CVE-2026-2908: Tenda HG9 Buffer Overflow Vulnerability

CVE-2026-2908 Overview

A stack-based buffer overflow vulnerability has been identified in the Tenda HG9 router firmware version 300001138. This vulnerability exists within the Loopback Detection Configuration Endpoint, specifically in the handling of the Ethtype argument within the /boaform/formLoopBack file. The flaw allows attackers to overflow the stack buffer through improper input validation, potentially leading to remote code execution or denial of service.

The vulnerability can be exploited remotely over the network, making it particularly dangerous for internet-exposed devices. The exploit has been disclosed publicly and may be actively used in attacks targeting vulnerable Tenda HG9 routers.

Critical Impact

Remote attackers can exploit this stack-based buffer overflow to potentially execute arbitrary code or crash affected Tenda HG9 routers, compromising network security and device availability.

Affected Products

• Tenda HG9 Firmware version 300001138
• Tenda HG9 Hardware devices running affected firmware
• Network environments utilizing vulnerable Tenda HG9 routers

Discovery Timeline

• 2026-02-22 - CVE-2026-2908 published to NVD
• 2026-02-23 - Last updated in NVD database

Technical Details for CVE-2026-2908

Vulnerability Analysis

This vulnerability is classified under CWE-119 (Improper Restriction of Operations within the Bounds of a Memory Buffer), which encompasses various buffer overflow conditions. The flaw resides in the Loopback Detection Configuration Endpoint's processing of the Ethtype argument.

When the affected endpoint receives a specially crafted request containing a malicious Ethtype value, the firmware fails to properly validate the input length before copying data to a stack-allocated buffer. This allows an attacker to overwrite adjacent memory regions on the stack, including critical control data such as return addresses and saved frame pointers.

The network-accessible nature of this vulnerability, combined with low attack complexity and the requirement for only low-level authentication, makes this a significant risk for organizations deploying Tenda HG9 devices in their network infrastructure.

Root Cause

The root cause of this vulnerability is insufficient input validation and boundary checking in the /boaform/formLoopBack endpoint. The firmware does not properly verify the length of the Ethtype parameter before processing, allowing oversized input to overflow the fixed-size stack buffer. This is a common coding error in embedded device firmware where memory-safe programming practices are not consistently applied.

Attack Vector

The attack vector for CVE-2026-2908 is network-based, requiring no user interaction. An authenticated attacker with low-level privileges can send a malicious HTTP request to the /boaform/formLoopBack endpoint with a crafted Ethtype parameter. The oversized payload triggers the buffer overflow, potentially allowing the attacker to:

1. Overwrite the return address on the stack to redirect execution flow
2. Inject and execute arbitrary code with device privileges
3. Cause a denial of service by crashing the router firmware

The vulnerability exploitation involves sending HTTP requests to the router's web management interface with manipulated parameters designed to exceed buffer boundaries. Technical details of the exploit methodology have been disclosed in the GitHub Issue Report.

Detection Methods for CVE-2026-2908

Indicators of Compromise

• Unusual HTTP POST requests to /boaform/formLoopBack with abnormally long Ethtype parameter values
• Router crashes, unexpected reboots, or unresponsive web management interfaces
• Anomalous network traffic patterns originating from or directed at Tenda HG9 devices
• Log entries indicating failed authentication attempts followed by successful loopback configuration changes

Detection Strategies

• Deploy network intrusion detection systems (IDS) with signatures for buffer overflow patterns targeting Tenda devices
• Monitor HTTP traffic to router management interfaces for requests containing oversized parameters
• Implement web application firewalls (WAF) to filter malicious requests to /boaform/formLoopBack
• Enable logging on network devices to capture and analyze access to router configuration endpoints

Monitoring Recommendations

• Establish baseline network behavior for Tenda HG9 devices and alert on deviations
• Monitor router CPU and memory utilization for signs of exploitation attempts
• Implement SIEM rules to correlate multiple failed requests to loopback configuration endpoints
• Review router access logs regularly for unauthorized configuration change attempts

How to Mitigate CVE-2026-2908

Immediate Actions Required

• Restrict network access to the router's web management interface using firewall rules
• Disable remote management features if not required for operations
• Segment Tenda HG9 devices on isolated network VLANs to limit exposure
• Implement strong authentication credentials and limit administrative access
• Monitor for firmware updates from Tenda addressing this vulnerability

Patch Information

At the time of publication, no official patch from Tenda has been confirmed for this vulnerability. Organizations should monitor the Tenda Official Website for security advisories and firmware updates. Additional vulnerability tracking information is available through VulDB #347217.

Workarounds

• Configure access control lists (ACLs) to restrict management interface access to trusted IP addresses only
• Place affected routers behind a properly configured firewall that blocks external access to port 80/443
• Consider replacing vulnerable devices with alternative hardware until a patch is available
• Implement network segmentation to isolate potentially vulnerable devices from critical systems

# Example firewall rule to restrict management interface access
# Adjust interface and IP ranges according to your network configuration
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