CVE-2026-4565: Tenda AC21 Buffer Overflow Vulnerability

CVE-2026-4565 Overview

A buffer overflow vulnerability has been identified in Tenda AC21 routers running firmware version 16.03.08.16. The vulnerability exists in the formSetQosBand function within the file /goform/SetNetControlList. An attacker can exploit this flaw by manipulating the list argument, leading to a buffer overflow condition. This vulnerability can be triggered remotely and a public exploit is available.

Critical Impact

Remote attackers can exploit this buffer overflow vulnerability to potentially execute arbitrary code or cause denial of service on affected Tenda AC21 routers with firmware version 16.03.08.16.

Affected Products

• Tenda AC21 Firmware version 16.03.08.16

Discovery Timeline

• 2026-03-23 - CVE-2026-4565 published to NVD
• 2026-03-23 - Last updated in NVD database

Technical Details for CVE-2026-4565

Vulnerability Analysis

This vulnerability is classified under CWE-119 (Improper Restriction of Operations within the Bounds of a Memory Buffer). The formSetQosBand function in the Tenda AC21 router firmware fails to properly validate the length of user-supplied input passed through the list parameter. When processing requests to /goform/SetNetControlList, the function copies user-controlled data into a fixed-size buffer without adequate bounds checking.

The vulnerability is remotely exploitable over the network and requires low-privilege authentication to access the affected endpoint. Since the exploit is publicly available, the risk of active exploitation increases significantly for unpatched devices.

Root Cause

The root cause of this vulnerability is improper memory buffer operations in the formSetQosBand function. The function does not implement proper bounds checking when processing the list argument from incoming HTTP requests. This allows an attacker to supply an oversized input value that exceeds the allocated buffer size, resulting in adjacent memory being overwritten.

Attack Vector

The attack can be initiated remotely over the network. An authenticated attacker with low privileges can send a crafted HTTP request to the /goform/SetNetControlList endpoint with a maliciously constructed list parameter. The oversized input triggers a buffer overflow condition in the formSetQosBand function.

The vulnerability mechanism involves sending HTTP POST requests to the vulnerable endpoint with an excessively long list parameter value. When the formSetQosBand function processes this input without proper length validation, it overflows the destination buffer, potentially corrupting adjacent memory structures. This can lead to denial of service through application crash or potentially allow code execution if the attacker can precisely control the overwritten memory. For technical details and proof-of-concept information, refer to the GitHub Issue #14 and the GitHub PoC Release.

Detection Methods for CVE-2026-4565

Indicators of Compromise

• Unusual or malformed HTTP POST requests to /goform/SetNetControlList with abnormally long list parameter values
• Router crashes or unexpected reboots that may indicate buffer overflow exploitation attempts
• Network traffic anomalies targeting the router's web management interface

Detection Strategies

• Monitor HTTP traffic to /goform/SetNetControlList for requests containing unusually large list parameter values
• Implement network intrusion detection rules to flag oversized POST requests to Tenda AC21 management endpoints
• Review router logs for access attempts to the vulnerable endpoint from unauthorized IP addresses

Monitoring Recommendations

• Enable logging on network firewalls and IDS/IPS systems to capture traffic destined for router management interfaces
• Implement rate limiting on management interface access to slow down potential exploitation attempts
• Monitor for firmware version 16.03.08.16 across the network and flag devices requiring updates

How to Mitigate CVE-2026-4565

Immediate Actions Required

• Restrict access to the router's web management interface to trusted IP addresses only
• Disable remote management access if not required for operations
• Implement network segmentation to isolate affected routers from untrusted networks
• Monitor for firmware updates from Tenda and apply patches as soon as they become available

Patch Information

At the time of publication, no official patch information is available from Tenda. Organizations should monitor the Tenda Website for security updates and firmware releases. Additional vulnerability details can be found at VulDB #352402.

Workarounds

• Configure firewall rules to block external access to port 80/443 on the router's management interface
• Use access control lists (ACLs) to limit which hosts can connect to the router's web interface
• Consider deploying an upstream firewall or security gateway to filter malicious requests before they reach the vulnerable device

# Example: Restrict management access via iptables on upstream firewall
iptables -A FORWARD -d <router_ip> -p tcp --dport 80 -j DROP
iptables -A FORWARD -d <router_ip> -p tcp --dport 443 -j DROP
# Allow only trusted management hosts
iptables -I FORWARD -s <trusted_admin_ip> -d <router_ip> -p tcp --dport 80 -j ACCEPT