CVE-2026-24109: Tenda W20e Firmware Buffer Overflow Flaw

CVE-2026-24109 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 handling of the picName parameter, where user-controlled input is passed to sprintf without proper validation of variable sizes. This unsafe memory operation can lead to a classic stack-based buffer overflow, potentially enabling remote attackers to execute arbitrary code or cause denial of service on affected devices.

Critical Impact

This network-accessible buffer overflow vulnerability requires no authentication or user interaction, allowing remote attackers to potentially gain complete control over affected Tenda W20E routers.

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-2026-24109 published to NVD
• 2026-03-05 - Last updated in NVD database

Technical Details for CVE-2026-24109

Vulnerability Analysis

CVE-2026-24109 is classified as CWE-120 (Buffer Copy without Checking Size of Input), a classic buffer overflow vulnerability. The flaw originates from the firmware's improper handling of the picName parameter value. When this user-controlled input is processed using the sprintf function, the code fails to validate the length of the input data against the destination buffer's capacity.

The sprintf function in C is inherently unsafe because it does not perform bounds checking. When an attacker supplies an excessively long string for picName, the function will write beyond the allocated buffer boundaries, corrupting adjacent memory regions including potentially critical control structures like return addresses and saved registers on the stack.

Given that this vulnerability is network-accessible without requiring authentication, an attacker can remotely craft malicious HTTP requests containing oversized picName values to trigger the overflow condition. Successful exploitation could lead to arbitrary code execution with the privileges of the web server process, typically running as root on embedded devices like the Tenda W20E.

Root Cause

The root cause of this vulnerability is the use of the unsafe sprintf function without implementing proper input validation or bounds checking. The firmware code directly incorporates user-supplied data from the picName parameter into a fixed-size buffer without verifying that the input length does not exceed the buffer's capacity. This is a common vulnerability pattern in C-based embedded device firmware where memory-safe alternatives like snprintf should be used instead.

Attack Vector

The attack vector is network-based, allowing remote exploitation without authentication. An attacker can send specially crafted HTTP requests to the Tenda W20E router's web management interface containing an oversized picName parameter. The malicious payload would overflow the destination buffer, potentially overwriting return addresses or other control data structures. By carefully crafting the overflow payload, an attacker could redirect program execution to attacker-controlled code, achieving remote code execution on the device.

The vulnerability mechanism involves passing user-controlled input from the picName parameter directly to sprintf without size validation. When the input exceeds the expected buffer size, memory corruption occurs. Additional technical details can be found in the GitHub CVE Report and the Tenda Security Advisory.

Detection Methods for CVE-2026-24109

Indicators of Compromise

• Unexpected router reboots or crashes that may indicate exploitation attempts
• Anomalous HTTP requests to the router's web interface containing unusually long picName parameter values
• Unauthorized configuration changes or new user accounts on the router
• Unusual outbound network traffic from the router indicating potential command and control communication

Detection Strategies

• Deploy network intrusion detection systems (IDS) with signatures for buffer overflow patterns in HTTP requests targeting Tenda W20E devices
• Monitor HTTP traffic to router management interfaces for requests with excessively long parameter values (>256 bytes in picName field)
• Implement web application firewall (WAF) rules to block malformed requests to Tenda router endpoints
• Configure logging on network perimeter devices to capture and analyze traffic destined for router management ports

Monitoring Recommendations

• Enable comprehensive logging on network devices monitoring traffic to and from Tenda W20E routers
• Set up alerts for authentication failures or unusual access patterns to router management interfaces
• Monitor for unexpected firmware changes or configuration modifications on affected devices
• Implement network segmentation to isolate router management interfaces from untrusted networks

How to Mitigate CVE-2026-24109

Immediate Actions Required

• Restrict access to the router's web management interface to trusted internal networks only
• Disable remote management features if not required for operations
• Implement network segmentation to isolate affected Tenda W20E devices from untrusted network segments
• Monitor affected devices for signs of compromise while awaiting vendor patch

Patch Information

No official patch has been confirmed at this time. Organizations should consult the Tenda Security Advisory for the latest firmware update information. Until a patch is available, implement the recommended workarounds to reduce exposure.

Workarounds

• Disable remote management and web interface access from WAN/external networks
• Implement strict access control lists (ACLs) limiting management interface access to specific trusted IP addresses
• Deploy a firewall or network security device in front of the Tenda W20E to filter malicious requests
• Consider replacing the affected device with a router that has received security updates if extended use is required

# Example firewall rule to restrict management access (iptables)
# Block external access to router management ports
iptables -A INPUT -i eth0 -p tcp --dport 80 -s 192.168.1.0/24 -j ACCEPT
iptables -A INPUT -i eth0 -p tcp --dport 80 -j DROP
iptables -A INPUT -i eth0 -p tcp --dport 443 -s 192.168.1.0/24 -j ACCEPT
iptables -A INPUT -i eth0 -p tcp --dport 443 -j DROP