CVE-2026-9401: Edimax BR-6675nD Buffer Overflow Flaw

CVE-2026-9401 Overview

CVE-2026-9401 is a buffer overflow vulnerability in the Edimax BR-6675nD router running firmware version 1.12. The flaw resides in the formWanTcpipSetup function within /goform/formWanTcpipSetup, part of the device's POST Request Handler component. Attackers manipulate the pppUserName parameter to overflow a fixed-size buffer. The issue is classified under [CWE-119], improper restriction of operations within the bounds of a memory buffer. The exploit has been disclosed publicly and the attack can be initiated remotely over the network. The vendor was contacted but did not respond to the disclosure, leaving affected devices without an official patch.

Critical Impact
Remote attackers with low privileges can trigger memory corruption in the router's web management interface, potentially leading to denial of service or arbitrary code execution on the embedded device.

Affected Products

Edimax BR-6675nD router
Firmware version 1.12
/goform/formWanTcpipSetup POST Request Handler component

Discovery Timeline

2026-05-24 - CVE-2026-9401 published to NVD
2026-05-26 - Last updated in NVD database

Technical Details for CVE-2026-9401

Vulnerability Analysis

The vulnerability is a classic stack-based buffer overflow in the web administration interface of the Edimax BR-6675nD router. The formWanTcpipSetup handler processes WAN configuration data submitted via HTTP POST requests. When the handler copies the pppUserName parameter into a fixed-size buffer, it fails to validate the input length, allowing an attacker-supplied value to write beyond the buffer boundary.

This behavior maps to [CWE-119], improper restriction of operations within the bounds of a memory buffer. On embedded MIPS or ARM-based routers, such overflows commonly overwrite saved return addresses on the stack, enabling control-flow hijacking. The attacker requires low privileges on the device, consistent with an authenticated session to the router's management portal.

Root Cause

The root cause is the absence of bounds checking before copying user-controlled POST data into a stack buffer. The pppUserName field, intended to hold a PPPoE username for WAN authentication, accepts arbitrary-length input. Embedded web servers in consumer routers frequently use unsafe string functions such as strcpy or sprintf without length validation, which is the likely implementation pattern in this firmware.

Attack Vector

An attacker with access to the router's management interface submits a crafted HTTP POST request to /goform/formWanTcpipSetup containing an oversized pppUserName value. The malformed request triggers memory corruption inside the request handler. Because the management interface is reachable over the local network and sometimes exposed to the WAN, remote exploitation is feasible. Successful exploitation can crash the device or, with reliable payload crafting, execute attacker-supplied code with the privileges of the web server process.

No verified public exploit code is available in the references. Technical details are documented in the Notion writeup and the VulDB vulnerability entry.

Detection Methods for CVE-2026-9401

Indicators of Compromise

HTTP POST requests to /goform/formWanTcpipSetup containing abnormally long pppUserName field values.
Unexpected reboots, crashes, or watchdog resets of Edimax BR-6675nD devices coinciding with management interface traffic.
Outbound connections originating from the router to unknown hosts following suspicious POST activity.

Detection Strategies

Inspect HTTP request bodies destined for the router's management interface and flag pppUserName parameters that exceed reasonable PPPoE username lengths (typically under 64 bytes).
Use network intrusion detection signatures targeting POST requests to /goform/formWanTcpipSetup with oversized form fields.
Correlate device availability metrics with web-management traffic to identify exploitation attempts that result in crashes.

Monitoring Recommendations

Log all authentication events and configuration changes on the router's administrative interface.
Monitor syslog output from the device for segmentation faults or process restarts on the HTTP server component.
Restrict and log access to TCP ports hosting the web management interface from both LAN and WAN segments.

How to Mitigate CVE-2026-9401

Immediate Actions Required

Disable remote WAN-side administration on the BR-6675nD if enabled, limiting the management interface to trusted LAN hosts only.
Place the router behind a network segment that restricts which clients can reach the management portal.
Change default and weak administrator credentials, since exploitation requires authenticated access at low privilege.
Consider replacing the BR-6675nD with a supported device, given the lack of vendor response to disclosure.

Patch Information

No vendor patch is available. According to the disclosure, Edimax was contacted early about this vulnerability but did not respond. Users running firmware 1.12 on the BR-6675nD should treat the device as unpatched and plan for compensating controls or replacement. Monitor the VulDB entry for CVE-2026-9401 for updates if the vendor issues firmware later.

Workarounds

Block external access to the router's HTTP management port using upstream firewall rules.
Apply network access control lists that limit POST requests to /goform/formWanTcpipSetup to administrator workstations only.
Place the device on an isolated VLAN to reduce the population of hosts that could authenticate and trigger the overflow.
Where feasible, replace end-of-life hardware with a vendor-supported router that receives security updates.

bash

# Example: restrict access to router management interface via upstream firewall
# Replace 192.168.1.1 with router IP and 192.168.1.10 with admin workstation
iptables -A FORWARD -p tcp -d 192.168.1.1 --dport 80 -s 192.168.1.10 -j ACCEPT
iptables -A FORWARD -p tcp -d 192.168.1.1 --dport 80 -j DROP