CVE-2026-2871: Tenda A21 Firmware Buffer Overflow Flaw

CVE-2026-2871 Overview

A stack-based buffer overflow vulnerability has been identified in the Tenda A21 wireless router firmware version 1.0.0.0. This vulnerability affects the fromSetIpMacBind function within the /goform/SetIpMacBind file, where improper handling of the list argument enables remote attackers to trigger a buffer overflow condition. The exploit has been publicly disclosed, increasing the risk of active exploitation against vulnerable devices.

Critical Impact

Remote attackers can exploit this stack-based buffer overflow to potentially execute arbitrary code, crash the device, or gain unauthorized access to the router, compromising network security.

Affected Products

• Tenda A21 Firmware version 1.0.0.0
• Tenda A21 Hardware Device

Discovery Timeline

• 2026-02-21 - CVE-2026-2871 published to NVD
• 2026-02-23 - Last updated in NVD database

Technical Details for CVE-2026-2871

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 fromSetIpMacBind function, which processes user-supplied input from the list parameter without adequate bounds checking. When an attacker submits a crafted request with an oversized list value, the function writes beyond the allocated stack buffer boundaries, corrupting adjacent memory structures.

The network-accessible nature of this vulnerability combined with low attack complexity makes it particularly dangerous for internet-exposed Tenda A21 routers. An authenticated attacker with low privileges can exploit this flaw remotely without user interaction. Successful exploitation can lead to complete compromise of device confidentiality, integrity, and availability.

Root Cause

The root cause of CVE-2026-2871 is insufficient input validation in the fromSetIpMacBind function. The function fails to properly validate the length of the list argument before copying it to a fixed-size stack buffer. This classic buffer overflow scenario occurs when the input data exceeds the buffer's allocated space, overwriting critical stack data including return addresses and saved frame pointers.

Attack Vector

The attack vector is network-based, targeting the /goform/SetIpMacBind endpoint accessible through the router's web management interface. An attacker can craft a malicious HTTP request containing an oversized list parameter value. When the vulnerable fromSetIpMacBind function processes this request, the overflow occurs on the stack, potentially allowing the attacker to:

1. Overwrite the function's return address to redirect execution flow
2. Inject and execute shellcode on the device
3. Cause a denial of service by crashing the router's web service

The vulnerability exploitation details are documented in the GitHub Issue Discussion and VulDB Entry #347108.

Detection Methods for CVE-2026-2871

Indicators of Compromise

• Unusual HTTP POST requests to /goform/SetIpMacBind with abnormally large list parameter values
• Router crashes, reboots, or unresponsive web management interface
• Unexpected network traffic originating from the router to external IP addresses
• Modified router configuration or unauthorized administrative accounts

Detection Strategies

• Monitor HTTP traffic to Tenda A21 devices for requests containing oversized parameters in the SetIpMacBind endpoint
• Implement intrusion detection signatures to flag buffer overflow attack patterns targeting Tenda router firmware
• Deploy network anomaly detection to identify unusual POST request sizes to router management interfaces
• Review router logs for repeated failed authentication attempts or access to administrative endpoints

Monitoring Recommendations

• Enable logging on the Tenda A21 router if supported and forward logs to a centralized SIEM
• Monitor network traffic patterns for suspicious outbound connections from IoT devices
• Implement network segmentation to isolate vulnerable router management interfaces from untrusted networks
• Set up alerts for any access attempts to the /goform/SetIpMacBind endpoint from external sources

How to Mitigate CVE-2026-2871

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 firewall rules to block external access to the router's administrative ports
• Monitor the Tenda Official Website for firmware updates addressing this vulnerability

Patch Information

At the time of publication, no official patch has been released by Tenda for CVE-2026-2871. Organizations should monitor vendor communications and apply security updates as soon as they become available. The vulnerability details have been submitted to VulDB (Submission #754630 and Submission #754631) for tracking and coordination.

Workarounds

• Configure network access control lists (ACLs) to restrict access to the router management interface to specific trusted IP addresses only
• Place the Tenda A21 router behind a properly configured firewall that blocks unauthorized access to management ports
• Consider replacing the vulnerable device with an alternative router if a patch is not released in a timely manner
• Use VPN tunneling for any required remote administrative access rather than exposing the management interface directly

# Example firewall rule to restrict access to router management (adjust IP ranges as needed)
# Block external access to common router management ports
iptables -A INPUT -p tcp --dport 80 -s ! 192.168.1.0/24 -j DROP
iptables -A INPUT -p tcp --dport 443 -s ! 192.168.1.0/24 -j DROP