CVE-2026-5687: Tenda CX12L Buffer Overflow Vulnerability

CVE-2026-5687 Overview

A stack-based buffer overflow vulnerability has been identified in Tenda CX12L router firmware version 16.03.53.12. This vulnerability affects the fromNatStaticSetting function within the /goform/NatStaticSetting endpoint. By manipulating the page argument, an attacker can trigger a stack-based buffer overflow condition. The vulnerability can be exploited remotely over the network, making it a significant security concern for affected devices.

Critical Impact

Remote attackers can exploit this buffer overflow vulnerability to potentially execute arbitrary code or cause denial of service on affected Tenda CX12L routers, compromising network security and device integrity.

Affected Products

• Tenda CX12L firmware version 16.03.53.12

Discovery Timeline

• April 6, 2026 - CVE-2026-5687 published to NVD
• April 7, 2026 - Last updated in NVD database

Technical Details for CVE-2026-5687

Vulnerability Analysis

This vulnerability is classified under CWE-119 (Improper Restriction of Operations within the Bounds of a Memory Buffer). The fromNatStaticSetting function in the Tenda CX12L router firmware fails to properly validate the length of user-supplied input for the page argument before copying it to a fixed-size buffer on the stack. When an attacker provides an oversized input value, the function writes beyond the allocated buffer boundaries, corrupting adjacent stack memory including potentially critical data such as saved return addresses and frame pointers.

The attack can be initiated remotely through the network interface, requiring low-level authentication. An attacker who successfully exploits this vulnerability could achieve complete compromise of confidentiality, integrity, and availability of the affected device.

Root Cause

The root cause of this vulnerability is insufficient input validation in the fromNatStaticSetting function. The firmware does not implement proper boundary checks when processing the page parameter received via the /goform/NatStaticSetting HTTP endpoint. This allows user-controlled data to overflow the stack buffer, enabling attackers to overwrite critical stack memory structures.

Attack Vector

The vulnerability is exploitable over the network by sending a crafted HTTP request to the /goform/NatStaticSetting endpoint on the affected Tenda CX12L device. The attacker must include a maliciously crafted page parameter with excessive length to trigger the stack-based buffer overflow.

The attack flow involves:

1. An attacker identifies a vulnerable Tenda CX12L device accessible over the network
2. The attacker crafts a malicious HTTP request targeting /goform/NatStaticSetting
3. The page argument is populated with an oversized payload designed to overflow the stack buffer
4. Upon processing, the fromNatStaticSetting function copies the malicious input without proper bounds checking
5. The overflow corrupts stack memory, potentially allowing code execution or causing device crash

For detailed technical analysis and proof-of-concept information, refer to the GitHub Issue Discussion and the VulDB Vulnerability Report.

Detection Methods for CVE-2026-5687

Indicators of Compromise

• Abnormally large HTTP POST requests to /goform/NatStaticSetting containing oversized page parameter values
• Unexpected router reboots or crashes that may indicate exploitation attempts
• Network traffic anomalies showing repeated connections to the router's web management interface
• Evidence of unauthorized configuration changes on the affected device

Detection Strategies

• Implement network intrusion detection rules to monitor for HTTP requests to /goform/NatStaticSetting with unusually large payload sizes
• Configure web application firewall rules to block requests with page parameter values exceeding expected lengths
• Deploy SentinelOne Singularity to detect and block exploitation attempts targeting embedded devices and IoT infrastructure

Monitoring Recommendations

• Enable logging on network security devices to capture all HTTP traffic directed at Tenda router management interfaces
• Monitor for patterns of repeated failed or malformed requests that may indicate reconnaissance or exploitation attempts
• Set up alerts for device availability issues that could indicate successful DoS exploitation

How to Mitigate CVE-2026-5687

Immediate Actions Required

• Restrict network access to the Tenda CX12L web management interface using firewall rules or network segmentation
• Disable remote administration features if not required for operations
• Place affected devices behind a properly configured firewall that blocks external access to management ports
• Monitor Tenda's official channels for firmware updates addressing this vulnerability

Patch Information

At the time of publication, no official patch from Tenda has been confirmed for this vulnerability. Users should monitor the Tenda Official Website and security advisory channels for firmware updates. Organizations should prioritize applying security patches as soon as they become available from the vendor.

For additional vulnerability details, refer to the VulDB Submission Record and VulDB CTI Details.

Workarounds

• Isolate affected Tenda CX12L devices on a separate network segment with restricted access
• Implement access control lists (ACLs) to limit which IP addresses can access the router's web interface
• Consider deploying an alternative router solution if the device handles critical network functions and no patch is available
• Use VPN or jump host solutions to access device management interfaces rather than exposing them directly

# Example firewall rule to restrict access to router management interface
# Block external access to the web management port (adjust port as needed)
iptables -A INPUT -p tcp --dport 80 -s ! 192.168.1.0/24 -j DROP
iptables -A INPUT -p tcp --dport 443 -s ! 192.168.1.0/24 -j DROP