CVE-2026-5685: Tenda CX12L Buffer Overflow Vulnerability

CVE-2026-5685 Overview

A stack-based buffer overflow vulnerability has been identified in Tenda CX12L firmware version 16.03.53.12. This vulnerability affects the fromAddressNat function within the /goform/addressNat file. The flaw stems from improper handling of the page argument, which can be manipulated to trigger a stack-based buffer overflow condition. As a network-accessible vulnerability, it poses significant risk to affected devices as exploitation can be initiated remotely by authenticated attackers.

Critical Impact

Remote attackers with low privileges can exploit this stack-based buffer overflow to potentially achieve code execution, compromise device confidentiality and integrity, and disrupt device availability. A public exploit is reportedly available, increasing the risk of active exploitation.

Affected Products

• Tenda CX12L with firmware version 16.03.53.12

Discovery Timeline

• 2026-04-06 - CVE-2026-5685 published to NVD
• 2026-04-07 - Last updated in NVD database

Technical Details for CVE-2026-5685

Vulnerability Analysis

This vulnerability is classified under CWE-119 (Improper Restriction of Operations within the Bounds of a Memory Buffer). The flaw resides in how the fromAddressNat function processes user-supplied input through the page parameter. When an attacker provides specially crafted input that exceeds the expected buffer size, the function fails to properly validate the input boundaries, resulting in data being written beyond the allocated stack buffer.

The network-accessible nature of this endpoint, combined with the low attack complexity, makes this vulnerability particularly concerning for organizations deploying affected Tenda routers. An authenticated attacker can leverage this flaw to overwrite adjacent stack memory, potentially including return addresses and saved registers, which could lead to arbitrary code execution with the privileges of the affected process.

Root Cause

The root cause lies in insufficient bounds checking within the fromAddressNat function when processing the page argument. The function does not properly validate the length of user-supplied data before copying it to a fixed-size stack buffer, allowing memory corruption to occur when oversized input is provided.

Attack Vector

The attack is network-based and can be executed remotely against the device's web management interface at the /goform/addressNat endpoint. An attacker with low-level authentication can send a malicious HTTP request containing an oversized page parameter value. The lack of proper input validation allows the overflow to corrupt stack memory.

The exploitation mechanism involves crafting a request to the vulnerable endpoint with a page parameter containing more data than the destination buffer can accommodate. When the fromAddressNat function processes this input, the excess data overwrites adjacent stack memory, potentially allowing an attacker to control program execution flow.

Detection Methods for CVE-2026-5685

Indicators of Compromise

• Unusual HTTP POST requests to /goform/addressNat containing abnormally long page parameter values
• Device crashes or unexpected reboots that may indicate exploitation attempts
• Anomalous network traffic patterns to the Tenda CX12L web management interface
• Unexpected changes to device configuration or behavior following web interface access

Detection Strategies

• Monitor web server logs for requests to /goform/addressNat with unusually large parameter sizes
• Implement network intrusion detection rules to identify buffer overflow payload patterns targeting Tenda devices
• Deploy deep packet inspection to analyze HTTP traffic destined for the router's management interface
• Configure alerting for repeated authentication attempts followed by requests to the vulnerable endpoint

Monitoring Recommendations

• Establish baseline network traffic patterns for Tenda device management interfaces
• Enable logging on network segments where Tenda CX12L devices are deployed
• Monitor for unusual outbound connections from router devices that may indicate post-exploitation activity
• Review authentication logs for the device web interface for suspicious login patterns

How to Mitigate CVE-2026-5685

Immediate Actions Required

• Restrict network access to the Tenda CX12L web management interface to trusted IP addresses only
• Implement firewall rules to block external access to the /goform/addressNat endpoint
• Consider disabling the web management interface if not required for operations
• Monitor for firmware updates from Tenda that address this vulnerability

Patch Information

At the time of publication, no official patch has been confirmed from Tenda. Organizations should monitor the Tenda official website for security advisories and firmware updates addressing this vulnerability. Additional technical details are available through the VulDB vulnerability entry and the GitHub issue discussion.

Workarounds

• Disable remote management access to the Tenda CX12L device entirely
• Place the device behind a properly configured firewall that restricts access to the web management interface
• Implement network segmentation to isolate the management interface from untrusted networks
• Use a VPN for administrative access if remote management is required

# Example: Restrict access to management interface via iptables
# 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 only from trusted management network
iptables -I FORWARD -s <TRUSTED_MGMT_SUBNET> -d <ROUTER_IP> -p tcp --dport 80 -j ACCEPT