CVE-2026-3809: Tenda FH1202 Buffer Overflow Vulnerability

CVE-2026-3809 Overview

A stack-based buffer overflow vulnerability has been identified in Tenda FH1202 wireless router firmware version 1.2.0.14(408). The vulnerability exists in the fromNatStaticSetting function within the /goform/NatSaticSetting endpoint. By manipulating the page argument, a remote attacker can trigger a buffer overflow condition that may lead to arbitrary code execution or denial of service on the affected device.

Critical Impact

Remote attackers with low privileges can exploit this vulnerability over the network to potentially execute arbitrary code, compromise device integrity, or cause system crashes on affected Tenda FH1202 routers.

Affected Products

• Tenda FH1202 Firmware version 1.2.0.14(408)
• Tenda FH1202 Hardware

Discovery Timeline

• 2026-03-09 - CVE-2026-3809 published to NVD
• 2026-03-09 - Last updated in NVD database

Technical Details for CVE-2026-3809

Vulnerability Analysis

This vulnerability is classified as CWE-119 (Improper Restriction of Operations within the Bounds of a Memory Buffer), manifesting as a stack-based buffer overflow in the Tenda FH1202 router's web management interface. The flaw resides in the fromNatStaticSetting function, which handles NAT static setting configurations through the /goform/NatSaticSetting endpoint.

When the page parameter is processed by this function, insufficient bounds checking allows an attacker to supply an oversized input that exceeds the allocated stack buffer. This overflow condition can corrupt adjacent memory regions, including saved return addresses and other critical stack data structures.

The vulnerability is accessible over the network without requiring user interaction, making it particularly dangerous in environments where router management interfaces are exposed. A proof-of-concept exploit has been published, indicating active research interest in this vulnerability.

Root Cause

The root cause of this vulnerability is improper input validation in the fromNatStaticSetting function. The function fails to adequately verify the length of the page argument before copying it into a fixed-size stack buffer. This allows an attacker to provide an excessively long input value that overflows the buffer boundary, corrupting adjacent stack memory and potentially allowing control flow hijacking.

Attack Vector

The attack vector is network-based, targeting the web management interface of the Tenda FH1202 router. An attacker can exploit this vulnerability by sending a specially crafted HTTP request to the /goform/NatSaticSetting endpoint with a maliciously crafted page parameter containing excessive data. The vulnerability requires low privileges to exploit, indicating that some level of authentication may be required to access the vulnerable endpoint.

The exploitation mechanism involves overflowing the stack buffer to overwrite the return address, potentially redirecting execution to attacker-controlled shellcode or leveraging return-oriented programming (ROP) techniques to achieve arbitrary code execution. Technical details and proof-of-concept code are available in the GitHub PoC Repository.

Detection Methods for CVE-2026-3809

Indicators of Compromise

• Abnormally large HTTP POST requests to /goform/NatSaticSetting endpoint containing oversized page parameter values
• Unexpected router crashes, reboots, or unresponsive behavior following web management interface access
• Suspicious network traffic patterns targeting router management ports from unauthorized sources
• Evidence of unauthorized configuration changes or new administrative accounts on the device

Detection Strategies

• Implement network intrusion detection rules to monitor for HTTP requests to /goform/NatSaticSetting with abnormally long parameter values
• Deploy web application firewall (WAF) rules to block requests exceeding expected parameter lengths for router management endpoints
• Monitor router syslog output for crash dumps or memory corruption indicators
• Use network traffic analysis to identify reconnaissance or exploitation attempts targeting Tenda router management interfaces

Monitoring Recommendations

• Enable logging on network perimeter devices to capture all traffic to router management interfaces
• Implement alerting for repeated failed authentication attempts or unusual access patterns to router administration pages
• Monitor for firmware integrity changes or unauthorized modifications to router configuration
• Establish baseline behavior patterns for router management traffic to detect anomalies

How to Mitigate CVE-2026-3809

Immediate Actions Required

• Restrict access to the router's web management interface to trusted IP addresses only using firewall rules
• Disable remote management access if not strictly required for operations
• Implement network segmentation to isolate management interfaces from untrusted networks
• Monitor for exploitation attempts using the detection strategies outlined above

Patch Information

At the time of publication, no official patch from Tenda has been identified for this vulnerability. Administrators should monitor the Tenda Official Website for firmware updates that address CVE-2026-3809. Additional vulnerability intelligence is available through VulDB.

Workarounds

• Restrict web management interface access to localhost or trusted internal networks only
• Implement strong access control lists (ACLs) on upstream network devices to filter traffic to router management ports
• Consider replacing affected devices with models from vendors with better security update track records if patches are not forthcoming
• Deploy a VPN solution to secure remote management access instead of exposing the management interface directly

# Example iptables rules to restrict management interface access
# Allow management access only from trusted admin subnet
iptables -A INPUT -p tcp --dport 80 -s 192.168.1.0/24 -j ACCEPT
iptables -A INPUT -p tcp --dport 80 -j DROP

# Block external access to goform endpoints
iptables -A INPUT -p tcp --dport 80 -m string --string "/goform/" --algo bm -j DROP