CVE-2026-4904: Tenda AC5 Buffer Overflow Vulnerability

CVE-2026-4904 Overview

A critical stack-based buffer overflow vulnerability has been discovered in Tenda AC5 wireless routers running firmware version 15.03.06.47. The vulnerability exists in the formSetCfm function within the /goform/setcfm POST request handler component. An authenticated attacker can remotely exploit this vulnerability by manipulating the funcpara1 argument, leading to memory corruption that could result in arbitrary code execution or denial of service on affected devices.

Critical Impact

This stack-based buffer overflow vulnerability allows remote attackers with low privileges to compromise the confidentiality, integrity, and availability of the affected Tenda AC5 router. Successful exploitation could lead to complete device takeover, enabling attackers to intercept network traffic, modify configurations, or use the router as a pivot point for further attacks.

Affected Products

• Tenda AC5 Firmware version 15.03.06.47
• Tenda AC5 Hardware version 1.0

Discovery Timeline

• 2026-03-27 - CVE-2026-4904 published to NVD
• 2026-03-31 - Last updated in NVD database

Technical Details for CVE-2026-4904

Vulnerability Analysis

The vulnerability resides in improper input handling within the formSetCfm function, which processes POST requests directed to the /goform/setcfm endpoint. When processing the funcpara1 parameter, the function fails to properly validate the length of user-supplied input before copying it to a stack-allocated buffer. This lack of bounds checking allows an attacker to supply a specially crafted request containing an oversized funcpara1 value that exceeds the buffer's allocated size.

As a stack-based buffer overflow (CWE-787: Out-of-bounds Write, CWE-119: Improper Restriction of Operations within the Bounds of a Memory Buffer), successful exploitation could allow an attacker to overwrite adjacent stack memory, including saved return addresses and function pointers. This could enable arbitrary code execution with the privileges of the web server process running on the router.

The exploit has been publicly disclosed, increasing the risk of active exploitation in the wild. The network-accessible nature of this vulnerability, combined with the low attack complexity and minimal privilege requirements, makes it particularly dangerous for exposed devices.

Root Cause

The root cause of this vulnerability is insufficient input validation in the formSetCfm function when handling the funcpara1 argument from POST requests. The function copies user-supplied data into a fixed-size stack buffer without verifying that the input length does not exceed the buffer's capacity. This represents a classic example of improper memory bounds checking that has been a persistent issue in embedded device firmware development.

Attack Vector

The attack can be launched remotely over the network by sending a malicious HTTP POST request to the /goform/setcfm endpoint. The attacker needs low-level authentication to the router's web interface. The attack does not require user interaction.

The exploitation methodology involves:

1. Authenticating to the Tenda AC5 web management interface
2. Crafting a malicious POST request to /goform/setcfm
3. Including an oversized funcpara1 parameter designed to overflow the stack buffer
4. Overwriting the return address with attacker-controlled values
5. Redirecting execution flow to malicious shellcode or existing code gadgets

The vulnerability mechanism involves improper bounds checking in the formSetCfm function when processing the funcpara1 POST parameter. When an attacker sends an HTTP POST request to /goform/setcfm with an excessively long funcpara1 value, the function copies this input into a stack-allocated buffer without validating the length, causing adjacent memory to be overwritten. For detailed technical analysis, refer to the Notion Documentation for Tenda AC5.

Detection Methods for CVE-2026-4904

Indicators of Compromise

• Unexpected HTTP POST requests to /goform/setcfm with abnormally large funcpara1 parameter values
• Unusual router crashes or reboots without administrative action
• Unauthorized configuration changes on the router
• Anomalous outbound network connections from the router to unknown destinations

Detection Strategies

• Implement network intrusion detection rules to monitor for HTTP POST requests to /goform/setcfm containing oversized parameters
• Deploy web application firewall rules to inspect and block malformed requests targeting the vulnerable endpoint
• Monitor router logs for repeated authentication attempts followed by POST requests to the /goform/setcfm path
• Configure alerts for unexpected router restarts that may indicate exploitation attempts

Monitoring Recommendations

• Enable comprehensive logging on the Tenda AC5 router if supported by the firmware
• Implement network traffic analysis at the perimeter to detect suspicious requests to IoT devices
• Monitor for changes to router DNS settings or firewall rules that could indicate compromise
• Establish baseline behavioral profiles for router traffic patterns to detect anomalous activity

How to Mitigate CVE-2026-4904

Immediate Actions Required

• Restrict access to the router's web management interface to trusted internal networks only
• Disable remote management access if not explicitly required
• Implement network segmentation to isolate IoT devices from critical network segments
• Change default credentials and implement strong authentication for the router administration panel

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 firmware updates addressing CVE-2026-4904. Additional vulnerability details are available through VulDB #353655.

Workarounds

• Configure firewall rules to restrict access to the /goform/setcfm endpoint from untrusted networks
• Place the Tenda AC5 router behind a dedicated firewall that can filter malicious HTTP requests
• Consider replacing the affected device with a router from a vendor with more responsive security patching
• Implement strict network access controls to limit which hosts can communicate with the router's management interface

# Example iptables rule to restrict access to router management interface
# Apply on upstream firewall or gateway device
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 management access only from specific trusted host
iptables -I FORWARD -s <TRUSTED_ADMIN_IP> -d <ROUTER_IP> -p tcp --dport 80 -j ACCEPT