CVE-2026-6016: Tenda AC9 Buffer Overflow Vulnerability

CVE-2026-6016 Overview

A stack-based buffer overflow vulnerability has been identified in Tenda AC9 routers running firmware version 15.03.02.13. The vulnerability exists within the decodePwd function located in the /goform/WizardHandle endpoint of the POST Request Handler component. An attacker can exploit this flaw by manipulating the WANS argument, leading to a buffer overflow condition that could allow arbitrary code execution or denial of service on affected devices.

Critical Impact

Remote attackers with low privileges can exploit this network-accessible vulnerability to potentially achieve code execution or crash affected Tenda AC9 routers, compromising network security and device availability.

Affected Products

• Tenda AC9 Router Firmware Version 15.03.02.13
• Tenda AC9 devices with /goform/WizardHandle endpoint exposed

Discovery Timeline

• April 10, 2026 - CVE-2026-6016 published to NVD
• April 13, 2026 - Last updated in NVD database

Technical Details for CVE-2026-6016

Vulnerability Analysis

This vulnerability is classified under CWE-119 (Improper Restriction of Operations within the Bounds of a Memory Buffer). The decodePwd function within the WizardHandle component fails to properly validate the length of user-supplied input provided through the WANS parameter in POST requests. When an attacker sends a specially crafted request with an oversized WANS argument value, the function copies this data into a fixed-size stack buffer without adequate bounds checking.

The vulnerability can be triggered remotely over the network, requiring only low-level privileges to exploit. Once triggered, the overflow corrupts adjacent stack memory, potentially overwriting critical control structures such as the return address or saved frame pointer.

Root Cause

The root cause stems from improper input validation and memory management in the decodePwd function. The function allocates a fixed-size buffer on the stack to store decoded password data derived from the WANS parameter but fails to enforce length restrictions on the incoming data before copying it into this buffer. This classic stack-based buffer overflow pattern allows attackers to write beyond the allocated buffer boundaries.

Attack Vector

The attack is network-based and targets the /goform/WizardHandle endpoint, which handles POST requests during the router's setup wizard process. An authenticated attacker with low privileges can craft a malicious HTTP POST request containing an excessively long WANS parameter value. When the decodePwd function processes this parameter, the oversized input overflows the stack buffer, potentially enabling the attacker to:

1. Crash the router service (denial of service)
2. Overwrite the return address to redirect execution flow
3. Execute arbitrary code in the context of the router's web service

The exploit has been publicly disclosed, increasing the risk of active exploitation in the wild. Technical details are available in the Notion Documentation Resource.

Detection Methods for CVE-2026-6016

Indicators of Compromise

• Unexpected POST requests to /goform/WizardHandle with abnormally large WANS parameter values
• Router crashes or unexpected reboots following web interface access
• Anomalous network traffic patterns targeting Tenda AC9 management interfaces
• Memory corruption errors or segmentation faults in router logs

Detection Strategies

• Deploy network intrusion detection rules to monitor for oversized POST parameters targeting /goform/WizardHandle
• Implement web application firewall (WAF) rules to validate and limit the size of the WANS parameter
• Enable verbose logging on router management interfaces to capture suspicious request patterns
• Monitor for repeated connection attempts to the router's web management port from external sources

Monitoring Recommendations

• Establish baseline traffic patterns to the router's administrative interface and alert on deviations
• Configure SIEM alerts for multiple failed or malformed requests to Tenda device endpoints
• Implement rate limiting on the /goform/ endpoint to prevent brute-force exploitation attempts
• Review router access logs regularly for signs of reconnaissance or exploitation activity

How to Mitigate CVE-2026-6016

Immediate Actions Required

• Restrict access to the router's web management interface to trusted internal networks only
• Disable remote management features if not explicitly required
• Implement network segmentation to isolate affected Tenda AC9 devices from untrusted networks
• Monitor for firmware updates from Tenda and apply patches as soon as they become available

Patch Information

At the time of this publication, no official patch has been confirmed from Tenda. Administrators should monitor the Tenda Official Website for security advisories and firmware updates addressing this vulnerability. Additional vulnerability details are available through VulDB Vulnerability #356572.

Workarounds

• Configure firewall rules to block external access to the router's management interface (typically port 80/443)
• Use access control lists (ACLs) to whitelist only authorized IP addresses for management access
• Consider deploying a reverse proxy with input validation in front of the management interface
• If possible, disable the WizardHandle functionality through configuration if it is not actively needed

# Example iptables rules to restrict management interface access
# Allow management access only from trusted admin network
iptables -A INPUT -p tcp --dport 80 -s 192.168.1.0/24 -j ACCEPT
iptables -A INPUT -p tcp --dport 443 -s 192.168.1.0/24 -j ACCEPT
# Block all other access to management ports
iptables -A INPUT -p tcp --dport 80 -j DROP
iptables -A INPUT -p tcp --dport 443 -j DROP