CVE-2026-2874: Tenda A21 Buffer Overflow Vulnerability

CVE-2026-2874 Overview

A stack-based buffer overflow vulnerability has been discovered in Tenda A21 firmware version 1.0.0.0. The vulnerability affects the form_fast_setting_wifi_set function located in the /goform/fast_setting_wifi_set endpoint. By manipulating the ssid argument, an attacker can trigger a stack-based buffer overflow condition. This vulnerability can be exploited remotely over the network, making it particularly dangerous for exposed devices.

Critical Impact

Remote attackers can exploit this buffer overflow to potentially execute arbitrary code, crash the device, or gain unauthorized control over the Tenda A21 router by sending malicious requests to the vulnerable endpoint.

Affected Products

• Tenda A21 Firmware version 1.0.0.0
• Tenda A21 Hardware Device

Discovery Timeline

• 2026-02-21 - CVE-2026-2874 published to NVD
• 2026-02-23 - Last updated in NVD database

Technical Details for CVE-2026-2874

Vulnerability Analysis

This vulnerability is classified as CWE-119 (Improper Restriction of Operations within the Bounds of a Memory Buffer). The flaw exists in the form_fast_setting_wifi_set function, which handles WiFi configuration requests through the /goform/fast_setting_wifi_set endpoint.

When processing the ssid parameter, the function fails to properly validate the input length before copying data to a fixed-size stack buffer. This allows an attacker to supply an overly long SSID value that exceeds the allocated buffer space, resulting in a classic stack-based buffer overflow condition.

The network-accessible nature of this vulnerability, combined with low attack complexity and no user interaction requirements, makes it a significant security concern for organizations using Tenda A21 devices.

Root Cause

The root cause of this vulnerability is insufficient bounds checking on user-supplied input in the form_fast_setting_wifi_set function. The ssid parameter is processed without verifying that its length fits within the destination buffer on the stack. This is a common vulnerability pattern in embedded device firmware where memory-safe programming practices may not be consistently applied.

Attack Vector

The attack can be executed remotely over the network by sending a crafted HTTP POST request to the /goform/fast_setting_wifi_set endpoint with a maliciously long ssid parameter value. The attacker needs to have low-level privileges (authenticated access) to reach the vulnerable endpoint.

The exploitation flow involves:

1. Attacker identifies a Tenda A21 device accessible on the network
2. Attacker sends a crafted HTTP request to /goform/fast_setting_wifi_set with an oversized ssid parameter
3. The vulnerable function copies the malicious input to a stack buffer without bounds checking
4. Stack memory is corrupted, potentially overwriting the return address
5. This can lead to arbitrary code execution or denial of service

The vulnerability has been publicly disclosed and exploit information is available, increasing the risk of active exploitation. For detailed technical information, refer to the GitHub Issue Discussion and VulDB entry #347111.

Detection Methods for CVE-2026-2874

Indicators of Compromise

• Unusual HTTP POST requests to /goform/fast_setting_wifi_set with abnormally long ssid parameter values
• Device crashes or unexpected reboots of Tenda A21 routers
• Anomalous network traffic patterns targeting the router's web management interface
• Unexpected changes to WiFi configuration settings

Detection Strategies

• Implement network intrusion detection rules to monitor for oversized HTTP parameters targeting /goform/fast_setting_wifi_set
• Deploy web application firewall rules to limit parameter length for requests to the vulnerable endpoint
• Monitor Tenda A21 device logs for crash events or service restarts that may indicate exploitation attempts
• Use network traffic analysis to identify reconnaissance activity targeting Tenda devices

Monitoring Recommendations

• Enable logging on network firewalls and IDS/IPS systems for traffic to and from Tenda A21 devices
• Implement alerting for any access to the /goform/ path on Tenda router management interfaces
• Regularly review device stability metrics to detect potential denial of service conditions
• Consider implementing network segmentation to isolate IoT devices from critical network assets

How to Mitigate CVE-2026-2874

Immediate Actions Required

• Restrict network access to the Tenda A21 web management interface to trusted IP addresses only
• Disable remote management features if not required for operations
• Place affected devices behind a firewall with strict ingress filtering
• Implement network segmentation to isolate vulnerable devices from sensitive network segments
• Monitor for firmware updates from Tenda

Patch Information

At the time of 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 that address CVE-2026-2874. Additional technical details and community discussion can be found in the GitHub Issue Discussion.

Workarounds

• Disable the web management interface if remote configuration is not required
• Implement access control lists (ACLs) on upstream network devices to restrict access to the router's management port
• Use a VPN to access the management interface rather than exposing it directly to untrusted networks
• Consider replacing the affected device with an alternative that receives regular security updates

# Example: Block external access to router management on upstream firewall
# Adjust IP addresses and interface names for your environment
iptables -A FORWARD -d <TENDA_A21_IP> -p tcp --dport 80 -i <WAN_INTERFACE> -j DROP
iptables -A FORWARD -d <TENDA_A21_IP> -p tcp --dport 443 -i <WAN_INTERFACE> -j DROP