1.0.0.9
• Tenda FH451 wireless routers running vulnerable firmware

Discovery Timeline

• 2026-03-22 - CVE-2026-4535 published to NVD
• 2026-03-23 - Last updated in NVD database

Technical Details for CVE-2026-4535

Vulnerability Analysis

This vulnerability is classified under CWE-119 (Improper Restriction of Operations within the Bounds of a Memory Buffer). The WrlclientSet function in the Tenda FH451 firmware fails to properly validate the length of user-supplied input passed through the GO argument before copying it to a fixed-size stack buffer. When an attacker supplies an oversized value, the function writes beyond the allocated buffer boundaries, corrupting adjacent memory regions on the stack.

The network-accessible nature of this vulnerability significantly increases its risk profile. Attackers can reach the vulnerable endpoint through the device's web management interface without requiring physical access. The exploitation complexity is considered low, as the attack can be executed with minimal prerequisites beyond low-level authentication.

Root Cause

The root cause of this vulnerability lies in insufficient input validation within the WrlclientSet function. The function accepts user-controlled data through the GO parameter and processes it without adequate bounds checking. This classic buffer overflow pattern occurs when the firmware copies input data to a stack-allocated buffer using functions that do not enforce length restrictions, allowing the input to overflow into adjacent stack memory including saved return addresses and other control data.

Attack Vector

The attack vector for CVE-2026-4535 is network-based, targeting the /goform/WrlclientSet endpoint on vulnerable Tenda FH451 devices. An attacker with low-level privileges can craft a malicious HTTP request containing an oversized GO parameter value. When the vulnerable function processes this request, the excessive data overwrites critical stack structures.

The exploitation flow typically involves:

1. Identifying a vulnerable Tenda FH451 device exposed on the network
2. Crafting an HTTP request to the /goform/WrlclientSet endpoint with a malicious GO parameter
3. Sending the crafted request to trigger the buffer overflow
4. Potentially achieving code execution by overwriting the return address with attacker-controlled values

The exploit has been publicly disclosed, and technical details are available in the GitHub Vulnerability Repository. Organizations should review this disclosure for detailed exploitation mechanics.

Detection Methods for CVE-2026-4535

Indicators of Compromise

• Unusual HTTP POST requests to /goform/WrlclientSet with abnormally large GO parameter values
• Router crashes, unexpected reboots, or unresponsive behavior following web management access
• Suspicious outbound network connections originating from Tenda router devices
• Unexpected changes to router configuration or firmware

Detection Strategies

• Implement network intrusion detection rules to identify oversized HTTP requests targeting /goform/WrlclientSet
• Monitor for HTTP requests containing excessively long parameter values directed at Tenda device endpoints
• Deploy web application firewall rules to block requests with abnormal payload sizes to router management interfaces
• Conduct regular vulnerability scans of network infrastructure to identify unpatched Tenda devices

Monitoring Recommendations

• Enable logging on network firewalls and IDS/IPS systems to capture traffic to and from Tenda router management interfaces
• Monitor device health metrics for unexpected reboots or service interruptions on Tenda FH451 units
• Implement network segmentation to isolate IoT and network infrastructure devices from general network traffic
• Review access logs for the router's web management interface for suspicious activity patterns

How to Mitigate CVE-2026-4535

Immediate Actions Required

• Restrict network access to the Tenda FH451 web management interface using firewall rules or access control lists
• Disable remote management features if not required for operational purposes
• Isolate vulnerable Tenda devices on a dedicated network segment with strict ingress and egress filtering
• Monitor the Tenda Official Website for firmware updates addressing this vulnerability

Patch Information

At the time of publication, no official patch information has been released by Tenda. Organizations should monitor vendor communications and apply firmware updates as soon as they become available. Additional technical details and vulnerability tracking information can be found at VulDB #352323.

Workarounds

• Configure firewall rules to block external access to the router's web management interface (typically port 80/443)
• Implement IP whitelisting to allow management access only from trusted administrative workstations
• Consider deploying a VPN solution for remote management access instead of exposing the web interface directly
• Evaluate replacing vulnerable devices with alternative router hardware if patches are not forthcoming

# Example firewall rules to restrict management access
# Block external access to router management interface
iptables -A INPUT -p tcp --dport 80 -s ! 192.168.1.0/24 -j DROP
iptables -A INPUT -p tcp --dport 443 -s ! 192.168.1.0/24 -j DROP

# Allow management only from specific admin IP
iptables -A INPUT -p tcp --dport 80 -s 192.168.1.100 -j ACCEPT
iptables -A INPUT -p tcp --dport 443 -s 192.168.1.100 -j ACCEPT