CVE-2026-4041: Tenda i12 Buffer Overflow Vulnerability

CVE-2026-4041 Overview

A stack-based buffer overflow vulnerability has been discovered in Tenda i12 router firmware version 1.0.0.6(2204). The vulnerability exists in the vos_strcpy function within the /goform/exeCommand endpoint. An attacker can exploit this flaw by manipulating the cmdinput argument, causing a stack-based buffer overflow that can be triggered remotely. The exploit has been publicly disclosed and may already be used in active attacks targeting vulnerable devices.

Critical Impact

Remote attackers can exploit this buffer overflow vulnerability to potentially execute arbitrary code, compromise device integrity, and gain unauthorized control over affected Tenda i12 routers. The network-accessible attack vector significantly increases exposure risk for internet-facing devices.

Affected Products

• Tenda i12 Firmware Version 1.0.0.6(2204)

Discovery Timeline

• 2026-03-12 - CVE CVE-2026-4041 published to NVD
• 2026-03-12 - Last updated in NVD database

Technical Details for CVE-2026-4041

Vulnerability Analysis

This vulnerability is classified under CWE-119 (Improper Restriction of Operations within the Bounds of a Memory Buffer). The flaw resides in the web management interface of the Tenda i12 router, specifically in how the vos_strcpy function handles user-supplied input from the cmdinput parameter.

The vos_strcpy function appears to copy user-controlled input without adequate boundary checks, allowing an attacker to overflow the stack buffer. Since this endpoint is accessible via network requests, authenticated attackers with low privileges can craft malicious requests to trigger the overflow condition.

Stack-based buffer overflows in embedded devices like routers are particularly dangerous because they often lack modern exploit mitigations such as ASLR, stack canaries, or DEP that are common in desktop operating systems. Successful exploitation could allow attackers to overwrite return addresses or function pointers, redirecting execution flow to attacker-controlled code.

Root Cause

The root cause of this vulnerability is the unsafe use of the vos_strcpy function to handle the cmdinput parameter without proper input length validation. The function copies user-supplied data into a fixed-size stack buffer without verifying that the input length does not exceed the buffer capacity. This allows attackers to supply oversized input that overwrites adjacent stack memory, including return addresses and saved registers.

Attack Vector

The attack can be launched remotely over the network by sending a specially crafted HTTP request to the /goform/exeCommand endpoint. The attacker must have low-level privileges (likely basic authentication to the router's web interface) to reach the vulnerable function. By providing an excessively long string in the cmdinput parameter, the attacker triggers the buffer overflow condition.

The vulnerability mechanism involves sending malicious input to the /goform/exeCommand endpoint that exceeds the expected buffer size in the vos_strcpy function. Technical details and proof-of-concept information can be found in the GitHub CVE Issue Discussion and VulDB CTI Incident Report.

Detection Methods for CVE-2026-4041

Indicators of Compromise

• Unusual HTTP POST requests to /goform/exeCommand with abnormally long cmdinput parameter values
• Router crashes or unexpected reboots that may indicate exploitation attempts
• Unexpected network traffic patterns originating from the router to external addresses
• Modified router configurations or unauthorized administrative access

Detection Strategies

• Deploy network intrusion detection rules to flag HTTP requests to /goform/exeCommand with payload sizes exceeding normal thresholds
• Monitor web server logs on Tenda i12 devices for requests containing excessively long cmdinput values
• Implement deep packet inspection on network segments containing vulnerable devices
• Use endpoint detection solutions capable of monitoring embedded device behavior anomalies

Monitoring Recommendations

• Enable verbose logging on Tenda i12 devices if supported by the firmware
• Set up network monitoring to alert on any traffic patterns indicative of buffer overflow exploitation
• Regularly audit device configurations for unauthorized changes
• Monitor for unusual outbound connections from router IP addresses

How to Mitigate CVE-2026-4041

Immediate Actions Required

• Restrict access to the router's web management interface to trusted IP addresses only
• Disable remote administration features if not required for operations
• Implement network segmentation to isolate affected devices from critical network resources
• Monitor for firmware updates from Tenda and apply patches immediately when available

Patch Information

At the time of publication, no official patch has been confirmed from Tenda for this vulnerability. Organizations should monitor the Tenda Official Site for security advisories and firmware updates. Until a patch is available, implementing the recommended workarounds is critical to reduce exposure.

Workarounds

• Configure firewall rules to block external access to the /goform/exeCommand endpoint
• Disable the web management interface entirely and use console access where possible
• Place affected devices behind a VPN to limit exposure to authenticated users only
• Consider replacing affected devices with alternative hardware if no patch becomes available

# Configuration example - Firewall rule to restrict access to management interface
# Apply on upstream firewall/router to block external access to Tenda i12 web interface
iptables -A FORWARD -d <TENDA_ROUTER_IP> -p tcp --dport 80 -j DROP
iptables -A FORWARD -d <TENDA_ROUTER_IP> -p tcp --dport 443 -j DROP
# Allow only trusted management IPs
iptables -I FORWARD -s <TRUSTED_MGMT_IP> -d <TENDA_ROUTER_IP> -p tcp --dport 80 -j ACCEPT