CVE-2026-2873: Tenda A21 Buffer Overflow Vulnerability

CVE-2026-2873 Overview

A stack-based buffer overflow vulnerability has been identified in the Tenda A21 router firmware version 1.0.0.0. This security flaw exists in the setSchedWifi function located in the /goform/openSchedWifi file. The vulnerability can be triggered by manipulating the schedStartTime and schedEndTime arguments, leading to a buffer overflow condition that can be exploited remotely by authenticated attackers.

Critical Impact

Remote attackers with low-level privileges can exploit this buffer overflow vulnerability to potentially execute arbitrary code, compromise device integrity, and gain full control of affected Tenda A21 routers.

Affected Products

• Tenda A21 Firmware version 1.0.0.0
• Tenda A21 Hardware

Discovery Timeline

• 2026-02-21 - CVE-2026-2873 published to NVD
• 2026-02-23 - Last updated in NVD database

Technical Details for CVE-2026-2873

Vulnerability Analysis

This vulnerability falls under CWE-119 (Improper Restriction of Operations within the Bounds of a Memory Buffer). The setSchedWifi function in the Tenda A21 router firmware fails to properly validate the length of user-supplied input for the schedStartTime and schedEndTime parameters before copying them into a fixed-size stack buffer.

When an attacker sends specially crafted HTTP requests to the /goform/openSchedWifi endpoint with oversized values for these scheduling parameters, the function writes data beyond the allocated buffer boundaries on the stack. This classic stack-based buffer overflow can corrupt adjacent memory regions, potentially overwriting return addresses and enabling arbitrary code execution.

The vulnerability is remotely exploitable over the network, requiring only low-privilege authentication to access the affected functionality. No user interaction is required for exploitation, making this a significant threat to router security.

Root Cause

The root cause of this vulnerability is insufficient input validation and lack of proper bounds checking in the setSchedWifi function. The function accepts the schedStartTime and schedEndTime parameters from HTTP POST requests without verifying that the input lengths do not exceed the size of the destination stack buffers. This allows attackers to supply arbitrarily long strings that overflow the allocated memory space.

Attack Vector

The attack vector is network-based, allowing remote exploitation. An attacker with low-level authentication to the router's web interface can send malicious HTTP requests to the /goform/openSchedWifi endpoint. By crafting requests with excessively long schedStartTime or schedEndTime values, the attacker can trigger the buffer overflow condition.

The exploitation technique typically involves:

1. Authenticating to the Tenda A21 router web interface
2. Sending a crafted POST request to /goform/openSchedWifi
3. Including oversized data in the schedStartTime or schedEndTime parameters
4. Overflowing the stack buffer to corrupt program control flow

The exploit for this vulnerability is publicly available. For detailed technical information, refer to the GitHub Issue #4 Discussion and the VulDB entry #347110.

Detection Methods for CVE-2026-2873

Indicators of Compromise

• Unusual or malformed HTTP POST requests targeting /goform/openSchedWifi with abnormally long parameter values
• Unexpected router reboots or crashes that may indicate exploitation attempts
• Anomalous network traffic patterns from the router's management interface
• Evidence of unauthorized configuration changes or firmware modifications

Detection Strategies

• Monitor HTTP traffic to Tenda A21 devices for requests to /goform/openSchedWifi with oversized schedStartTime or schedEndTime parameters
• Implement web application firewall (WAF) rules to detect and block requests with excessively long parameter values
• Deploy network intrusion detection systems (NIDS) with signatures for buffer overflow attack patterns targeting embedded devices
• Enable logging on network perimeter devices to capture suspicious traffic destined for router management interfaces

Monitoring Recommendations

• Establish baseline network behavior for Tenda A21 routers and alert on deviations
• Monitor router management interface access logs for unusual authentication patterns
• Set up alerts for multiple failed exploitation attempts or router instability
• Implement network segmentation to isolate IoT/router management traffic for easier monitoring

How to Mitigate CVE-2026-2873

Immediate Actions Required

• Restrict network access to the router's web management interface using firewall rules
• Disable remote administration if not required for operations
• Implement strong authentication and change default credentials immediately
• Isolate affected Tenda A21 devices on a separate network segment until patched

Patch Information

At the time of this publication, users should check the Tenda official website for firmware updates addressing this vulnerability. Monitor security advisories and apply patches as soon as they become available.

For additional technical details and vulnerability tracking, refer to:

• GitHub Issue #4 Discussion
• VulDB Entry #347110

Workarounds

• Disable the scheduled WiFi functionality if not required for business operations
• Restrict access to the /goform/openSchedWifi endpoint using network-level controls
• Place router management interfaces behind a VPN or jump host requiring additional authentication
• Consider replacing vulnerable devices with models from vendors with better security update practices

# Example: Restrict access to router management interface (iptables)
# 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 80 -j DROP

# Block external access to vulnerable endpoint path
iptables -A INPUT -p tcp --dport 80 -m string --string "/goform/openSchedWifi" --algo bm -j DROP