CVE-2026-1637: Tenda AC21 Buffer Overflow Vulnerability

CVE-2026-1637 Overview

A stack-based buffer overflow vulnerability has been identified in Tenda AC21 routers running firmware version 16.03.08.16. The vulnerability exists in the fromAdvSetMacMtuWan function within the /goform/AdvSetMacMtuWan endpoint. Successful exploitation of this vulnerability allows remote attackers to execute arbitrary code on affected devices through network-based attacks. The exploit has been publicly disclosed and may already be in use.

Critical Impact

Remote attackers can exploit this stack-based buffer overflow to gain complete control over affected Tenda AC21 routers, potentially compromising entire network segments and enabling lateral movement within victim environments.

Affected Products

• Tenda AC21 Router - Firmware version 16.03.08.16

Discovery Timeline

• 2026-01-29 - CVE-2026-1637 published to NVD
• 2026-01-29 - Last updated in NVD database

Technical Details for CVE-2026-1637

Vulnerability Analysis

This vulnerability is classified as a stack-based buffer overflow (CWE-119: Improper Restriction of Operations within the Bounds of a Memory Buffer). The flaw resides in the fromAdvSetMacMtuWan function, which handles HTTP requests to the /goform/AdvSetMacMtuWan endpoint on the router's web management interface.

When processing incoming requests, the vulnerable function fails to properly validate the length of user-supplied input before copying it to a fixed-size buffer on the stack. This allows an attacker to supply oversized input that overwrites adjacent memory locations, including the function's return address. By carefully crafting the malicious payload, an attacker can redirect program execution to arbitrary code.

The network attack vector means this vulnerability can be exploited remotely without requiring physical access to the device. While authentication is required (low privileges), the simplicity of exploitation and the potential for full system compromise make this a serious security concern for organizations using affected Tenda routers.

Root Cause

The root cause of this vulnerability is improper input validation and insufficient bounds checking in the fromAdvSetMacMtuWan function. The function accepts user-controlled input from HTTP parameters but does not verify that the input length falls within the expected boundaries before performing memory copy operations. This is a classic buffer overflow pattern commonly found in embedded device firmware where memory-safe programming practices may not be consistently applied.

Attack Vector

The attack vector for CVE-2026-1637 is network-based, targeting the router's web management interface. An attacker with network access to the affected router can send specially crafted HTTP POST requests to the /goform/AdvSetMacMtuWan endpoint. The malicious request contains oversized input parameters designed to overflow the stack buffer and overwrite critical memory structures.

The exploitation process typically involves:

1. Identifying a vulnerable Tenda AC21 router on the network
2. Crafting an HTTP POST request with malicious payload targeting the vulnerable function
3. Sending the request to trigger the buffer overflow
4. Executing arbitrary code with the privileges of the web server process (typically root on embedded devices)

Since this vulnerability affects a router's management interface, successful exploitation could allow attackers to modify router configurations, intercept network traffic, establish persistent backdoors, or use the compromised device as a pivot point for further attacks within the network.

Detection Methods for CVE-2026-1637

Indicators of Compromise

• Unexpected HTTP POST requests to /goform/AdvSetMacMtuWan with unusually large parameter values
• Router crashes or unexpected reboots that may indicate exploitation attempts
• Unauthorized configuration changes or new user accounts on the router
• Unusual outbound network connections originating from the router

Detection Strategies

• Monitor web server logs on Tenda AC21 devices for abnormal requests to the /goform/AdvSetMacMtuWan endpoint
• Implement network intrusion detection rules to identify buffer overflow patterns in HTTP traffic destined for router management interfaces
• Deploy honeypot routers with vulnerable firmware versions to detect active exploitation campaigns
• Utilize firmware integrity monitoring to detect unauthorized modifications

Monitoring Recommendations

• Establish baseline network behavior for router management traffic and alert on anomalies
• Configure SIEM alerts for repeated failed or suspicious requests to Tenda router endpoints
• Monitor for signs of router compromise including DNS modifications or traffic interception
• Track firmware versions across all network devices to maintain visibility of vulnerable systems

How to Mitigate CVE-2026-1637

Immediate Actions Required

• Restrict access to the router's web management interface to trusted IP addresses only
• Disable remote management access from the WAN interface if not required
• Implement network segmentation to isolate router management traffic from general network access
• Monitor for vendor firmware updates and apply them immediately when available

Patch Information

At the time of publication, no official patch has been released by Tenda. Organizations should monitor the Tenda Official Website for security updates and firmware releases addressing this vulnerability. Additional technical details are available through the VulDB entry and the GitHub CVE Issue Discussion.

Workarounds

• Configure firewall rules to block external access to the router's web management interface on port 80/443
• Use a VPN for remote administration rather than exposing the management interface directly
• Consider replacing affected devices with alternative hardware if no patch becomes available
• Implement strong authentication and change default credentials on all network devices

# Example: Restrict management interface access using iptables on upstream firewall
# Block external access to router management interface
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 only from trusted admin workstation
iptables -I FORWARD -s <ADMIN_IP> -d <ROUTER_IP> -p tcp --dport 80 -j ACCEPT
iptables -I FORWARD -s <ADMIN_IP> -d <ROUTER_IP> -p tcp --dport 443 -j ACCEPT