CVE-2026-2872: Tenda A21 Buffer Overflow Vulnerability

CVE-2026-2872 Overview

A critical stack-based buffer overflow vulnerability has been identified in Tenda A21 router firmware version 1.0.0.0. This vulnerability exists within the set_device_name function located in the /goform/setBlackRule endpoint, which is part of the MAC Filtering Configuration component. Remote attackers can exploit this flaw by manipulating the devName or mac arguments to trigger a buffer overflow condition, potentially leading to arbitrary code execution or device compromise.

Critical Impact

Remote attackers can exploit this stack-based buffer overflow to potentially execute arbitrary code on vulnerable Tenda A21 routers, compromising network security and enabling further attacks on connected devices.

Affected Products

• Tenda A21 Firmware version 1.0.0.0
• Tenda A21 Hardware

Discovery Timeline

• 2026-02-21 - CVE-2026-2872 published to NVD
• 2026-02-23 - Last updated in NVD database

Technical Details for CVE-2026-2872

Vulnerability Analysis

This vulnerability is classified as CWE-119 (Improper Restriction of Operations within the Bounds of a Memory Buffer). The set_device_name function in the Tenda A21 router firmware fails to properly validate the length of user-supplied input when processing the devName and mac parameters through the /goform/setBlackRule endpoint. This MAC Filtering Configuration endpoint is designed to allow administrators to configure device blacklisting rules, but the lack of boundary checking creates a dangerous attack surface.

When an attacker submits an HTTP request with oversized values for the devName or mac parameters, the vulnerable function copies this data onto the stack without verifying that the destination buffer is large enough to accommodate it. This results in adjacent stack memory being overwritten, including potentially critical data such as return addresses and saved registers.

Root Cause

The root cause of this vulnerability stems from insufficient input validation in the set_device_name function. The function allocates a fixed-size buffer on the stack for processing device name and MAC address values but does not implement proper bounds checking before copying user-supplied data. This classic buffer overflow pattern allows attackers to write beyond the allocated buffer boundaries, corrupting stack memory and potentially hijacking program execution flow.

Attack Vector

The attack can be performed remotely over the network by sending specially crafted HTTP POST requests to the /goform/setBlackRule endpoint. An attacker with low-level privileges (authenticated access to the router's web interface) can exploit this vulnerability by:

1. Accessing the MAC Filtering Configuration interface
2. Submitting a crafted request with oversized devName or mac parameter values
3. Triggering the stack-based buffer overflow in the set_device_name function
4. Potentially achieving code execution or causing a denial of service condition

The exploit has been publicly disclosed, increasing the risk of active exploitation in the wild. Technical details regarding this vulnerability can be found in the GitHub Issue Discussion and additional context is available through VulDB #347109.

Detection Methods for CVE-2026-2872

Indicators of Compromise

• Unexpected HTTP POST requests to /goform/setBlackRule containing unusually long devName or mac parameter values
• Router instability, crashes, or unexpected reboots that may indicate exploitation attempts
• Unusual network traffic patterns originating from or directed at Tenda A21 devices
• Unauthorized changes to MAC filtering rules or device blacklist configurations

Detection Strategies

• Implement web application firewall (WAF) rules to detect and block requests with excessively long parameter values targeting /goform/setBlackRule
• Monitor router logs for repeated failed or malformed requests to the MAC Filtering Configuration endpoint
• Deploy network-based intrusion detection systems (IDS) with signatures for buffer overflow attack patterns targeting IoT devices
• Enable SentinelOne Singularity™ for IoT visibility and anomaly detection on network segments containing vulnerable devices

Monitoring Recommendations

• Establish baseline network behavior for Tenda A21 devices and alert on deviations
• Configure centralized logging for all web interface access attempts to router management endpoints
• Implement network segmentation to isolate IoT devices from critical infrastructure
• Monitor for unexpected outbound connections from router devices that may indicate compromise

How to Mitigate CVE-2026-2872

Immediate Actions Required

• Restrict access to the router's web management interface to trusted networks only
• Implement network-level access controls to limit who can reach the /goform/setBlackRule endpoint
• Consider disabling the web management interface if not actively needed for administration
• Place affected Tenda A21 devices behind a firewall with strict ingress filtering

Patch Information

At the time of this publication, no official patch has been released by Tenda for this vulnerability. Organizations should monitor the Tenda Official Website for security updates and firmware releases addressing CVE-2026-2872. Review the GitHub Issue Discussion for the latest information on vulnerability status and potential mitigations.

Workarounds

• Disable remote management access and restrict web interface access to local, trusted networks only
• Implement strong network segmentation to isolate vulnerable routers from sensitive systems
• Deploy upstream firewall rules to filter malicious requests targeting the vulnerable endpoint
• Consider replacing affected devices with alternative hardware that receives regular security updates

# Example: Restrict access to router management interface using iptables on upstream firewall
# Block external access to router web interface (adjust IP as needed)
iptables -A FORWARD -d 192.168.1.1 -p tcp --dport 80 -j DROP
iptables -A FORWARD -d 192.168.1.1 -p tcp --dport 443 -j DROP

# Allow management access only from trusted admin workstation
iptables -I FORWARD -s 192.168.1.100 -d 192.168.1.1 -p tcp --dport 80 -j ACCEPT