CVE-2026-4254: Tenda AC8 Buffer Overflow Vulnerability

CVE-2026-4254 Overview

A stack-based buffer overflow vulnerability has been identified in Tenda AC8 routers up to firmware version 16.03.50.11. This vulnerability affects the doSystemCmd function within the /goform/SysToolChangePwd HTTP endpoint. By manipulating the local_2c argument, an unauthenticated remote attacker can trigger a stack-based buffer overflow, potentially leading to arbitrary code execution or denial of service on the affected device.

Critical Impact

Remote attackers can exploit this vulnerability over the network without authentication to potentially execute arbitrary code on vulnerable Tenda AC8 routers, compromising network security and device integrity.

Affected Products

• Tenda AC8 firmware versions up to 16.03.50.11
• Devices with the vulnerable /goform/SysToolChangePwd HTTP endpoint exposed

Discovery Timeline

• 2026-03-16 - CVE-2026-4254 published to NVD
• 2026-03-17 - Last updated in NVD database

Technical Details for CVE-2026-4254

Vulnerability Analysis

This vulnerability is classified under CWE-119 (Improper Restriction of Operations within the Bounds of a Memory Buffer). The flaw exists in how the doSystemCmd function processes user-supplied input through the local_2c argument when handling requests to the /goform/SysToolChangePwd endpoint.

The HTTP endpoint, designed for system password management functionality, fails to properly validate the length of input data before copying it to a fixed-size stack buffer. This allows an attacker to supply oversized input that overflows the allocated buffer space on the stack, potentially overwriting adjacent memory including saved return addresses and other critical data structures.

Root Cause

The root cause stems from insufficient bounds checking on user-controlled input within the doSystemCmd function. When processing password change requests, the function allocates a fixed-size buffer on the stack but does not verify that incoming data fits within these boundaries. This classic buffer overflow pattern is common in embedded device firmware where memory-safe programming practices may not be consistently applied.

Attack Vector

The vulnerability is remotely exploitable over the network. An attacker can craft a malicious HTTP POST request to the /goform/SysToolChangePwd endpoint containing an oversized payload in the local_2c parameter. Since the attack requires no authentication, any attacker with network access to the router's web management interface can attempt exploitation.

The attack flow involves sending a specially crafted HTTP request that exceeds the expected buffer size, overwriting stack memory and potentially redirecting program execution flow. Successful exploitation could allow the attacker to gain full control of the router, modify its configuration, intercept network traffic, or use the device as a pivot point for further attacks within the network.

Technical details and proof-of-concept information are available in the GitHub CVE Report.

Detection Methods for CVE-2026-4254

Indicators of Compromise

• Unexpected HTTP POST requests to /goform/SysToolChangePwd with abnormally large payload sizes
• Router crashes, unexpected reboots, or service instability without clear cause
• Unusual outbound network connections originating from the router
• Modified router configuration settings that were not authorized by administrators

Detection Strategies

• Deploy network intrusion detection rules to monitor for oversized HTTP POST requests targeting /goform/SysToolChangePwd
• Implement web application firewall (WAF) rules to block requests with excessively long parameter values
• Monitor router logs for repeated failed authentication attempts or malformed requests
• Use endpoint detection and response (EDR) solutions on network segments to identify anomalous traffic patterns

Monitoring Recommendations

• Establish baseline network traffic patterns for router management interfaces and alert on deviations
• Configure SIEM alerts for high-volume requests to vulnerable endpoints from external sources
• Regularly audit router firmware versions and maintain an inventory of potentially affected devices
• Enable verbose logging on router management interfaces where supported

How to Mitigate CVE-2026-4254

Immediate Actions Required

• Restrict access to the router's web management interface to trusted internal networks only
• Disable remote management functionality if not required for operations
• Implement network segmentation to isolate vulnerable devices from untrusted network segments
• Monitor Tenda's official channels for firmware updates addressing this vulnerability

Patch Information

As of the last NVD update on 2026-03-17, no official patch has been confirmed from Tenda. Organizations should monitor the Tenda Official Website and vendor security advisories for patch availability. Additional technical details and tracking information can be found at VulDB #351212.

Workarounds

• Configure firewall rules to block external access to the /goform/SysToolChangePwd endpoint
• Limit management interface access to specific trusted IP addresses using access control lists (ACLs)
• Consider replacing vulnerable devices with alternative hardware if patches are not forthcoming
• Use a VPN for remote management rather than exposing the web interface directly

# Example iptables rule to restrict access to management interface
# Replace eth0 with your WAN interface and adjust IP as needed
iptables -A INPUT -i eth0 -p tcp --dport 80 -j DROP
iptables -A INPUT -i eth0 -p tcp --dport 443 -j DROP

# Allow management access only from trusted management subnet
iptables -A INPUT -s 192.168.1.0/24 -p tcp --dport 80 -j ACCEPT