CVE-2026-4181: D-Link DIR-816 Buffer Overflow Vulnerability

CVE-2026-4181 Overview

A stack-based buffer overflow vulnerability has been discovered in D-Link DIR-816 router firmware version 1.10CNB05. This vulnerability affects the /goform/form2RepeaterStep2.cgi file within the goahead component, where improper handling of the key1, key2, key3, key4, and pskValue arguments can lead to a stack-based buffer overflow condition. The attack can be launched remotely over the network without requiring authentication, potentially allowing an attacker to execute arbitrary code or cause a denial of service. The exploit has been publicly disclosed and may be used in active attacks.

Critical Impact

This vulnerability affects an end-of-life product no longer supported by D-Link, meaning no official patches will be released. Remote exploitation could allow attackers to gain complete control over affected routers, compromising network security and all connected devices.

Affected Products

• D-Link DIR-816 Firmware version 1.10CNB05
• D-Link DIR-816 Hardware

Discovery Timeline

• 2026-03-16 - CVE-2026-4181 published to NVD
• 2026-03-19 - Last updated in NVD database

Technical Details for CVE-2026-4181

Vulnerability Analysis

This vulnerability is classified as a stack-based buffer overflow (CWE-787: Out-of-bounds Write) with an underlying improper restriction of operations within memory buffer bounds (CWE-119). The flaw exists in the goahead web server component of the D-Link DIR-816 router firmware, specifically within the repeater configuration functionality.

When processing HTTP requests to /goform/form2RepeaterStep2.cgi, the firmware fails to properly validate the length of user-supplied input for the wireless key parameters (key1, key2, key3, key4) and the pre-shared key value (pskValue). By supplying oversized input to these parameters, an attacker can overflow the stack buffer, potentially overwriting the return address and gaining control of program execution.

The vulnerability is particularly dangerous because it requires no authentication and can be exploited remotely over the network. Additionally, this product has reached end-of-life status and is no longer receiving security updates from D-Link.

Root Cause

The root cause of CVE-2026-4181 is insufficient bounds checking on user-supplied input to the repeater step 2 configuration handler. The key1, key2, key3, key4, and pskValue parameters are copied into fixed-size stack buffers without verifying that the input length does not exceed the allocated buffer size. This allows an attacker to write data beyond the buffer boundary, corrupting adjacent stack memory including saved return addresses.

Attack Vector

This vulnerability can be exploited remotely via crafted HTTP requests to the router's web management interface. An attacker on the same network, or remotely if the management interface is exposed to the internet, can send specially crafted POST requests to /goform/form2RepeaterStep2.cgi with oversized values for the vulnerable parameters.

The attack does not require any authentication or user interaction. Upon successful exploitation, the attacker may achieve arbitrary code execution with the privileges of the web server process, typically running as root on embedded devices. This could result in complete device compromise, allowing the attacker to intercept network traffic, modify DNS settings, pivot to internal network resources, or use the device as part of a botnet.

For detailed technical documentation of this vulnerability, refer to the GitHub Vulnerability Documentation or the VulDB entry #351085.

Detection Methods for CVE-2026-4181

Indicators of Compromise

• Unusual HTTP POST requests to /goform/form2RepeaterStep2.cgi with excessively long parameter values
• Unexpected router reboots or service crashes indicating potential exploitation attempts
• Modified router configuration, DNS settings, or firmware without administrative action
• Abnormal outbound network connections originating from the router

Detection Strategies

• Monitor web server logs for requests to /goform/form2RepeaterStep2.cgi with unusually large key1, key2, key3, key4, or pskValue parameters
• Deploy network intrusion detection signatures to identify buffer overflow patterns in HTTP traffic destined for D-Link routers
• Use network traffic analysis to detect POST requests with payload sizes exceeding normal operational parameters for router configuration endpoints

Monitoring Recommendations

• Implement network segmentation to isolate vulnerable D-Link DIR-816 devices from critical infrastructure
• Configure SIEM rules to alert on suspicious web traffic patterns targeting known vulnerable endpoints on router management interfaces
• Regularly audit router configurations for unauthorized changes that may indicate successful exploitation

How to Mitigate CVE-2026-4181

Immediate Actions Required

• Replace affected D-Link DIR-816 routers with actively supported models as this device is end-of-life with no patches available
• Disable remote management interfaces and restrict access to the web management console to trusted internal networks only
• Implement network access controls to limit which hosts can communicate with the router's management interface
• Consider deploying a firewall or access control list in front of the device to filter malicious requests

Patch Information

No official patch is available for this vulnerability. D-Link has designated the DIR-816 as an end-of-life product that is no longer receiving security updates. For more information, visit the D-Link Official Website. The VulDB entry provides additional tracking information for this vulnerability.

Workarounds

• Disable the web management interface entirely if not required for normal operations
• Restrict management interface access using MAC address filtering or IP-based access controls
• Place the router behind a properly configured firewall that can filter malicious HTTP requests
• Replace the vulnerable device with a currently supported router model that receives security updates

# Example: Restrict management interface access via iptables on an upstream firewall
# Block external access to the router management interface (adjust IP as needed)
iptables -A FORWARD -d 192.168.0.1 -p tcp --dport 80 -j DROP
iptables -A FORWARD -d 192.168.0.1 -p tcp --dport 443 -j DROP

# Allow only trusted admin workstation
iptables -I FORWARD -s 192.168.0.100 -d 192.168.0.1 -p tcp --dport 80 -j ACCEPT