CVE-2025-5623: D-Link DIR-816 Buffer Overflow Vulnerability

CVE-2025-5623 Overview

A critical stack-based buffer overflow vulnerability has been identified in D-Link DIR-816 router firmware version 1.10CNB05. This vulnerability affects the qosClassifier function located in the /goform/qosClassifier file. The flaw can be exploited through manipulation of the dip_address or sip_address parameters, allowing attackers to trigger a stack-based buffer overflow condition. The vulnerability is remotely exploitable and public exploit information has been disclosed.

Critical Impact
This vulnerability enables remote attackers to execute arbitrary code or cause denial of service on affected D-Link DIR-816 routers. The affected product is no longer supported by D-Link, meaning no official patches will be released.

Affected Products

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

Discovery Timeline

2025-06-05 - CVE-2025-5623 published to NVD
2025-06-06 - Last updated in NVD database

Technical Details for CVE-2025-5623

Vulnerability Analysis

This vulnerability is classified as a stack-based buffer overflow (CWE-787: Out-of-bounds Write) with additional implications under CWE-119 (Improper Restriction of Operations within the Bounds of a Memory Buffer). The vulnerable function qosClassifier fails to properly validate the length of user-supplied input through the dip_address and sip_address parameters before copying the data to a fixed-size stack buffer.

When an attacker sends a specially crafted HTTP request containing oversized data in these address fields, the function writes beyond the allocated buffer boundaries on the stack. This overflow can overwrite adjacent memory locations including saved return addresses, function pointers, or other critical stack data, potentially enabling arbitrary code execution with the privileges of the web service process running on the router.

Root Cause

The root cause of CVE-2025-5623 lies in insufficient input validation within the qosClassifier function. The function accepts IP address parameters (dip_address and sip_address) from HTTP form data processed via the /goform/qosClassifier endpoint without verifying that the input length does not exceed the destination buffer size. This classic buffer overflow pattern allows attackers to inject malicious payloads that corrupt stack memory.

Attack Vector

The attack can be initiated remotely over the network without requiring authentication. An attacker with network access to the router's web management interface can submit a malicious HTTP POST request to the /goform/qosClassifier endpoint. The request would contain specially crafted oversized values in the dip_address or sip_address form fields.

The exploitation mechanism involves overflowing the stack buffer to overwrite the return address, allowing the attacker to redirect execution flow to attacker-controlled code. On embedded devices like the DIR-816 router, which may lack modern exploit mitigations such as ASLR or stack canaries, exploitation success rates are typically higher.

For technical details on the vulnerability and exploitation methodology, refer to the GitHub Vulnerability Documentation and VulDB advisory #311109.

Detection Methods for CVE-2025-5623

Indicators of Compromise

Unusual or malformed HTTP POST requests targeting /goform/qosClassifier with excessively long dip_address or sip_address parameters
Router crashes, unexpected reboots, or unresponsive web management interface following suspicious network activity
Unexpected outbound network connections from the router indicating potential compromise
Modified router configuration settings or unauthorized firmware changes

Detection Strategies

Deploy network intrusion detection systems (IDS) with rules to detect oversized HTTP form parameters targeting D-Link router endpoints
Monitor for HTTP POST requests to /goform/qosClassifier containing unusually long IP address field values (exceeding typical IPv4/IPv6 address lengths)
Implement deep packet inspection to identify buffer overflow attack patterns in web traffic destined for router management interfaces
Alert on repeated connection attempts to router web management ports from external or untrusted networks

Monitoring Recommendations

Enable logging on network firewalls and capture traffic to and from router management interfaces
Regularly review router access logs for anomalous requests or failed authentication attempts
Set up alerts for any unexpected router behavior including spontaneous reboots or configuration changes
Monitor for DNS queries or outbound connections to suspicious domains originating from router IP addresses

How to Mitigate CVE-2025-5623

Immediate Actions Required

Disable remote web management access to the D-Link DIR-816 router immediately
Restrict access to the router's web management interface to trusted internal networks only using firewall rules
Consider replacing the affected D-Link DIR-816 router with a currently supported model that receives security updates
Segment the network to isolate vulnerable IoT and network devices from critical systems

Patch Information

D-Link has indicated that the DIR-816 router is an end-of-life (EOL) product and is no longer supported. As a result, no official security patch will be released to address CVE-2025-5623. Organizations using this device should plan for immediate device replacement with a supported router model.

For additional information, visit the D-Link Official Website.

Workarounds

Implement network-level access controls to restrict management interface access to specific trusted IP addresses
Deploy a web application firewall (WAF) or reverse proxy in front of the router management interface to filter malicious requests
Disable the QoS classifier functionality if not required for network operations
Use a VPN for any necessary remote management access rather than exposing the web interface directly

bash

# Example firewall rule to restrict router management access (iptables)
# Replace 192.168.1.1 with your router's IP and 192.168.1.100 with trusted admin IP
iptables -A INPUT -d 192.168.1.1 -p tcp --dport 80 -s 192.168.1.100 -j ACCEPT
iptables -A INPUT -d 192.168.1.1 -p tcp --dport 80 -j DROP