CVE-2026-2870: Tenda A21 Firmware Buffer Overflow Flaw

CVE-2026-2870 Overview

A stack-based buffer overflow vulnerability has been discovered in Tenda A21 router firmware version 1.0.0.0. This security flaw affects the set_qosMib_list function within the /goform/formSetQosBand endpoint. An authenticated attacker can exploit this vulnerability by manipulating the list argument, leading to a stack-based buffer overflow condition. The attack can be executed remotely over the network, and exploitation details have been publicly disclosed.

Critical Impact

Remote attackers with low privileges can exploit this buffer overflow to potentially execute arbitrary code or cause denial of service on affected Tenda A21 routers, compromising network security and device integrity.

Affected Products

• Tenda A21 Firmware version 1.0.0.0
• Tenda A21 Hardware devices running vulnerable firmware

Discovery Timeline

• 2026-02-21 - CVE-2026-2870 published to NVD
• 2026-02-23 - Last updated in NVD database

Technical Details for CVE-2026-2870

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 QoS (Quality of Service) management functionality of the Tenda A21 router's web interface. When processing QoS bandwidth configuration requests, the set_qosMib_list function fails to properly validate the length of user-supplied input in the list parameter before copying it to a fixed-size stack buffer.

The vulnerability is accessible remotely over the network and requires only low-privilege authentication to exploit. No user interaction is required, making it particularly dangerous in environments where router administrative interfaces are exposed or where compromised credentials could provide initial access.

Root Cause

The root cause of this vulnerability stems from insufficient input validation in the set_qosMib_list function. The function processes data submitted to the /goform/formSetQosBand endpoint without adequately checking the size of the list parameter against the allocated buffer size. This allows an attacker to supply an oversized input that exceeds the stack buffer boundaries, overwriting adjacent memory including return addresses and other critical stack data.

Attack Vector

The attack is network-based and targets the web management interface of the Tenda A21 router. An attacker must have valid credentials (low privilege level required) to access the QoS configuration endpoint. The exploitation flow involves:

1. Authenticating to the router's web interface with valid credentials
2. Sending a crafted HTTP POST request to /goform/formSetQosBand
3. Including a maliciously oversized value in the list parameter
4. The overflow corrupts the stack, potentially enabling arbitrary code execution or device crash

The vulnerability has been publicly disclosed with exploitation details available through the GitHub Issue Discussion, increasing the risk of exploitation in the wild.

Detection Methods for CVE-2026-2870

Indicators of Compromise

• Anomalous HTTP POST requests to /goform/formSetQosBand with unusually large list parameter values
• Router crashes or unexpected reboots following QoS configuration access
• Suspicious network traffic originating from the router to unknown external hosts
• Unexpected changes to router configuration or firmware

Detection Strategies

• Monitor web server logs for POST requests to /goform/formSetQosBand containing excessively long parameter values
• Implement intrusion detection rules to flag HTTP requests with abnormally large payload sizes targeting QoS endpoints
• Deploy network monitoring to detect unexpected outbound connections from router management interfaces
• Establish baseline behavior for router management traffic and alert on deviations

Monitoring Recommendations

• Enable logging on the Tenda A21 administrative interface and forward logs to a centralized SIEM
• Configure network segmentation to isolate router management interfaces from untrusted networks
• Monitor for firmware integrity changes using scheduled configuration backups and comparison tools

How to Mitigate CVE-2026-2870

Immediate Actions Required

• Restrict access to the router's web management interface to trusted IP addresses only
• Ensure strong, unique credentials are used for router administration
• Disable remote management if not required, limiting access to the local network
• Monitor the Tenda Official Website for firmware updates addressing this vulnerability

Patch Information

As of the last update on 2026-02-23, no official patch has been confirmed from Tenda. Organizations should monitor vendor announcements and security advisories for updates. Additional technical details are available through VulDB #347107.

Workarounds

• Implement firewall rules to block external access to the router's web management interface on port 80/443
• Use a VPN or jump host to access router administration from remote locations instead of exposing the interface directly
• Consider deploying a web application firewall (WAF) in front of the management interface to filter malicious requests
• Regularly audit router access logs for unauthorized authentication attempts

# Example: iptables rule to restrict management interface access to trusted subnet
iptables -A INPUT -p tcp --dport 80 -s 192.168.1.0/24 -j ACCEPT
iptables -A INPUT -p tcp --dport 80 -j DROP
iptables -A INPUT -p tcp --dport 443 -s 192.168.1.0/24 -j ACCEPT
iptables -A INPUT -p tcp --dport 443 -j DROP