CVE-2026-2140: Tenda TX9 Firmware Buffer Overflow Flaw

CVE-2026-2140 Overview

A buffer overflow vulnerability has been identified in Tenda TX9 routers running firmware versions up to 22.03.02.10_multi. The vulnerability exists in the sub_4223E0 function within the /goform/setMacFilterCfg endpoint, where improper handling of the deviceList argument allows attackers to trigger a buffer overflow condition. This firmware vulnerability can be exploited remotely over the network by authenticated attackers, potentially leading to device compromise, arbitrary code execution, or denial of service.

Critical Impact

Successful exploitation of this buffer overflow vulnerability could allow remote attackers to execute arbitrary code on affected Tenda TX9 routers, potentially gaining full control of the device and compromising network security.

Affected Products

• Tenda TX9 Firmware versions up to 22.03.02.10_multi
• Tenda TX9 Hardware devices running vulnerable firmware
• All Tenda TX9 Pro router variants with affected firmware versions

Discovery Timeline

• 2026-02-08 - CVE-2026-2140 published to NVD
• 2026-02-10 - Last updated in NVD database

Technical Details for CVE-2026-2140

Vulnerability Analysis

This vulnerability is classified as CWE-119 (Improper Restriction of Operations within the Bounds of a Memory Buffer). The flaw resides in the sub_4223E0 function, which processes MAC filter configuration requests through the /goform/setMacFilterCfg endpoint. When the deviceList parameter receives maliciously crafted input, the function fails to properly validate the length of the incoming data before copying it into a fixed-size buffer, resulting in a classic buffer overflow condition.

The network-accessible nature of this vulnerability makes it particularly dangerous for IoT devices like routers, which often sit at network perimeters. Exploitation requires low-privilege authentication but no user interaction, making it feasible for attackers who have gained any level of access to the router's web interface.

Root Cause

The root cause of this vulnerability stems from insufficient bounds checking in the sub_4223E0 function when processing the deviceList argument. The function appears to allocate a fixed-size buffer for storing device list entries but fails to verify that the incoming data does not exceed this allocated space. This lack of input validation allows an attacker to supply an oversized deviceList parameter that overwrites adjacent memory regions, potentially corrupting critical data structures or overwriting return addresses on the stack.

Attack Vector

The attack vector for CVE-2026-2140 is network-based, targeting the router's web management interface. An attacker with low-level authentication to the Tenda TX9 router can send a specially crafted HTTP POST request to the /goform/setMacFilterCfg endpoint. By including an excessively long or malformed deviceList parameter in the request, the attacker can trigger the buffer overflow in the sub_4223E0 function.

The vulnerability is exploited through the following attack flow:

1. Attacker authenticates to the router's web interface with minimal privileges
2. A crafted POST request is sent to /goform/setMacFilterCfg containing an oversized deviceList parameter
3. The sub_4223E0 function processes the malicious input without proper bounds checking
4. Memory corruption occurs as data overflows the allocated buffer
5. Depending on the payload, this can result in denial of service, code execution, or device compromise

Proof-of-concept information and technical documentation are publicly available. For detailed exploitation techniques, see the GitHub IoT Vulnerability Documentation.

Detection Methods for CVE-2026-2140

Indicators of Compromise

• Unusual HTTP POST requests to /goform/setMacFilterCfg with abnormally large deviceList parameters
• Router crashes, unexpected reboots, or unresponsive web management interfaces
• Unexpected changes to MAC filtering configurations on affected devices
• Network traffic anomalies originating from the router's management IP address

Detection Strategies

• Implement network intrusion detection rules to identify HTTP requests to /goform/setMacFilterCfg containing excessively long parameters
• Monitor router syslog output for crash events or memory-related errors in the httpd process
• Deploy web application firewall (WAF) rules to inspect and block malformed requests to vulnerable endpoints
• Configure alerts for authentication attempts followed by rapid configuration change requests

Monitoring Recommendations

• Enable comprehensive logging on Tenda TX9 routers and forward logs to a centralized SIEM solution
• Monitor for repeated access attempts to the /goform/ directory from unusual source addresses
• Implement baseline analysis for normal router behavior to detect anomalous activity patterns
• Consider network segmentation to isolate IoT devices from critical network assets

How to Mitigate CVE-2026-2140

Immediate Actions Required

• Restrict access to the router's web management interface to trusted IP addresses only
• Disable remote management features if not required for operations
• Implement strong, unique credentials for router administration accounts
• Place affected routers behind a firewall that blocks external access to management ports
• Consider replacing affected devices with alternatives from vendors with better security track records

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 firmware updates addressing this security issue. Given Tenda's historical response to security vulnerabilities, organizations should evaluate whether continued use of affected devices aligns with their security requirements.

Additional technical details and vulnerability tracking information are available through the VulDB entry for this vulnerability.

Workarounds

• Disable the web-based management interface entirely and manage the device through other means if available
• Implement network-level access controls (ACLs) to restrict which hosts can reach the router's management ports
• Deploy an upstream firewall or IPS to filter malicious requests targeting the vulnerable endpoint
• Monitor and audit all administrative access to the affected devices until a patch is available
• Consider network isolation for affected devices to limit potential lateral movement if compromised

# Example: Restrict management interface access using upstream firewall (iptables)
# Block external access to router management port (typically 80/443)
iptables -A FORWARD -d <router_ip> -p tcp --dport 80 -j DROP
iptables -A FORWARD -d <router_ip> -p tcp --dport 443 -j DROP

# Allow only trusted management hosts
iptables -I FORWARD -s <trusted_admin_ip> -d <router_ip> -p tcp --dport 80 -j ACCEPT
iptables -I FORWARD -s <trusted_admin_ip> -d <router_ip> -p tcp --dport 443 -j ACCEPT