CVE-2026-3976: Tenda W3 Buffer Overflow Vulnerability

CVE-2026-3976 Overview

A stack-based buffer overflow vulnerability has been identified in the Tenda W3 router firmware version 1.0.0.3(2204). The vulnerability exists in the formWifiMacFilterSet function located in the /goform/WifiMacFilterSet endpoint of the POST Parameter Handler component. Remote attackers can exploit this vulnerability by manipulating the index or GO parameters, potentially leading to arbitrary code execution or device compromise.

Critical Impact

This vulnerability allows remote attackers to trigger a stack-based buffer overflow through malicious POST requests, potentially enabling complete device takeover, persistent backdoor installation, or use of the compromised router as a pivot point for further network attacks.

Affected Products

• Tenda W3 Router Firmware version 1.0.0.3(2204)

Discovery Timeline

• 2026-03-12 - CVE-2026-3976 published to NVD
• 2026-03-12 - Last updated in NVD database

Technical Details for CVE-2026-3976

Vulnerability Analysis

The vulnerability is classified under CWE-119 (Improper Restriction of Operations within the Bounds of a Memory Buffer). The formWifiMacFilterSet function in the Tenda W3 firmware fails to properly validate the length of user-supplied input through the index and GO POST parameters. When processing HTTP POST requests to the /goform/WifiMacFilterSet endpoint, the function copies user-controlled data into a fixed-size stack buffer without adequate bounds checking. This improper memory operation allows attackers to overwrite adjacent stack memory, including return addresses and other critical stack data.

The attack can be performed remotely over the network without requiring user interaction, though it does require low-level privileges (authentication to the router's web interface). Successful exploitation can result in complete compromise of confidentiality, integrity, and availability of the affected device.

Root Cause

The root cause of this vulnerability is insufficient input validation in the formWifiMacFilterSet function. The firmware does not implement proper boundary checks when handling the index and GO parameters from POST requests. When these parameters contain data exceeding the expected buffer size, the excess data overflows into adjacent stack memory. This is a classic example of unsafe memory handling in embedded device firmware, where resource constraints often lead developers to skip security validations.

Attack Vector

The attack vector is network-based, targeting the web management interface of the Tenda W3 router. An attacker with network access to the device's management interface can craft a malicious HTTP POST request to /goform/WifiMacFilterSet containing oversized values in the index or GO parameters. The exploitation process involves:

1. Identifying a vulnerable Tenda W3 device on the network
2. Crafting a POST request with an oversized index or GO parameter value
3. Sending the malicious request to trigger the buffer overflow
4. Overwriting the return address to redirect execution flow
5. Achieving arbitrary code execution on the device

Public proof-of-concept repositories demonstrate both the index parameter and GO parameter overflow variants. Technical details and PoC code are available in the Tenda W3 formWifiMacFilterSet index buffer overflow repository and Tenda W3 formWifiMacFilterSet GO buffer overflow repository.

Detection Methods for CVE-2026-3976

Indicators of Compromise

• Unexpected HTTP POST requests to /goform/WifiMacFilterSet with abnormally large parameter values
• Router crashes, reboots, or unresponsive web management interface following suspicious network activity
• Anomalous outbound connections from the router to unknown external IP addresses
• Unauthorized configuration changes or new administrative accounts on the device

Detection Strategies

• Implement network intrusion detection rules to identify POST requests to /goform/WifiMacFilterSet with oversized index or GO parameters
• Monitor router logs for repeated authentication attempts followed by web interface crashes
• Deploy web application firewall rules to block requests with parameter values exceeding expected lengths for the affected endpoint
• Use network traffic analysis to detect anomalous patterns targeting router management interfaces

Monitoring Recommendations

• Enable verbose logging on the Tenda W3 router if available and monitor for web interface access anomalies
• Implement network segmentation to isolate router management interfaces from untrusted network segments
• Deploy behavioral analysis tools to detect unusual router activity patterns such as unexpected reboots or configuration changes
• Regularly audit network traffic to and from router management ports for suspicious activity

How to Mitigate CVE-2026-3976

Immediate Actions Required

• Disable remote management access to the Tenda W3 router immediately if not required for operations
• Restrict access to the router's web management interface to trusted IP addresses only using firewall rules
• Place the router's management interface on an isolated network segment inaccessible from untrusted networks
• Monitor for and apply any firmware updates released by Tenda addressing this vulnerability

Patch Information

At the time of publication, no official patch has been released by Tenda to address CVE-2026-3976. Users should monitor the Tenda official website for firmware updates. Additional vulnerability details are tracked in VulDB #350411.

Workarounds

• Configure firewall rules to block external access to the router's web management interface (typically port 80/443)
• Use VPN or other secure access methods for remote administration instead of exposing the web interface directly
• Consider replacing the vulnerable device with a router from a vendor that provides regular security updates if a patch is not forthcoming
• Implement network access control to ensure only authorized devices can reach the management interface

# Example iptables rule to restrict management interface access
# Allow management access only from trusted admin subnet
iptables -A INPUT -p tcp --dport 80 -s 192.168.1.0/24 -j ACCEPT
iptables -A INPUT -p tcp --dport 80 -j DROP
iptables -A INPUT -p tcp --dport 443 -s 192.168.1.0/24 -j ACCEPT
iptables -A INPUT -p tcp --dport 443 -j DROP