CVE-2026-5686: Tenda CX12L Buffer Overflow Vulnerability

CVE-2026-5686 Overview

A stack-based buffer overflow vulnerability has been discovered in Tenda CX12L router firmware version 16.03.53.12. This vulnerability affects the fromRouteStatic function within the /goform/RouteStatic file. Manipulation of the page argument allows an attacker to trigger a stack-based buffer overflow condition. The vulnerability is remotely exploitable, and a public exploit has been disclosed, increasing the risk of active exploitation.

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

• 2026-04-06 - CVE-2026-5686 published to NVD
• 2026-04-07 - Last updated in NVD database

Technical Details for CVE-2026-5686

Vulnerability Analysis

This vulnerability is classified under CWE-119 (Improper Restriction of Operations within the Bounds of a Memory Buffer), a fundamental memory safety issue that allows operations to occur outside the intended memory boundaries. The flaw resides in the fromRouteStatic function, which processes user-supplied input via the page parameter without adequate bounds checking.

When a malicious request is submitted to the /goform/RouteStatic endpoint with a specially crafted page parameter, the function copies the input into a fixed-size stack buffer without validating the input length. This allows an attacker to overwrite adjacent memory on the stack, including critical control data such as return addresses and saved registers.

The network-accessible nature of this vulnerability significantly increases its risk profile, as attackers can target vulnerable devices without requiring local access or user interaction. The public disclosure of exploit code further elevates the threat level, as it provides a ready-made attack vector for malicious actors.

Root Cause

The root cause is inadequate input validation and bounds checking in the fromRouteStatic function when processing the page parameter. The function allocates a fixed-size buffer on the stack and copies user-supplied data without verifying that the input length does not exceed the buffer capacity. This is a classic buffer overflow condition resulting from unsafe string handling practices commonly found in embedded device firmware.

Attack Vector

The attack can be initiated remotely over the network by sending a malicious HTTP request to the /goform/RouteStatic endpoint on an affected Tenda CX12L router. The attacker crafts a request with an oversized page parameter value designed to overflow the stack buffer. By carefully constructing the overflow payload, an attacker may be able to:

1. Overwrite the function's return address to redirect execution flow
2. Inject and execute shellcode on the device
3. Cause the device to crash, resulting in denial of service
4. Potentially gain persistent access to the device

The vulnerability requires low-privilege authentication to exploit, meaning an attacker would need some level of access to the router's web interface. Technical details and exploit information have been published to the GitHub Issue Discussion and documented in VulDB #355513.

Detection Methods for CVE-2026-5686

Indicators of Compromise

• HTTP POST requests to /goform/RouteStatic with unusually large page parameter values
• Unexpected router reboots or crashes indicating potential exploitation attempts
• Anomalous network traffic patterns to or from the router's management interface
• Unauthorized configuration changes on the affected device

Detection Strategies

• Deploy network intrusion detection rules to monitor for malformed HTTP requests targeting /goform/RouteStatic with oversized parameters
• Implement web application firewall (WAF) rules to block requests containing excessively long page parameter values
• Monitor router logs for repeated access attempts to the vulnerable endpoint
• Use firmware integrity verification to detect unauthorized modifications

Monitoring Recommendations

• Enable logging on the router if available and forward logs to a centralized SIEM solution
• Monitor for unusual outbound connections from the router that may indicate compromise
• Implement network segmentation to limit exposure of router management interfaces
• Conduct periodic vulnerability scans of network infrastructure devices

How to Mitigate CVE-2026-5686

Immediate Actions Required

• Restrict access to the router's web management interface to trusted IP addresses only
• Disable remote management if not required for operations
• Implement network segmentation to isolate the router from untrusted networks
• Monitor the Tenda Official Website for firmware updates addressing this vulnerability

Patch Information

As of the last update on 2026-04-07, no official patch has been released by Tenda for this vulnerability. Organizations should monitor Tenda's official channels and security advisories for firmware updates that address CVE-2026-5686. Additional technical information is available through VulDB Submission #792783.

Workarounds

• Configure firewall rules to restrict access to the /goform/RouteStatic endpoint from untrusted networks
• Disable the web-based management interface entirely if feasible and use alternative management methods
• Place the router behind a dedicated firewall that can filter and inspect HTTP traffic
• Consider replacing the affected device with an alternative that is not vulnerable until a patch is available

# Example firewall rule to restrict access to router management interface
# Block external access to the management interface on port 80/443
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 trusted admin subnet
iptables -I FORWARD -s 192.168.1.0/24 -d <router_ip> -p tcp --dport 80 -j ACCEPT