CVE-2026-24111: Tenda W20e Firmware Buffer Overflow Flaw

CVE-2026-24111 Overview

A critical buffer overflow vulnerability has been discovered in Tenda W20E V4.0br_V15.11.0.6 router firmware. The vulnerability exists in the addAuthUser function where user-supplied input through the userInfo parameter is processed by sscanf without proper size validation. This classic buffer overflow condition allows remote attackers to potentially execute arbitrary code or cause denial of service on affected devices.

Critical Impact

Remote attackers can exploit this buffer overflow vulnerability over the network without authentication, potentially gaining complete control of the affected Tenda W20E router or causing device crashes.

Affected Products

• Tenda W20E Firmware version 15.11.0.6
• Tenda W20E Hardware version 4.0
• Tenda W20E V4.0br_V15.11.0.6

Discovery Timeline

• 2026-03-02 - CVE-2026-24111 published to NVD
• 2026-03-05 - Last updated in NVD database

Technical Details for CVE-2026-24111

Vulnerability Analysis

This vulnerability is classified as CWE-120 (Buffer Copy without Checking Size of Input), a classic buffer overflow condition. The flaw resides in the firmware's authentication user management functionality, specifically within the addAuthUser function.

When processing user authentication requests, the affected function accepts a userInfo parameter from network requests. This parameter is then parsed using the sscanf function, a C standard library function known for its lack of built-in bounds checking. The absence of size validation before or during the sscanf operation allows an attacker to supply an oversized input that exceeds the destination buffer's allocated memory space.

The network-accessible nature of this vulnerability combined with no authentication requirements makes it particularly dangerous for exposed devices. Successful exploitation could allow attackers to overwrite adjacent memory, potentially hijacking program execution flow to achieve remote code execution, or at minimum causing the device to crash.

Root Cause

The root cause of this vulnerability is improper input validation in the addAuthUser function. The function uses sscanf to parse the userInfo parameter without first verifying that the input length does not exceed the destination buffer size. This is a common programming error in C/C++ codebases, particularly prevalent in embedded systems and IoT device firmware where memory-safe languages are less commonly used.

The use of sscanf without corresponding format string length specifiers (such as %Ns where N limits input size) allows unbounded data to be written to fixed-size stack or heap buffers.

Attack Vector

The attack vector for CVE-2026-24111 is network-based, requiring no authentication or user interaction. An attacker can craft a malicious HTTP request to the router's web management interface containing an oversized userInfo parameter value.

The exploitation process involves sending a specially crafted request where the userInfo field contains data exceeding the expected buffer size. When the vulnerable addAuthUser function processes this input through sscanf, the excess data overwrites adjacent memory regions. Depending on the memory layout and the attacker's payload, this can result in:

1. Device crash - Overwriting critical data structures causing service disruption
2. Code execution - Overwriting return addresses or function pointers to redirect execution to attacker-controlled shellcode

Technical details and proof-of-concept information can be found in the GitHub CVE-2026-24111 Report.

Detection Methods for CVE-2026-24111

Indicators of Compromise

• Unusual or malformed HTTP requests to the router's web management interface with oversized userInfo parameters
• Unexpected router crashes or reboots that may indicate exploitation attempts
• Anomalous network traffic patterns targeting the router's administrative ports
• Evidence of unauthorized configuration changes on the device

Detection Strategies

• Monitor network traffic for HTTP requests containing abnormally large userInfo parameter values targeting Tenda W20E devices
• Implement network intrusion detection rules to identify buffer overflow attack patterns against router management interfaces
• Deploy web application firewall (WAF) rules to filter and inspect requests to router administrative endpoints
• Establish baseline behavior monitoring for Tenda W20E devices to detect anomalous activity

Monitoring Recommendations

• Enable logging on network perimeter devices to capture all traffic to/from Tenda W20E router management interfaces
• Configure alerts for repeated authentication failures or malformed requests to router endpoints
• Monitor device uptime and unexpected reboots which may indicate active exploitation attempts
• Review access logs for any suspicious IP addresses attempting to reach the router's web interface

How to Mitigate CVE-2026-24111

Immediate Actions Required

• Restrict network access to the Tenda W20E router management interface to trusted IP addresses only
• Disable remote management features if not required for operations
• Place the router behind a firewall that can filter and inspect incoming traffic
• Segment the network to isolate vulnerable devices from untrusted networks

Patch Information

At the time of publication, no official patch has been confirmed for this vulnerability. Device owners should monitor the Tenda support page for firmware updates. Given the critical severity of this vulnerability, organizations should prioritize monitoring for vendor patches and apply them immediately when available.

Consider the following when planning remediation:

• Check for firmware updates on Tenda's official website regularly
• Subscribe to security advisories from Tenda for notification of patches
• If no patch is available, evaluate whether the device should be replaced with a more secure alternative

Workarounds

• Implement strict access control lists (ACLs) to limit which IP addresses can reach the router's management interface
• Deploy network-level filtering to block requests with oversized parameters from reaching the device
• Use a VPN for remote management rather than exposing the web interface directly to the internet
• Consider deploying a reverse proxy with input validation capabilities in front of the device management interface

# Example iptables rules 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 443 -s 192.168.1.0/24 -j ACCEPT
iptables -A INPUT -p tcp --dport 80 -j DROP
iptables -A INPUT -p tcp --dport 443 -j DROP